How to water marijuana plants in soil

Which is the best way to water marijuana plants in soil?

Novice growers often ask themselves which is the best method for watering their cannabis crops in soil. While it is true that marijuana needs a lot of water, it is also true that it does not tolerate over watering, so it is easy to make mistakes. By following these tips you will ensure optimum results from your crops.

Cannabis is a plant that does not like continuous irrigation. It is imperative to water abundantly our crop and then wait until the substrate gets dry to water again, either with fertilizers or with tap water.

Follow these rules to calculate the needed amount of water per plant:

A good sprinkler for a good watering

A good sprinkler for a good watering

The volume of water needed in each irrigation must be, at least,  a quarter the total volume of the pot or container.


For a 1L pot, use 0.25L of water in each irrigation
For a 4L pot, use 1L of water in each irrigation
For a 12L pot, use 3L of water in each irrigation
For a 30L pot, use 7.5L of water in each irrigation

Usually, we will have to water 2 or 3 times, slowly and uniformly over the surface, to properly moisten all the substrate. Place pot plates under the pots so they will be filled with water once we irrigate the plants; this water will be absorved by capilarity in less than one day. It is also very important to use a quality substrate not only with good retention capacity but also with good drainage (our substrate should contain clay pebbles, perlite and/or coco coir,…)

Once watered, you can notice that your pots/containers are now very heavy, being difficult to move them. What you should do now is weighing the containers every day – you can do it with your hands -. You can use a pot filled up with dry substrate to compare both weights. Once they have lost most of their weight, it is time to water them again.

In effect, as water weighs much more than soil, a pot saturated with water weighs a lot more than another ready to be watered. With a little practice, you will quickly learn when is the appropriate time for watering your crops.

This technique is extremely easy and efficient, and suitable for any kind of potted plants! Using this method, you will probably water your plants every 3-6 days. Several factors may require a more frequent irrigation, such as small pots, dry and hot climate, sun exposure,etc.

Drip irrigation does not offer good results in soil, and it should be only used in exceptional cases (holidays). If you want a functional drip irrigation system, you should forget about soil and use any hydroponic substrate available on the market (coco coir, rockwool, clay pebbles, etc).

This plant needs a good watering

This plant needs a good watering


We have to wait until the substrate is dry to water again. Over watered plant.

We have to wait until the substrate is dry to water again. Over watered plant.

 Which is the best water to irrigate marijuana?

You can use tap water as long as its EC value is <0.4. Otherwise, you should dilute it with pure water (EC = 0.0) such as rainwater, or osmosis/demineralized water in order to get an EC value of about 0.3

Indeed, cannabis plants don’t like chlorine, limy (hard) water or other substances contained in tap water. It is recommended to allow your water to evaporate all chlorine (rest for 24hrs) at room temperature.

  • Osmosis water is perfect for cannabis growing as long as we add a few supplements. It can be easily found in pet/fish shops, or can be produced by using an osmosis system. We should add Calcium/Magnesium/Trace elements since most fertilizers are made for using with tap water.
  • Rainwwater is also very useful, but it is recommended – specially indoors – to put it in the microwave for a few seconds to sterilize it from bacteria, eggs, fungi spores, etc.
  • The water pH value must be between 6 and 7. Otherwise, it should be adjusted with a pH tester and a pH reducer.


Tap water dilutions table

Tap water dilutions table

Marijuana watering tips

Progressively reducing irrigations at the end of flowering increases resin production

Progressively reducing irrigations at the end of flowering increases resin production

During the last weeks before harvest, the plant’s need for water descreases. We must then progressively reduce the amount of water in each irrigation, because we can have problems with molds developing in the buds if the substrate takes too long to dry.  Furthermore, a slight “water stress” at the end of flowering increases resin production.

However, don’t forget to flush your plants with water (without any fertilizer) 3-4 weeks before harvest.

Use fertilizers every 2 irrigations, or two times out of three. Flushing your plants with water (eventually, you can add enzymes or humic/fulvic acids) dilutes and “recycles” those nutrients accumulated in the soil, which can cause an intoxication of marijuana plants .

After a few irrigations, a hard layer may appear in the surface of the substrate. Break it gently and regularly so water can moisten the substrate correctly. You can use natural humectants with your irrigation water to avoid this issue (Yuca, Aloe Vera…), which also allows better water absorption and nutrient assimilation.

Naturally, the bigger the plant, the more water it needs. If you notice that a specific plant needs less water than the others, you can reduce the amount of water per irrigation so you can water all your plants the same day. The same happens with plants that demand more water than the others.

First symptoms of over watering

First symptoms of over watering

Spraying the plants with water makes the plant to develop less roots, so you shoul avoid spraying them more than 1-2 times per week. Since plants get water from their leafs, they don’t develop a nice rootball to find it in the soil.

The first symptoms of over watering are similar to drought. Weigh your pots to know what happens: if the pot weighs too much, you are over watering your plants!

Happy irrigations and harvests!!


August 11, 2014 | Cannabis grow guide

30 comments on “How to water marijuana plants in soil

  1. Tony Montana

    Is their a comprehensive guide on feeding and watering I mean really something that is like text book. Were it gives me a clear understanding when to just water and when to add nutrients and water together for feeding I’ve watched videos but they don’t show the feeding regimens, their not tutorial or a book were someone can get a clear understanding or a video which you would recommend

    1. Dani Alchimia Post author

      Hi Tony,

      Basically, you have to water when the soil is still humid but not wet, and water until you see nutrient solution draining from the pot. You can also raise the pot to check its weight, this is a good way to make sure that the soil is being properly irrigated.

      We have the book Hydroponics for Everybody available in the shop; while it is about hydroponic systems, it explains perfectly the watering needs of cannabis plants and shows different irrigation systems.

      All the best!

  2. Susan

    I will look it up on Google, but a definition of “substrate” would be good for a beginning pot waterer.

    1. Dani Alchimia Post author

      Hi Susan,

      With “substrate” we mean any media in which plants grow and develop attached by the roots. In this sense, substrates can be fertile (most soils used for pot plants) or inert (coco coir, clay pebbles, rockwool, mapito, etc.).

      Hope it helped! 😉

  3. andrew clark

    First time grower and must state that when you germinate your seeds,do so with no nutrients and use baby grow soil ,if using coca soil don’t introduce coca until at least 2,3 weeks old as when you germinate seeds they come fully loaded with it’s built in nutrients for at least 2 weeks and should only take 48 hrs to 72hrs to break it’s shell.if you use my method by putting kitchen paper warm wet ideal temp 22/25c.inbetween two small plates,so it makes like a sealed dome,better to use transparent plates so you can see what’s going down in town.mine auto seeds from sensi seeds amsterdam,cracked there shell and sprouted within 48 hrs. Then into small very small use real small pots as this will make a ball root,very very important for plant growth,flowing,and big now you have a small but great looking baby plant about three to five inchs high with root ball and masses of other white looking single vine roots great to now plant in second dive pots then from about five inch pots to it’s main growing pot soil and this will be its finall transfer to main pot.i myself find that changing pots three to four times in it’s life time upsets the plant in some mad i myself start in a 2 inch pot wait for seeds to make a root ball and loads of roots coming out from pot bottom,then transfer to 9 to 13 inch pots.using coca,plus root grow for it’s first 2/5 weeks.if using coca remember it’s from coca trees that grow next to rich beach and sea lines so are loaded with nutrients anyway,you can wash out your coca to reduce nutrients and dry out.but don’t let to dry out completely as this kills it’s nutrients place under lamp,i am using cfl 300 with red and Blue and white on week three plants are now 7ins tall with about ten leaves dark green which is good.but getting a little yellowing and drying up on some tips of leaves.hope it’s just nutrition burn where nutrients levels are too high.made up new mix and now doing fine,would you belive when yellow started coming on tips used a mix of water p.h down and miracle grow very small amount as miracle grow is high in most of it’s made up nutrients but great for nitrogen,plants love this but be very careful on how strong to make solution.yellow on plants cleared up within three days then stopped using miracle grow.and started another treatment.

    1. Dani Alchimia Post author

      Hi andrew,

      Thanks for your comment and tips, they are always welcome! As Nitrogen is a mobile element, as soon as you see the lower leaves of the plant turn yellow it means that it is lacking Nitrogen on the younger shoots so it is taking it from the leaves. A bit of nitrogen (or a nitrogen-based growth fertiliser) in the nutrient solution will fix the problem.

      All the best!

  4. Rosie larkin

    Hi, I’m just starting to use a very small grow tent 60cm high with a CFL( it was advertised as a 200watt, the box says 5500lumen grow 5U125w 6400 K)
    . A bit worried about water from irrigation or spray getting on the CFL. I see you recommend the light for 18 hours on 6 off . If water drops are a concern how long do I need to wait for the CFL to cool down, thinking here of having to water either very early or very late in the day in case I shatter the CFL with stray water drops.
    Many thanks

    1. Dani Alchimia Post author

      Hi Rosie,

      First of all, HID lamps like HPS or MH bulbs are much more sensitive to this type of problems than CFLs or fluorescent tubes, since they reach much higher temperatures.

      I don’t think you’ll have much trouble with irrigation water, but you should be a bit careful when spraying: notice that most insecticides/fungicides are photosensitive, so it’s always better spraying the plants with the lights off. In this way, we also avoid the problem of shattering our bulb with the water drops. Thus, and as you say, spraying the plants when the lights turn off or about 30 minutes before they turn on is best.

      All the best!

  5. Rosie larkin

    Many thanks!

  6. Yeoolnypotbroo

    Will cucumber water hurt or kill my weed plants. I read it kills spider mite too but I just thought about cucumber water but want to see if it okay

    1. Dani Alchimia Post author

      Hi Yeoolnypotbroo,

      Sorry, I’ve never heard of that method. We have neem-based insecticides and also products especially formulated to kill spider mite made of coriander and canola oil.

      Please tell us how it works if you try it! 😉

      All the best!

  7. sampath

    Thanks a lot

  8. Nancie

    My plants are outside in the ground how often should I water ? Temps have been in the high 90’s.

    1. Dani Alchimia Post author

      Hi Nancy,

      Check the topsoil layer and water your plants when the first 3-4 cm are dry. Never wait to see symptoms of tension loss on the leaves. If your plant looks like it needs to be watered, then it means that you should have watered before. You’ll also realize that the more your plant grows, the more water it’ll demand. This article about growing directly in the ground may be useful for you.

      Hope it helped!

  9. Mikee

    So, I’m kinda dumb and would totally appreciate so help. I use foxfarm nutrients and it has a quide for watering. Now, I mix nutrients and water then I pH to around 6.5. When my plants r young I give them just pH water. I normally wait a couple weeks before I give nutrients. My confusion lies with the watering with x amount of nutrients. My grow big mix sez 2-3 tspns in a gallon every other watering. If I water to the point it starts to drain without using the entire gallon am I still doing OK?
    I haven’t encountered droopy leaves ( over watering?) But I worry that my plants aren’t getting enough nutrients. Any tips and/or suggestions r greatly appreciated.

    1. Dani Alchimia Post author

      Hi Mikee,

      You are watering your plants correctly, it doesn’t matter how much nutrient solution you need to water your plants when you’re mixing the nutrients. You have to follow the instructions of the fertilizer company so you don’t overfeed your plants, although using EC meters greatly reduce the risk ofoverfeeding (they measure the amount of salts – nutrients – dissolved in the water). Here you have more info about electro conductivity (EC) and cannabis.

      About your second question: droopy leaves often come from overwatering your plants. You should water the plants when the upper layer of the growing medium is dry (but do not let all the substrate to dry up!). I would carefully check the composition of your nutrients and see if you have all the macro and micro elements, vitamins, amino acids, etc. needed for proper development. Also, I’d get an EC meter (one of the most useful tools for growing cannabis) and check if you’re watering with the corect EC level.

      Hope it helped!

  10. adam

    Can i use de mineralised water and then add back in the essential minerlas

    1. Dani Alchimia Post author

      Hi adam,

      Normally, professional growers use RO water and then add all the minerals (nutrients) needed for the plants development, there is no problem at all. If you use distilled water or RO water keep in mind that you should add some extra calcium and magnesium to your nutrient solution.

      All the best!

  11. teiar sharpe

    I just want to know should you water your leaves after your buds start coming in or 1and a half months left before harvesting

    1. Dani Alchimia Post author

      Hi teiar,

      Normally, growers stop spraying their plants when the buds start developing. In this way, you have less chances to get mold infections and you make sure that the taste of the buds will remain unaltered.

      All the best!

  12. John bair

    Can I use purified bottled water and add nutrient solution as needed or is purified water not good for them

    1. Tim Alchimia

      Hi John, sure you can use bottled water, although it may get expensive over time. Many growers use a Reverse Osmosis filter system to clean the tap water before using it to irrigate, which may be a more economical option in the long term.

  13. Bill king

    Is it ok to use the MITACLE GROW GRANUALS?

    1. Tim Alchimia

      Hi Bill, thanks for your question. Well, you can certainly use MIRACLE GROW GRANULES if there’s no other option available, although I’m not sure at what rate you’d apply them for cannabis, I’ve heard they’re quite powerful and can burn plants from over-fertilisation. With the huge variety of nutrient products available nowadays formulated specially to meet the nutritional needs of cannabis, why would you use anything else?

      Personally, I wouldn’t use Miracle Grow on anything I intended to smoke or eat. We always recommend using organic fertilisers over mineral/chemical ones, because they give far superior flavour and aroma, and are more respectful of the environment, both in their manufacture process and in their use.

      Another point to consider is that Scotts/Miracle Grow are owned by Monsanto, the GMO food multinational corporate giant, so there’s an ethical question as well, and while I understand that isn’t something that’s important to everyone, I do think we ought to buy responsibly and choose carefully where we spend our hard-earned cash.

      I hope that’s answered your question, happy growing!

  14. Sabine

    I’m growing outdoors in India, in pots. Now rains came. Don’t know if I should take them out of the rain, or if my feeling is right about rain being good and natural for all plants.

    1. Tim Alchimia

      Hi Sabine, thanks for your question. The answer would depend on a couple of factors, like what the genetics are, nd at what stage in the life cycle they are. The rain will only pose a problem if your plants are in mid to late flowering period, or if the soil is likely to become excessively waterlogged/flooded. If the plants are sativas, then they are more able to cope with any rain during flowering, Indica plants with their dense compact flowers are much more susceptible to fungal problems when humidity is high.

      But yes, as a general rule, the rain is perfectly natural for our plants and as long as it’s not so heavy as to break branches etc, then you’ll be fine! All the best for the season and happy growing!

  15. Louise

    My clones are rooted and I just took them out of the dome this morning…they seem to be doing great…I kept them I the shade all day …should I keep spraying them and should I keep them at a certain temperature…I will be growing them inside since the temperature will be getting cool soon..and are they too young to give them nutrient..if not what should I give them..first time doing this

    1. Tim Alchimia

      Hi Louise, thanks for your question. I’m glad your cuttings have rooted well. Keeping them in the shade at first is a good idea, I’d usually have them in dappled shade for a few days before putting them in full sun if growing them outside, well indoors it’s the same principal, introduce them to the full power of the lamp over a few days so they don’t get stressed.

      As for temperatures, anything fro 18-30ºC will be fine once they’re established, just try not to expose them to extreme heat and make sure the substrate is moist, spraying them can help a lot with that, but once they’re rooted it isn’t absolutely essential.

      Their nutrient requirements will depend on various factors, like the type of substrate, growth rate, size of cutting etc, but you can start them off with a quarter-strength dose of organic liquid fertiliser for vegetative growth, increasing the concentration over time as the plant grows. At this stage in the plants life, its main requirement is for Nitrogen. Have a look at our article on deficiencies and excesses of Nitrogen to make sure you’re not over or under-feeding your plants, and remember it’s easier to rectify a lack of nutrients than to fix the damage caused by excess feeding.

      All the best with your plants, happy growing!

  16. Seeley

    Newbie here. I was advised to plant established seedlings in a large pot with a piece of fish or fish head in the soil.
    Haven’t seen any reference to this elsewhere. Is it the wrong advice?

    1. Tim Alchimia

      Hi Seeley, thanks for your comment. I’m not going to say that the advice you were given is outright wrong, because I’m sure that it has worked well for many farmers over the years, however it’s not something I’d personally recommend at all. The idea of burying some un-composted fish under my plants doesn’t appeal at all, and I’m fairly sure it would attract unwanted attention from foxes, cats, dogs and other vermin, who would all want to dig up my plants to get to the stinky fish beneath them! Not to mention that the rotting process of the fish would not necessarily produce the correct conditions in the soil for plant development.

      If you want to use fish scraps so they don’t go to waste, then I’d try fermenting them in a jar with sugar and water to make some homemade Fish Emulsion. There are plenty of recipes and instructions online to help you. Otherwise there are plenty of Fish-based fertilisers available on the market, formulated to benefit plants and with accurate dosage instructions to avoid you over-feeding and burning plants.

      I hope that’s cleared things up, all the best and happy growing!

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