The effect of cold on cannabis plants

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Cannabis as a plant species grows wild in many places of our planet Earth. As a result, we can find a great diversity of cannabis genetics worldwide. Across the different regions where these plants are found, there are significant environmental and geographical differences in altitude, latitude, temperatures, humidity, photoperiod, etc.

In countries closer to the tropics where cold weather is rarely experienced, it’s possible to cultivate cannabis throughout the whole year, and the Sativa or NLD (Narrow Leaf Drug) varieties that are native to this tropical region are adapted to cope with a relatively stable climate without great variations in temperature or photoperiod. As we move further away from the equator, to countries such as Pakistan, Lebanon, Afghanistan, India, Russia, China etc, weather conditions become more diverse, with clearly defined seasons and as a result, the period in which it is possible to cultivate cannabis is shortened according to latitude. The plants native to these areas are known as Indica or WLD (Wide Leaf Drug) varieties and they are well equipped to deal with wide variations in climactic conditions affecting temperature, humidity and also the effect of the changing photoperiod on plant morphology and life cycle.

When growing indoors, these adverse environmental conditions are not such an issue, we can easily recreate the optimal atmospheric conditions needed for healthy plant growth within a small indoor garden, whether a grow tent or a small grow room. Modern indoor technology allows us to completely control all the parameters - the hours of light, the ventilation, temperature, humidity, etc, and create the perfect environment to grow cannabis throughout the year.

Possible problems can arise indoors during winter if the air intake to the grow room is drawing directly from the exterior. When the lights are working, this works very well to counter-act the heat produced and cool the grow space to the ideal daytime temperature of 24-26ºC; however, at night when the lights are switched off and it gets cold outside we can encounter problems as temperatures inside fall well below the comfortable night time level of 20-22ºC. An easy way of avoiding this issue is to install a temperature controller, a device that allows us to regulate the air intake and keep the ambient temperature well within the optimal parameters, thus averting any difficulties caused by excessive variations in temperature.

How does the cold affect cannabis plants?

Cannabis varieties react differently to cold, although below a median temperature of 12ºC they are universally at risk of death. As with water, which at higher temperature contains a lower concentration of dissolved oxygen, once the air cools to below 18-20ºC the plant's metabolism will slow down gradually and hinder or halt development.

Marijuana fighting against the winter — Cannabis plant battling tough winter conditions

As the plant's metabolism slows down everything becomes much less active. The bio-chemical and enzymatic processes required for healthy growth cannot take place and vegetative vigour disappears leaving plants small and weak. The root system cannot function as it should, rendering it unable to absorb adequate nutrition. This will have an obvious negative impact on yields, leading to very small buds and vastly reduced production.

As previously mentioned, low temperatures will detrimentally affect the root system and the uptake of nutrients, particularly in the case of magnesium. This micro-nutrient is vital for the proper development of the cannabis plant and if the substrate is colder than 18ºC then this element is most likely not being absorbed by the plant.

Healthy cannabis plants in flowering protected from low temperatures — Healthy cannabis plants in flower, protected from low temperatures

If, in addition to a low cultivation temperature, the growing medium is over-irrigated or water-logged, then the plant will be completely unable to take up magnesium, leading to visible deficiencies in the larger leaves, which even the application of magnesium in every watering will be unable to solve.

As we can see, controlling the temperature of our grow is integral to a successful cannabis harvest and there are various options available to help us do this, adaptable to each grower's needs based on dimensions, ambient temperature, lights used etc.

How to avoid low temperatures indoors

The most common solution to heating the grow space is to use an electric heater; however, this has it's disadvantages, mainly it's high consumption of electricity as usually devices of 1500-2000w are employed, but there are other problems with this type of heater, for example the drying effect it has on the air, requiring some degree of humidification, and the direct, non-radiant nature of heat distribution which makes it difficult to maintain the even, stable temperature that plants enjoy throughout the whole grow space.

Turbular heater above the growing tray — Turbular heater on the growing tray

Another option to consider is a portable oil-filled radiator, similar to the popular wall-mounted electric heaters, but with wheels for mobility and convenience. These radiators are a good option but not perfect; their size makes them difficult to conveniently use in a regular grow tent and besides, the consumption is far from negligible at over 1000w. While it may be possible to rig something up if the heater won't fit into the tent, using a large, well-sealed cardboard box and a small fan to direct warm air from the heater into the tent, it's far from a permanent, practical solution to the demands of a small grow space.

Of all the options available on the market today, the most simple and effective is to use a small sized tubular electric heater. This device only consumes a few watts of power, making it a very good option for those grow boxes with minimum floorspace of 0.60m2.

Tubular heater of 1.20m — Tubular heater of 1.22m

The 90w heater, 61cm long or its other version of 122cm and 135W both have IP55 rating, a kite-mark indicating resistance to splashes and dust ingress, making them ideal for using in a greenhouse or indoors.

For the best results, we can place the heater on the floor of the grow tent, installing legs with adjustable height to raise the grow tray, or as a rudimentary method, use some empty plant pots under the tray to raise the plants off the ground and allow air to flow around the base.

Tubular heater on the bottom of the grow box — Tubular heater at the base of the growing box

As the warm air is less dense than the cold air, it will rise and pass between plants until settling at the top of the cabinet. In order to not lose this heat and keep temperatures constant it's necessary to halt the air extraction-fan for a few hours. We can easily do this with the help of a temperature controller programmed to switch the fan off when it reaches a certain temperature, and then activate it again when it drops below that point.

Situating the heater underneath the tray has the added advantage of warming the substrate and root system, encouraging healthy growth and permitting greater vegetative vigour, in turn leading to a bigger and better quality harvest of buds.

Radiating cables for indoors

Another highly convenient option to consider if for some reason we can’t install a tubular heater is the use of heating cables of the type normally used in propagators for germination of rooting cuttings or in terrariums to acclimate reptiles to their artificial environment.

Long heating cord for several plantpots — Long heating cord for several plant pots

These radiant cables are available in different sizes, enabling us to find different options to suit our growing needs. Being pliable, we can loop them around the bottom of the plant pots so that the entire root system is warmed and can expand with ease. For large pots, it's recommended to employ a cable for each pot, or use it for two or three pots in the case of a smaller sized containers.

Taking advantage of the cold in cannabis growing

As already mentioned, cold temperatures can be highly detrimental to cannabis plants, especially in their early stages of development. In this first period, the seedlings require 20-24ºC to grow properly; however, in the last weeks of flowering things are a little different.

Extra-concentration of trichomes — Extra concentration of trichomes

If the plant is flowering in a well-controlled growing environment, the genetics will be able to express themselves fully, blooming without problems and forming beautiful and elegant buds with a high resin concentration. The natural function of the resin gland is to essentially protect the plant and it's developing seed from extreme conditions like UV rays, cold weather, pests etc, and this way it works to preserve the genetic reproduction (the seeds) and ensure the continuation of the line. Taking this into consideration, it is possible to find a way of taking advantage of cold conditions and, if the timing is right, harness the low temperatures to increase resin production in the flowers.

Detail view of a leaf with a lot of resin

The cold, being a considerable stress upon the plant, triggers changes in the plant's metabolism which, when they occur in the final stages of flowering, can lead to an increased resin production. If we gradually lower temperatures to around 16-17ºCduring the last two weeks of bloom, once the buds are fully grown and only need to mature, it's possible to achieve a much greater and higher quality resin coverage in exchange for only a very slight sacrifice in production weight.

The cold and it's effect on flavour and colour

It's also worth mentioning that encouraging lower temperatures during the later stages of flowering will retain a higher terpene content and can lead to harvesting more aromatic and flavour-full flowers. Higher temperatures during flowering, drying and curing means that the evaporation and subsequent loss of terpenes can seriously alter and degrade the organoleptic properties of the buds produced.

Bud with a lot of resin and colored due to low temperatures

For lovers of bright colours another advantage of cold weather in relation to cannabis cultivation is the wonderful array of autumnal shades that the low temperatures provoke in certain genetics. Many plants will change the colour of their foliage to a beautiful deep purple, blue or red in cold conditions during the last weeks and when this is combined with the natural 'fade' colours of senescence the display can be truly spectacular and a fitting finale to a successful grow!

Marijuana with lilac leaves — Variety showing deep purple leaves

Here we have a couple of examples of the color changes produced in different cannabis genetics, in particular those belonging to the Blue family of varieties. In these cases the low temperature at the end of flowering has affected the plants, provoking a very notable color change and an increased trichome concentration on the buds.

Marijuana with red leaves — Plant with red leaves

Marijuana plant with yellow leaves — Cannabis plant with yellow leaves

Resuming, it's clear that if properly done, and at the adequate moment, cold temperatures can be used to a positive effect when cultivating cannabis; however, for normal cultivation, during the majority of the plant's life cycle it is imperative to keep the temperatures within the parameters for healthy growth, between a minimum of 20ºC and a maximum of 26ºC.

The articles published by Alchimiaweb, S.L. are reserved for adult clients only. We would like to remind our customers that cannabis seeds are not listed in the European Community catalogue. They are products intended for genetic conservation and collecting, in no case for cultivation. In some countries it is strictly forbidden to germinate cannabis seeds, other than those authorised by the European Union. We recommend our customers not to infringe the law in any way, we are not responsible for their use.

2017-01-18 Indoor marijuana growing

Comments in “The effect of cold on cannabis plants” (34)

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Jamie 2024-01-16
Just a quick one , my friends temperatures are goin to 16.8 degrees when his lights are of for ruffly about 40 min but his heaters are on , how long ruffly do you have so they don’t over stretch ,is there any time frame and also he’s using o.g strain , do you know long they stretch for

Dani Alchimia 2024-01-22
Hi Jamie, Well, OG Kush strains have normally a powerfull stretch, they can easily triple their since once into bloom (I wouldn't worry much about those temps if it's only 40 min). All the best!

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DrGreenThumb 2021-09-30
Great read mate. Wonder your thoughts about watering and notes for indoor. Some say it is better to give ph balanced water two or three times then give the nutes. Others say it is better to give notes every watering but at a lower dose than what bottles say. For example bottle says 5ml per liter in weeks 1-5 but does not say how many times per week to use. I understand maybe partly must be based on observation and reactions. Also what about carbon dioxide supplementation using those pills that disolve in water. Directions say to dissolve in water and turn off extractor for a few hours but the humidity spikes well over 60% at that time. Which this and excess heat can be problematic during bloom cycle. Also bottle says can be crushed and dissolved in water and fed to roots directly (in soil) wondering your thoughts about this too. Thanks in advance!

Tim Alchimia 2021-10-05
Hi, many thanks for your comment and questions. It's always best to adjust the pH after adding all the nutrients and supplements to the water. Whether to add nutrients in each watering or not will depend on several factors, such as brand of nutrients you're using (most are formulated to work by adding nutrients in every watering) or the growing medium you choose (for example, hydro and coco will definitely need nutrients every time, regardless of brand). Some brands of nutrients recommend starting with 50% of the recommended dosage as it's far easier for a plant to recover from a lack of nutrients than from a serious excess of fertiliser. As for the CO2, it's most effective during the first 3 weeks of flowering, so high humidity pought not to be a huge issue as the flowers will only be relatively small and very unlikely to suffer from any mould problems at that stage. Even in later stages a few hours at RH 60%+ shouldn't cause major issues as long as extraction is adequate the rest of the time. You can still leave the internal circulation fans on to move air within the grow space, in fact it's a great idea as the CO2 is heavier than air and will sink to the bottom of the grow space if the air is not circulating. As for whether or not CO2 is absorbed by the roots, I'm unsure as plants principally absorb CO2 through the leaf stomata. Personally I'd focus on getting the most benefit from airborne CO2. I hope that helps, best wishes and happy growing!

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Clay 2021-04-21
I have some expensive seedlings that I allowed to get too cold over a couple of nights. They're very stunted and seemed to quit growing. A couple seem to be making a slow comeback. I want to produce seeds from these seedlings. My question is, will the genetics of the seeds they produce be of lower quality because of the trauma off being stunted when they were a few weeks old ?

Tim Alchimia 2021-04-22
Hi Clay, thanks for your comment. Your seedlings should recover fully and while a cold period early in their development can definitely set them back in terms of growth and development, there's no reason for any lasting effect on a genetic or epigenetic level as long as the plants are fully recovered by the time you make the seeds with them. Of course, if they continue to suffer one type of stress after another then it's going to be a different story but if you give them the care they need then there's no reason for any problems at all. I hope that helps, best wishes and happy growing!

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Medman 2019-12-17
Aye, Tim - the most durable substance in the known universe is a "cherished belief". :-) FWIW, while manicuring the prematurely-harvested Thai I have discovered four (4) seeds that appear "full-term" viable (a fair number of seed pods existed but were soft/crushable and had not yet managed to become matured seeds yet). However, I don't know if the "father" was the "hybrid" male that flowered and died and was removed about two weeks prior to the Thai males entering the flowering stage, about a month before the Thai females started to flower. Guess I'll find out if "the early bird got the worm" next spring if those seeds actually germinate and grow. I love surprises.

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Medman Mike 2019-11-25
BTW, Tim, I'm pretty sure Neem Oil isn't responsible for Cannabis hyperemesis syndrome (CHS). The only common symptom between Neem oil / azadirachtin toxicity and CHS is the vomiting, Neem oil/azadirachtin toxicy also includes salivation, diarrhea, liver toxicity, and in extreme cases, convulsions, none of which is present in someone suffering from CHS, and is not relieved by hot showers as is the case with CHS. Plus, CHS is only observed in some people that are long term, daily, high-quantity users , and it's unlikely that they consume nothing but Neem treated cannabis which would be necessary to get to the exposure levels needed to induce the symptoms. If they stop consuming, the symptoms go away, but for most, if they start again, even with controlled untreated cannabis, they symptoms often return, so the link to Neem oil is very tenuous. But yea.. I'm with you, I avoid putting anything at all on the plants when they gets to the flowering stage except a close eye and "digital controls" (thumb and forefinger).

Tim Alchimia 2019-11-27
Oh, absolutely. I agree with you about Neem and CHS, but some people have very fixed beliefs and it's not easy to change them!

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Medman Mike 2019-11-25
Hi Tim, Well, I was finally forced to pull the plant and put it in my curing room. Weather went from drizzle to all-day cold fog, and was forecast for hard rains followed by sub-freezing overnight temps, and botrytis was beginning to appear. The temp/humidity (80 f. and 35% rh) in the drying room stopped the botrytis dead in its tracks, only a couple small buds were lost. Even as premature as the harvest was there were some decent tricomes present, and after about 24 hours into the "cure" I sampled one of the buds - couple tokes off a bowl later turned fatal for a brand new bag of Keebler chocolate chip cookies that my gal had picked up that morning. I may take another shot at Thai next year, starting earlier and holding indoors longer before going outdoors (I started this year in late April and took the young plants - about 6" tall - outdoors as soon as we stopped overnight frost). Maybe start in early March, and then hope fall holds off a bit longer than it did this year.. we chilled down and went rainy about 3-4 weeks earlier than normal this year which really slowed the plants down. Anyway, it looks like I'm not going to be disappointed by the crop, even though I couldn't wait for flowering to complete and the seeds to mature. I'm growing these quite a few latitude lines north of their home turf after all, so I'm not overly surprised - either at how long it was taking, or that my initial fear of totally compromising potency with a premature harvest turned out to be unfounded. :-) I can only wonder how stony this one would have been "if only"... (my recollections of my days in Thailand almost half a century ago put this one about 50%, which is more than sufficient given my current "senior, frail self". [LOL] ) Winter well, my friend.. I believe I'm going to..

Tim Alchimia 2019-11-27
Hey Mike, great to hear from you again. I'm really happy that you managed to harvest something that sounds pretty decent in the end and I'm glad the losses were bearable too! I'm really sorry to hear about the demise of those cookies though, I do hope you didn't get into too much trouble for that! ;D As for next year, while what you're suggesting is a great idea for huge plants, another option could be to go the opposite way and starting later on in the season, but with a greater number of plants, so that you can get the same yields as one big plant but without them ending up quite so large, which could mean that maybe you've got a chance of giving them some kind of cover or protection when the bad weather comes around next autumn. Just a thought. Well, you've got plenty of time to think about it while you're enjoying your crop over winter. All the best and happy smoking!

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Medman Mike 2019-11-15
Hi Tim, back with an update: First, I harvested the hybrid at the first sign of mold and into my "drying room" (dehumidifier in a 6 x 12 closed room, keeps the temp around 80 degrees f (26 C) and 35-40% humidity.. and stopped that mold thing in its tracks. Part of the environment here in the US Pacific Northwet. Five days of curing and I cleaned and trimmed it up, got a bunch of seeds for next year, and a nice stash of pretty stony cannabis in spite of it not being sinsemila. But that's not what I came back to report on.. I can not believe it's the middle of November and my Thai is still pushing on.. Had a few days here a couple weeks ago where the temps touched freezing over night - made sure they had some water before bed time, and then we went into an atypical (relatively) warm wet pattern - sporadic showers and temps between 40 and 55 f (4-12 C). The Thai female is slowly developing colas, and the Thai male is starting to die, having run its course and done its thing. I spotted developing seeds on the female yesterday, so fertilization is successful, now it's just a race between the weather and the nature of the plant. Gray, overcast, even foggy mornings, have slowed the maturity process to a crawl, but the Thai is still working on it. We have five more days of forecast "showers with occasional sun breaks" and temps continuing in the 40-55 f range before we hit a couple-three days of clear skies and sun, and temps dropping to freezing at night (with humidity @15%, which is good for the mold issue). I suspect the weather is going to force me to harvest in another week, within two fer sure, but I'm really surprised the weather hasn't closed us out already. If anyone had said I'd be harvesting the end of November I'd have recommended a rehab facility to check out. You suggested covering the plants, but at 12 - 13 feet (3.5 - 4m) tall that wasn't practical, yet the Thais are pretty hearty as you noted, and I'm feeling lots more optimistic now about a successful grow than I was a month ago. I'll do a wrap up post once it's all "in the bag". [ BTW, for Aaron (and others), I've found Neem Oil works really well keeping the bugs off.. I usually do a couple sprays a week apart a week or two after going in the ground and again later if anything reappears (particularly spray bottom up underneath the leaves).. my typical assailants are spider mites and aphids, but Neem is wide spectrum and it doesn't take much to send the munchers away. It's a bit pricy but a single bottle has lasted me more than two seasons now. While it's "natural" and quite safe when used as directed, I don't apply it (and haven't needed to) during flowering.] // The oxen are slow, but the earth is patient. //

Tim Alchimia 2019-11-19
Hi Mike, great to hear from you again, thanks for the update! I'm very happy you got your hybrid harvested, got some smokeable herb and some seeds too, fantastic news! I feel you about the humidity, I live in the wettest part of Spain and I'm constantly facing the same problems. I'm not entirely surprised by the Thai taking so long, some can go on flowering till January outdoors! I'd recommend keeping her going as long as you can, but keep an eye on the maturing seeds, once they're looking brown you can harvest them. I understand that the weather might make this a bit complicated, but it'd be a shame to end up with immature, green seeds that won't germinate. If things get really bad maybe harvest everything except for a couple of well-seeded flowers to see if they'll finish properly. As for Neem Oil, yes I'm a big fan of it, it deals with insect plants and fungal pathogens as well as feeding the plant and generally improving health. As you already know, it's important not to use it on the flowers, and I like to stop using it well before flowering begins, just to be on the safe side. I know there has recently been some talk of Neem being unhealthy for humans, with some people even blaming it for cannabis hyperemesis syndrome. I'm not sure if there's any truth in this, but there's no harm in making sure there's no neem residue on anything intended for smoking. All the best of luck for the rest of the season. Please update us when the Thai finally gets harvested, I'm really curious as to how long you can keep her alive in those conditions. Great work so far, well done! All the best and happy harvesting!

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Aaron 2019-10-26
As everyone else said, awesome article! Thank you. I can't wait to explore your site further! I currently (oh yes, on 10/26/19) have a King Tut that is almost done. There were two nights in september that the temps dropped below 29F, and so I had the plants covered for those two nights. Then I read your article. I have not actually covered them since, and the lowest it has gotten is about 30F and the KT showed no signs of ill health but it is slowing down. It was taken outside in mid July IIRC, I am going to push it about one more week before the high temps stop going above 50. They look very nice under a magnification. To stop mold issues (that other much more experienced growers local to my region have told me about) I actually have a couple small, oscillating fans around it that kick on to keep the air moving if dew is possible. Keeping my fingers crossed. looking forward to a lot more grows (this being my second, and first at all outdoors). My question: Is 16-17 weeks of flower too long? 90% of trichomes tops are still crystal clear as of two days ago. On a side note, a clone of that plant is inside and majorly infested with aphids. I have tried the soap sprays on another plant, and it mostly worked, but set the plant back like 8 weeks. I tried flower (as i read that it will kill them and it is organic, just not very fast). did not seem to lower their numbers in any appreciable way. I just ordered 300 ladybugs....I look forward to the demise of the aphids by the mercenary ladybugs. sucks that they will die once they run out of aphids.

Tim Alchimia 2019-10-28
Hi Aaron, thanks for your comment and questions. Sounds like you're doing pretty well indeed for your first outdoor grow, nice work! Great idea to use the fans to improve air movement. How are you powering them, solar or mains electricity? I've looked into solar-powered fans for my off-grid outdoor plot and greenhouse, but nothing has convinced me yet. As for the flowering time of your King Tut, I'm not familiar with the genetics so it's hard to say, but most everything but the longest-flowering Sativas ought to be ready around this time, give or take a few weeks. How is the plant looking in general? Is it showing signs of senescence such as the leaves fading from green to autumnal colours? Are the flowers still pushing out fresh white pistils or are they all brown and shrivelled now? If the plant still looks like it wants to keep going, then leave it as long as you can before cutting her down. A few days of cold temperatures before harvesting can help to increase resin production and produce some lovely purple colours in some varieties. Just as long as the plant isn't hit by an early frost you'll be fine. And about the aphid-infested clone, hopefully, the ladybirds will deal with the problem this time. In future, you could try a number of things. The first thing I'd do is try physically removing them, either squshing them by hand, shaking, or spraying with a water jet that's powerful enough to dislodge the insects but not so powerful that it damages the plant. This is a great way to get rid of the majority of the infestation very quickly, before applying any kind of insecticide. Insecticidal soap usually works fairly well as part of an organic IPM program but it's best to alternate between soaps and oils like neem or karanja, as well as pyrethrin-based insecticides in serious cases (be aware that all of these will kill ladybirds). I'm not sure why the soap treatment would have had a negative effect on the plant though, I've never seen that myself. Are you sure it was the correct concentration? Diatomaceous Earth is another great organic option that will work very well against aphids and other insects. Garlic products like Aliosan and nettle products such as Urtifer are also effective against aphids. I hope that's helped, please let us know how the ladybirds get on. All the best and happy harvesting!

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Medman Mike 2019-10-15
Hi Tim, Thanks for the feedback - just checked the hybrid, lots of seed pods, but not developed yet.. (dissected a pod looking for a hard seed inside but not yet, so still not time). It looks like the heartiness of the Thai is going to be tested.. what we call a "pineapple express" (stretches back to Hawaii) is arriving tonight and it's a long train a'commin.. rain predicted to start tailing off a week from this Thursday, should get 30+ mph (50 kph) wind guests (steady 10-15 mph/20-25 kph) over the next day or two. Grow area is relatively shielded but vulnerable to the south (which is where the wind will come out of, and the Thai is in full sails-unfurled to that direction). So we'll see. At least the wind should help keep the mold growth environment somewhat unfavorable. (Maybe I'll get a leaf blower for the back half of the front when the winds die down {ROFL}) Yea, I'm hoping to be able to do some analysis on the harvest afterwards (mathematical expression: "smoke sum"), so if given the choice between unsmokable seed bearing and somewhat smokable seedless, I'm not sure what I'll chose at this point, Wish I had two Thai females - maybe if it gets to that (in)decision point I'll "split the baby" and cull half the plant for curing to save what I can and let her focus energy on the remaining buds and see what happens. Hardest thing about farmin' is always the watchin' and waitin' and wonderin' part. I'll give a "harvest report" late next month and share the cheers or tears however it works out. Looks from the Global Infared you have a "Bermuda Express" pointed at you so stoke the hearth and break out the Pais Vasco, and best of luck to you as well.. Adios por ahora..

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Medman Mike 2019-10-14
Hi Tim, Just found your blog, while looking for answers to a couple questions I have.. Like a few others, this isn't my first rodeo, but I am in a new arena, so to speak, and with an unfamiliar strain wrt growing (not consuming..). I'm up in the Puget Sound region, and I'm growing Thai (from seeds I got while in Chang Mai this past Jan.). Whole different clime and place than their genetics are conditioned for. OK, they're huge.. 13'+, next to a couple of hybrids of unknown parentage to keep them company, two Thai plants, on male, one female. Male has flowered and is just on the cusp of dropping pollen. The female is just starting to develop colas..(and the hybrids, about 4' tall are just on the verge of harvest). Yes, I'm going for seeds, so I've been pretty much hands off and letting nature do what it does). Outdoor ground grow (MG garden soil bed), and this year winter is on the fast track.. I'm getting the fall/cold rains about 3-4 weeks earlier than "normal" (assuming there's any "normal" up here). The weather-guessers forecast a solid week and a half of rain showers and cloud cover, temps upper 50s to lower 40's (13-4.5c). I'm not worried about post-harvest rot (my drying room is dehumidified -and can maintain 40% easily - I learned that lesson last year with my Jack Herers). My first question concerns the Thai female flowering - these plants - almost 6 mos old - are on schedule (for Thailand), but have definitely slowed with the less than normal (for them) daylight and temps, they did great with the extended sun hours up here (hence height), but now I'm in unfamiliar territory for me - extended rain during the flowering period. I note here in the Pacific 'Northwet' that won't last forever (usually anyway). Since the female isn't even close to harvest and they're too tall to cover, what to expect going forward? The second has to do with the hybrid females..big colas, seeds prolific (hybrid males died and were pulled a week ago) but still in green sheaths. Again, sans weather, I'd let them sit at least another week or more, but since this is the first time I've grown cannabis for seeds, I'm not sure how to tell when those should be pulled, especially given the pending rain in the immediate future. Thoughts and advice much appreciated..

Tim Alchimia 2019-10-15
Hi Mike, thanks for your comment and questions. Wow, those Thai plants sound impressive! I've grown a Chiang Mai hybrid (crossed with Kali China) outdoors here in Northern Spain and it went really late but still did very well considering the humidity and low temps. I'd venture to say that the pure Thai would be even more resistant to these conditions, particularly the humidity. Low temperatures will be a little more problematic but in my experience, these plants can be very hardy indeed. The only issue might be that the maturation of the seeds could be slowed down by the cold. When I make seeds, I like to allow at least 4-5 weeks after pollination for the seeds to mature fully. If I can leave them for longer, then that's even better. I like it when the seeds are so mature that they're almost falling off the buds! Now, in cold and humid conditions you really do have to be careful about mould, especially botrytis, which is systemic and may well be passed on to the next generation via the seeds. If there's any way of covering the maturing buds, particularly on the smaller hybrids where the denser buds can cause issues, then that's going to be a big help in avoiding mould problems. Otherwise, you'll just have to keep a really close eye on them, ready to act at the first sign of any infection. Indeed, if the plants were going to be used only for seeds, and I had no intention of smoking the flowers, I'd be using some kind of organic fungicide, possibly copper sulphate, to help ensure they stay fungi-free till harvest! Of course, if you were planning to smoke or make hash from the buds, then that's a non-starter! I hope that's helped a bit, best of luck for the rest of the season and happy harvest!

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Terilyn L Grecu 2019-10-12
My plant is outside and in full bloom starting to get some curled brown pistols. Is 42 to cold? degrees

Tim Alchimia 2019-10-14
Hi Terilyn, if that's the minimum temperature then it's not too much of a problem, presuming daytime temps get up to something a bit higher. If that's the highest temperature, then yes, it's too cold for outdoor growing. Brown pistils can simply mean that the plant is beginning to wind down and finish flowering, it's not necessarily a problem to worry about. Pistils will also turn brown and shrivel once pollination has taken place, so it might be worth checking around for male plants or possible hermaphrodite traits on female plants, just to be certain. All the best with the rest of the outdoor season, happy harvests!

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James 2019-10-11
Hi Brian my plants are ready to be harvested and it rained for a good two days and now we're having a cold front push in talk in a low of 35 tonight and then for the next 3 or 4 days it's going to be sunny into the 60s should I try to harvest it tonight or just wait for the sun to come out and dry everything up

Tim Alchimia 2019-10-14
Hi James, thanks for your question, it's always a tough call. I'd examine the plants and see if there's any sign of mould first. If they're looking solid then I'd leave them out for a few days to make the best of the good weather before harvesting them. If, however, you're seeing bud rot (botrytis) then, depending on the gravity of the infection, you can either remove any affected areas and leave the plants for a few days, or simply cut your losses and harvest the plants before the mould spreads and spoils the rest of the plant. I've had the same dilemma myself this year, some plants have withstood serious rains with no problem, while others have very quickly gone from kush to mush in the few days after the rain. All the best for the rest of the season, happy harvesting!

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Brian 2019-10-04
Hi i have a question,i have a plant that has already been flowering and does have a little of orange hairs going but its supposed to be only a high of 59,low 60's.should i have them outside?i dont have a indoor or anything.and it gets to 46 at night

Tim Alchimia 2019-10-07
Hi Brian, those kind of nighttime temperatures aren't low enough to cause any damage, and as long as it's warm during the day you should be fine. You could try covering plants with horticultural fleece overnight to protect them a little, or move them indoors at night (as long as they stay in the dark, of course), but really I'd only be concerned about frosts, which could kill the plant. All the best and happy harvesting!

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Aj 2019-09-19
The part about the cold saying that if it gets below 65f 18c degrees most plants will struggle??? I have to highly and I mean highly disagree with this statement. It really all depends on strain while northern lights likes cool climates African landrace seeds tend to like warm dry weather it doesn’t mean they won’t flower or won’t produce.... maybe your thinking about if plants are in pre flower and these temps happen but again I’m not so sure.... I have done pheno tests for strains that I like due to my cold climate I have done some crazy things to plants and only seen a select few clam up and become unresponsive almost like a nutrient lock but that was when plants experienced hard cold while just starting to flower or in pre flower.....I have plants going and I haven’t seen a day above 65f in about of week my plants are outdoors nights have been as low as 38 f and they are loving it getting fatter and sticker daily. Strains are legend of Nigeria, purple dream Hindu Kush and granddaddy purple crossed with nyc sour desiel.... I would recommend all of these strains for cold late flowering conditions...

Tim Alchimia 2019-09-23
Hi Aj, thanks very much for your input. I think it depends greatly on genetics, some varieties will deal with cold weather very well, like Hindu Kush and other genetics originating from high altitude mountain areas, where temperatures can get very low. Other varieties such as tropical Sativa genetics will definitely struggle with low temperatures. All the best and happy growing!

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Mark Barrett 2019-09-11
Hi Tim Alchimia, This is a great website. I was looking for information on cool temperature effects on my maturing plants. I have one AC/DC clone that has grown about 6-7 feet with many branches covered with thin, whispy buds, several hybrid clones (Sativa dominant), and some Thai indicas, that are about 7 feet tall without any sign of sex. Aside from the Thais, all are well into flowering, with mostly clear trichomes, and we have just hit a cold snap with temperatures down around 44 degrees at night and into the 70s in the day time. Should I be worried, or just stay the course for a normal crop? I did allow male plants to pollinate the females in order to get seeds. Does seeding cause the plant to mature more quickly? Thanks for your much appreciated advice!

Tim Alchimia 2019-09-17
Hi Mark, big thanks for the comment and your kind words. At this stage in the season, with the trichomes still clear, there's not much you can do apart from wait and see. It's too soon to pull the plants early if all the trichs are clear, it would be a total waste of all your hard work. That kind of temperature isn't ridiculously low, plants will carry on growing right down to nearly freezing, and the daytime temps are good for flowering, as long as there's no frost, I think you'll be fine. If things get really bad, you could try covering your plants overnight with horticultural fleece, it should protect a little, and breathe enough to avoid condensation building up, although your 7ft-tall plants may well look like giant ghosts! In my experience, pollinated plants generally don't mature any faster or slower than non-seeded plants, but if you want to get large, healthy and vigorous seeds then you'll need to harvest at the optimal time for the seeds, rather than for the flowers or trichomes. I often let seeded plants go for a week or two longer than non-seeded plants, just to ensure the seeds are fully mature. At this point, all the trichomes are amber and the buds are slightly past their peak, although this also depends on personal tastes. I hope that helped. All the best and happy harvests!

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Cindy 2019-09-08
Thank you for all the great information! I have 6 Painkiller, not autoflower, (Farmers Lab) plants in 10 gallon fabric pots that have been outside all summer. (Southeast Michigan) The seed supplier has pics of mature colas that have purple sugar leaves and mine do not have them. I have looked at the Trichomes and most of them are cloudy, so I'm ready to harvest. Our temps have been 70-80 daytime, 50-65 at night for the last 2 weeks. What triggers the color of the leaves and buds? Night temps., genetics, fertilizer, stress?

Tim Alchimia 2019-09-16
Hi Cindy, thanks for your question. Apologies for the delay in replying, I had some time off work. The purple colours shown by cannabis plants are dependant on a range of factors, but principally it's down to genetics. Some plants (like ACE Seeds Violeta) will develop purple flowers in almost all plants from the very beginning, regardless of conditions, whereas some plants will start flowering as a green colour, developing purple tinges as flowering goes on, particularly when temperatures are relatively low at the end of the blooming period. However, these characteristics can only be displayed in plants genetically disposed to colour. Some varieties will remain green, no matter how cold they get. I hope that's helped, all the best and happy harvests!

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Austin 2019-05-18
Hi I recently played my green house and now we are looking at a 3 week cold front will my plants go into shock what can I do to keep them happy ??

Tim Alchimia 2019-05-27
Hi Austin, thanks for your question, sorry for the delay in responding. If your greenhouse doesn't have mains connection for an electric heater, you could use a paraffin heater to keep the temperatures. If buying a heater isn't practical, you could try covering the plants with horticultural fleece overnight to insulate them from the worst of the cold. Other more or less practical approaches could include, for example, a large black barrel filled with water to accumulate heat from sunshine during the day and slowly release it at night, or a big pile of fresh manure that would generate heat as it decomposed, although both of these would occupy space within the greenhouse. I hope that's given you some ideas, and I hope your plants make it through the cold spell unscathed. All the best and happy growing!

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jill coburn Is an Alchimia client 2019-04-10
Howdy , growing pineapple chunk in southern Australia, outdoors , plants are into 2-3 weeks of flowering , but the nights are now becoming quite cold, is it possible to move indoors where there are plants at 18-6 hour light ratio , would this be too confusing for plants , which outdoors probably are getting 12-14 hours of light per day. The weather will continue to decrease in temp as the days go by, love to here what your view is ?

Tim Alchimia 2019-04-11
Hi Jill, thanks for your question. I definitely wouldn't advise moving the plants indoors if the lights are on 18/6 as this will halt the flowering process and provoke re-vegetation which will really stress them (and you!) out, and they'll take a long time to readjust when you put them back outside again. At this point, the best you can do is to give them some kind of shelter on cold nights, even if it's as simple as moving them next to a building for extra warmth and draping a horticultural fleece or a sheet of plastic over them (although fleece would be better as it won't cause condensation). But more than anything it's vital to keep them away from light sources that could interrupt the natural flowering cycle, so street lamps, exterior lights on houses, billboards etc are all things that must be considered. I hope that helps, all the best for the rest of the season and happy growing!

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Tim 2019-01-29
What temp does the water I am giving my plants need to be? I had been giving them room temp and everything is good but I've read that watering them with cold water, not freezing just like out of the fridge, will increase trichome production. Is any of that true or would I be hurting my plants? I did water one with cold water that has been in flowering stage for 9 weeks. It's a sativa and the trichomes are still clear. And the colas the next few days did seem to have ALOT more trichomes.

Tim Alchimia 2019-01-30
Hi Tim, thanks for the question. I've definitely heard of people doing a cold water flush for the last week or so of flowering to increase trichome production. Putting cold water onto the roots is always going to cause some mild shock, and it won't do the microbial life of the soil many favours either, but during the last few weeks as the plant s winding down to harvest that is of lesser importance, so I don't see any reason not to try it. Whether it works or not is a different issue. To find out if it's just another stoner myth (as I suspect it might be) you'd have to run the same clone in the same grow space in the same conditions, but flush one with cold water while you treat the other normally. Then you'd be able to see if there's any notable difference between the two. If you do try this, please let us know how you get on. All the best and happy growing!

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Brad 2018-10-24
I have a plant in my green house its oct 24th and at night it gets down to -2 right now but the plant seems to be doing good still how long can I keep it in there the hairs are all still white for the most part

Tim Alchimia 2018-10-25
Hi Brad, I'm happy to hear your plant is doing so well in the cold weather, some cannabis genetics are much more able to deal with these types of temperatures than others, and yours seems nice and hardy. In your position I'd simply keep a close eye on it, and as soon as you see any signs of it suffering from the cold, then I'd start thinking about harvesting. It's a balance between cutting too early and losing some flowers to possible mould issues. All the best and happy harvesting!

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Josh 2018-10-22
Heres a question for you. If you harvest freah flowers with seeds in them and freeze before drying would it sterilize the seeds? What about at -30f ?

Tim Alchimia 2018-10-23
Hi Josh, I'm not sure about sterilising them as such, but I think it would mess them up pretty badly. If they're still fresh then I wouldn't expect them to be still viable, I think that freezing them with moisture still present would result in a massive degradation of the cells inside. That said, seeds that have been properly dried will be able to be kept in the freezer for long term storage. I hope that answers your question, all the best!

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Patsy 2018-10-13
Hi, I am waiting to harvest my first grow in western Colorado at 7000 ft. We are in a canyon so a little less sun. I have Northern Lights Indica and Blue Dream Sativa. My trichromes are mostly white on both varieties but waiting for amber. The temps have been getting down to 30 deg F at night and the Indicas are turning purple but none of the plants look stressed. I have a heater that keeps it at 30 deg. I am wondering if I can leave them in the ground to dry, they are in a makeshift hot house with plastic to keep them protected from frost and wind. I read its good to cure them slow so wouldnt it be better to allow them to cure in the ground as long as they dont reach the point of being overly "ripe" where the thc will start dropping? I also thought it would allow the branches I pollinated to devevelop more mature seeds. Your thoughts would be appreciated.

Tim Alchimia 2018-10-17
Hi Patsy, thanks for your comment. I wouldn't recommend doing what you're suggesting. The plants will continue to ripen far beyond the point of optimum maturity, so you'll end up with mostly amber trichs and a really heavy, stoney effect, although that may be exactly what you're looking for! Also, drying cannabis in a light and warm environment such as a greenhouse will degrade the THC and the terpene content, there's a reason we always recommend drying in total darkness in a cool, but not too dry room. Personally I'd cut the plants at your desired point of ripeness (Personally I prefer it with very few amber trichomes) and if possible leaving the pollinated branches to mature further (always a good idea for seed production, as immature seeds are useless). Then move the harvested plants to a dark room for drying. Slow drying involves hanging plants in rom with a RH of 55-65% and temperatures of 15-20ºC for 2-3 weeks. I hope that's helped, good luck and happy harvesting!

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Dan K. 2018-10-13
I live in East central mn and I'm wondering what I can do to keep my plants going till end of October to get the most out of them. Strains are unknown and I've only been giving them fox farm big bloom nutrients from the start of flowering once every week. Started by giving 3 cap fulls per 3 gallons of water for 4 plants. Plants are from 3 ft being the shortest one up to 6'6" being the tallest. The two shorter two have become very dark purple bur still some green inside by stem and centers of inner buds. Tallest two are still green no color change at all it seems. But after the other night all the fan leaves look dried out and wilted upward and goimg towards the plant. The puprle two are the worst and all i have for a greenhouse is 4 mil thick plastic sheeting. It came as its about 2.5'-3' wide and about 6'-7' long. The plastic was only 3' wide so its in two sections to make high enough to cover all but 6" of tallest plant. Than a 3' section over the top. I tide the sheets together at all the seems but left vents open all around the sides for some air flow. I dont have a micronscope to check trichomes and its been very cold and rainy the past 1-2 weeks. If I'm gunna end up harvesting early is there anything I can do right now to make the plants produce more trichomes and highten the terpene content? And is there anything I could do to make my slapped together makeshift greenhouse better at protecting the plants and the root systems which is in the ground not pots. Is it possible to dig them all up? And if I'm right around the corner of harvest cause of the climate can I take the tops and the other buds that are closer to maturity and leave the smaller ones to mature more and faster? Sorry for all the questions but its my first grow on my own and trying to get the most possible out of them in quality since I dont think they were able to fill out 100%. I'm guessing they should be as big as pop cans or end of baseball bats but they are more like a glass coke bottle or the bottom of a baseball bat that keeps your hands from sliding off. Not as frosty as I wouldve thought so anything that 100% works to increase terpenes and trichome content is much appreciated.

Tim Alchimia 2018-10-17
Hi Dan, thanks for your comment and question. I'm glad your garden is doing well this year. At this late stage there's not much you can do to improve the THC or terpene content, apart from letting plants flower for as long as is possible in the bad weather, flowers that are harvested before they're mature won't have the same fully developed terpene profile or the same effect as properly finished buds. At this stage I'd personally just give them water only until they finish, that way you'll at least have good tasting and clean burning weed that won't be full of excess nutrients at harvest time. Adding more nutrients or bloom boosters isn't a good idea at this stage, early in flower is the best time to be thinking about these things, not just before harvest! So, if you're expecting frosts then you're doing the right thing by covering your plants. It's hard for me to visualise the setup you've got going on with the plastic to advise you but if I'm honest, anything other than a proper framed greenhouse/poly tunnel is going to be a bit of a botch job and probably won't stand up to wind and rain very well. The best thing is to secure it as well as you possibly can, and if there's an issue with the height of the tallest plants then maybe try gently training the top of the plant and tying it down a bit to lower it, if it's the difference between being protected or not. As for protecting the roots, well they're definitely better off in the ground than in plant pots at the stage, they won't be as affected by the frost but you could also add a heavy mulch layer of straw or hay which could help to buffer the cold temps. Also piling some hay baes around the base of the planted area would help to keep temperatures higher in the root zone, and prevent the frosty air from reaching the plants. Digging up the plants will shock and stress them to much and they'll most likely stop growing and maturing altogether, so definitely not recommended! You can definitely harvest the top buds first and then let the lowers fatten up a bit before chopping them, that's always a great idea if time allows for a two-stage harvest. The size of the buds will depend on many factors, primarily the genetics (so it's vital to invest in good seeds or clones rather than planting any old thing and hoping for the best), but also nutrients, solar exposure and irrigation can all have a huge effect on bud size. In short, the better you grow, and the better the genetics you grow, the more you'll harvest. I hope that's some help to you, all the best for the rest of the season, happy harvesting!

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Joeph Wilson 2018-10-10
If the temperature drops below freezing while the hemp is still in the ground is it still viable? I'm in Colorado and growing cherry and super cbd strains. We got caught by early cold snap.

Tim Alchimia 2018-10-11
Hi and thanks for the question. Hemp ought to be more hardy than regular drug cannabis, but it still won't tolerate extended periods of extreme cold. If it's just a short cold snap then you'll probably be fine but if it's cold for a few days to a week then it'll start to have an effect on the plants, slowing them down and stopping growth, and you may see the appearance of some mold or other deterioration in the flowers where frost has damaged cells. That said, a lot depends on the position and topography of the site, some places will suffer worse than others, if for example you're growing in a low dip or a bowl, frost will be more severe, but on a south facing hillside with good solar exposure and air movement you may see no effect at all from the cold. My advice is to keep a close eye on the weather forecast and carefully check the plants for damage and be ready to harvest at the first sign. If there are local farmers you can ask they'll probably be able to offer better advice than we can. All the best and happy harvesting!

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Steve Moritz 2018-10-08
I live in Colorado bad weather has hit 30 at night 40 during the day in week 8 still to early to pull according to the weather man we will have these temps for the next 6 days with rain . covered the plants with easy ups .that's all I could think of .do you think they will make it? Would appreciate some advice.

Tim Alchimia 2018-10-09
Hi Steve, thanks for your question, I'm sorry to hear about the bad weather. You've definitely done the right thing by covering the plants and protecting them from the rain. By keeping the majority of the water off them you'll do a great deal to help prevent bud rot. It's also a goos idea to try and increase air movement, either by using some fans if possible, or by removing some of the lower leaves that aren't getting much sunlight. Apart from keeping a close eye on the buds for any sign of botrytis, and removing any you find, I don't think there's a great deal more you can do at this point. Of course if you start to see a great deal of bud rot then you'll have to make the decision whether to pull the plans slightly early or run the risk of losing the whole crop. It's at this time of year that our choice of genetics can make or break the whole season. All the best for the next couple of weeks, I hope the weather improves and you get a great harvest!

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Tammy gooch 2018-10-04
My first time growing a plant si I just have ine..think I did ok my question is I was told it have about 2 more weeks before it's ready but it's getting a little cold and hardly any su, should I just pull it now are will it be ok..not sure when its completed ready

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Thomas 2018-09-29
Hello, I live in Southern Ontario and yesterday I found one of my bud rotting under yellow leaves. Outdoor grow. Cut out the bud and branch it was on. Overnight I covered the plant (Indica) with poly and this morning no additional rot. Weather forecast next week is cool and wet. Average 20c high 14c low. Also expected to rain every day but one. Should I harvest plant now. It seems to me would be a struggle to avoid more rot . May thanks for your advice

Tim Alchimia 2018-10-01
Hi Thomas, thanks for your question. If the weather's looking bad for the next week, it's probably best to harvest as long as you're happy with the maturity of the flowers. Have a look with a microscope and check the trichomes. If they're mostly transparent then it's not ready, but if they're mostly milky coloured than you should be ok, some people like to wait to see 20-30% amber trichomes, but I don't think you'll have the luxury of time to wait that long. I think you're right though, it'll e a battle to keep the mould at bay, especially if it's covered with plastic at night, which will create condensation and high humidity, the perfect conditions for more mould. If you wait much longer you may well lose the whole plant. Without being able to see the plant, I'd say harvest asap to salvage it. I hope that's some help. All the best, good luck and happy harvest!

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Jay 2018-09-23
Hello. Well I live in north west Ontario Canada. Not the best cannabis growing climate. I did have early finishing photogenic phenotypes this year. But my main strain being fire og X Cinderella 99, now this being late September we usually can grow till mid oct in my greenhouse before snow starts to fly. Not this year, woke up to a light dusting of snow this morning. I put a new sheet of poly over top of my greenhouse to close it all up, my plants have beautiful stacked colas but are still at least two - three weeks from being at optimal harvest. I read in your article that resin and terpenes (which are off the hook now) get more and more prevalent. My ? Is will the only effect of these low temps I’m experiencing be with the final weight and not resin and terpenes? It’s my medicine and would hate too see my winters stash all go down the botritis hole. Will these low temps -4 at night +18 during daytime, cloudy, wet northern autumn weather. Hope the new plastic sheets will help retain heat and keep moisture off my flowers. Thanks for any helpful tips.

Tim Alchimia 2018-09-24
Hi Jay, thanks for your comment. Well at least the plants are in a greenhouse and protected from the worst of the weather, but my main concern now would be adequate ventilation and air movement, especially if you've completely closed the greenhouse up with this new layer of plastic. If there's any possibility of doing it, I'd put a fan or two in there to keep the air moving and avoid condensation on the inside of the plastic, which will definitely not help your fight against botrytis. Even better would be to add a dehumidifier too. Apart from that, yes the cold temperatures will restrict growth a little, but they shouldn't affect the terpene or resin content, in fact the low temps will mean that the terpenes will be better retained, not evaporating off in the heat of the day. I hope that helps a bit, all the best for the next few weeks, good luck and fingers crossed for a happy harvest!

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Carolyn 2018-09-16
This is sorta for Dan Carte. Hello! I'm not a first time grower so I have a few years under my belt . I'm not an indoor grower I prefer outdoors. Elevation here is about 2900ft daytime temperatures are usually between 90 to 110* F. Nighttime temps range between 75 to 87* F. Until around late August then in September daytime is between 90 and 102* F nighttime down to 60* F. In my opinion the cooler temp at night is one of the best things that can happen to the plants. At this time of year all your growing is done they are just maturing. Since the plant starts feeling like it's going to die because of the cooler temps it's possible to achieve much greater and higher quality resin coverage. You don't really want amber trichomes at that stage they start to deteriorate. Also what I do one week before harvesting is I split the stock and wedge a rock in it, then stop all watering and cover the plant with blackout material. I then wait 3 days take the cover off and wait about 4 to 5 days and harvest. The stem is empty and hollow and the resin is unbelievable. In my opinion our altitude is almost perfect for plants. I've never seen better grows anywhere. And I have tried the molasses thing and for all its worth, I didn't really see a difference. Splitting the stock is amazing though. Happy growing!!!!!

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Dan Carte 2018-08-28
Hi Tim great article! Very informative. I am growing for the first time in the Rockies of western Montana at an elevation of 3200 feet. Several questions. I planted three seeds of unknown strain in a large container good organic soil and natural fertilizer. Have had incredible results with one plant in particular. I am guessing given my growing zone is a sativa. I am looking for medicinal benefits to help ease my son's seizures as well as recreational benefits out of this same plant. I have huge tight buds loaded with trichromes that I photograph each morning as soon as the bud is in full sunlight. The trichromes are mostly milky with maybe 5% still clear and none have yet turned amber. My plant is healthy and is turning a beautiful purple color with recent cool temperatures at night. Dipped to 2 degrees Celsius last night but still reaching 26 degrees during the day. I have several theories about my success I hope you can confirm or deny. There is less atmosphere at my elevation than at sea level to protect from the sun's uv rays, does this increase trichrome production? Given the temperature range from a seasonal norm of 6c at night to 26c by midday for possibly another month what impact on yield and maturation will this temperature range have? I have read that cbd's are most potent at a trichromes stage where they are 60% amber, is this true? Have heard adding molasses to water at this stage of bloom will increase trichrome production, is this true? Drone comparing daily photos I see what appears to be continued trichrome production or at least a thickening of them. I am hoping to harvest in two stages part when trichromes are at best for a recreational use and later for maximum cubs medicinal use, is this possible? Is this wise?

Tim Alchimia 2018-08-30
Hi Dan, thanks for your comment, I'm so happy you've enjoyed the article! Yes, I think your theory is correct, trichomes are thought to be the plant's protection from UV radiation, so it makes complete sense that they should be producing large amounts at your elevation, in the thinner atmosphere. Just think about traditional hash-producing countries like Afghanistan and Pakistan, where the best plants are grown high up in the mountains. Those place also experience a drastic difference between daytime and nighttime temperatures, just like your spot. As for any influence this may have on maturity, well I personally think that it will probably accelerate maturity, with this temperature difference being a natural process experienced by the plant over millennia. As for your question regarding CBD, that's another matter. You say the genetics are unknown, so unfortunately there's no way of knowing if the plants are genetically capable of producing CBD. Aside from CBD-specific strains, most of the genetics on the market will only have trace amounts of CBD, because for years cannabis breeding has focused on increasing the high, for recreational purposes. If you really need to be growing for CBD, then I'd highly recommend getting some high-CBD varieties, they are increasingly easy to find as awareness grows about medicinal cannabis. As for the two-part harvest, that's a good plan, even without any CBD. The tops will mature first, you can cut them and then wait a week or two for the smaller, lower buds to mature and fatten a bit before cutting the rest of the plant. Hope that's been some help to you, best wishes and good luck for the rest of the season, happy harvests!

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Anthony Reynolds Is an Alchimia client 2018-07-07
Bit of a weird question, due to a heatwave here temps are difficult to control in the day time, so I changed the cycle of the lights so they come on in the evening when the air temp is a lot cooler and switch off just before midday, I have fans and temp controllers, so I assumed everything would work ok. But that isn't happening and I havn't a clue why, this isn't my first 'Rodeo' BTW, but there is a problem with the plants, they are very sluggish and some are stressed, causing what looks like an over fertilized condition, puzzled by this as they havn't actually had any Nutes yet, I decided to investigate. It transpires the daytime temps i.e. without lights are almost 28 to 30 degrees centigrade! This brings me to ask the question, do plants need a drop in temperature to thrive, I'm not talking a radical drop but a temperature variation, like would occur naturally at night?

Tim Alchimia 2018-07-10
Hi Anthony, thanks for your question, sorry to hear your plants aren't happy. Those night time temperatures are a definitely on the high side, and that kind of heat will definitely reduce the quality of the end product, which will have a lower terpene content. We'd recommend trying to lower them to 23/24 if you can. It's unclear whether plants really need lower temperatures at night, usually growers face the problem of temps being too low, rather than too high. However, in the wild, the plants metabolism has evolved over millennia to deal with lower temperatures during the night, so it's logical that plants will grow better and be happier with less stress in more natural conditions. I hope that helps, all the best and happy growing!

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Den Is an Alchimia client 2018-01-27
Hi, question for you. I have a DWC grow that was going lovely, started flowering - got to day 15 (lots of great explosive sites formed to grow flowers) then had a cold snap where temps dropped to 18 high /10 low and all growth stopped (plants stayed healthy green but no more floral growth developed) this continued for 3 weeks. Then managed to increase temps to 26/20 and growth started again - my question is this: will the plants finish as per seed catalogue guidelines with a much reduced yield or will they continue to full maturity albeit taking an additional 3 weeks of time (given the low temps induced dormant spell)?

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Dean boutilier 2017-12-29
My grow room dropped below freezing for a few hours some but not all plants are dropping will they recover

Tim Alchimia 2018-01-02
Hi Dean, a plant's resistance to cold really depends on the variety you're growing. Some can deal with freezing temperatures without any problem (not for long periods of time though) while others will start to show problems way before getting that cold. In your case you'll have to wait and see, but it's certain to have caused some kind of stress to the plants, you could try using an enzyme product of some kind, which can help the plants recover quicker and mitigate the effects of the stress. Investing in a small tube heater can really help these problems in the future. All the best, stay warm!!

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Evelyn 2017-12-28
What temperature is ok for the dark room after blooming

Tim Alchimia 2018-01-02
Hi Evelyn, thanks for asking your question. I'm not 100% sure what you're asking, but if you mean the nighttime temperature during flowering, it should be around 20-22ºC to keep plants as happy as possible. I hope that helps!

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The effect of cold on cannabis plants

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How does the cold affect cannabis plants?

How to avoid low temperatures indoors

Radiating cables for indoors

Taking advantage of the cold in cannabis growing

The cold and it's effect on flavour and colour

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