Growing cannabis in Rockwool

How to grow  in Rockwool

  1. History of Rockwool
  2. How to stabilise Rockwool
  3. How to germinate seeds in Rockwool
  4. How to root cuttings in Rockwool
  5. How to grow in Rockwool
  6. Recycling Rockwool

1. History of Rockwool

Eruption of a volcano in Hawaii
Eruption of a volcano in Hawaii

Rockwool is a product that was first conceived during the 20th century in Hawaii by observing natural volcanic action there. Upon this discovery, a Danish company named Rockwool began to investigate and develop ways to commercialise this technique, and in the late 1930's released the first standardised stone wool product as we know it today.

To manufacture Rockwool, the raw material of basalt rock must go through several industrial processes, the first of which is melting of the rock in furnaces at extreme temperatures, up to 1600ºC, emulating the action of a volcano and leaving the rock in a natural state of liquid lava.

To obtain the fibres, an organic binder is added to the lava and the mixture is subjected to a mechanical process using centrifugal force, resulting in something resembling a woollen mattress. This wool is then compressed to varying degrees, the density and amount of air between the fibres depending on the projected end use, whether for acoustic or thermal insulation, fire-protection and, of course for it's use in agriculture.

2. How to stabilise Rockwool

Rockwool Cubes and Slabs
Rockwool Cubes and Slabs

Rockwool substrate is a product that needs to be treated before it's use. Given that its initial pH level is quite alkaline, nearly 7 points, we should stabilize it to enable us to grow without mishaps from the very beginning.

We must immerse the slabs for 24 hours in a nutrient solution that contains a pH level of 4.5 with EC levels of 0.5-0.6 - ideal to start growing with seeds without worrying about nutrient deficiencies.

When we start to grow the pH level of the slabs should be at 5.5, if that isn’t the case, we should immerse them again during a few more hours checking the pH regularly until it is stabilised. An easy way to know if the pH of the slab is stabilised is to irrigate with water of 5.5 pH and measure the run-off with a pH meter. If the run-off is at 5.5 we can proceed to plant the seeds.

3. How to germinate cannabis seeds in Rockwool

Germination in rockwool
Germination in rockwool

Once the Rockwool cubes have a stable pH as described above, we can proceed with the germination of the seeds we intend to use in our grow.

We will start the germination in two wet paper towels, placed between two dishes in order to maintain a constant humidity and facilitate the germination of the cannabis seeds. Once we see that the seed casing has cracked open and the root appears, we can very carefully transplant them into the stabilised Rockwool cubes and, once established, we can perform the transplant to the slabs.

To transplant the seedlings, we insert the small 3 x 3cm cubes into the 7.5 x 7.5cm Rockwool blocks taped to the slabs. As we do this, we must push gently and slowly until the small cube is well inserted into the larger block, taking great care not to break or damage any roots in the process.

4. How to root cuttings in Rockwool

The process of rooting cuttings in Rockwool is very similar to other cloning methods using jiffies or coco-coir (see how to take cuttings).

After the process of preparing the cuttings for cloning them, we place them in the Rockwool cubes (previously stabilised to pH 5.5) which should be moistened but not soaking wet, to avoid any problems with stems rotting.

Once the cuttings are placed in the cubes we can treat them exactly as if they were in jiffies or coco, monitoring the humidity every day and spraying the cubes to keep a constant moisture on them to ease the rooting. As time goes by it will be necessary to lower the relative humidity until we can finally remove the cover of the mini-greenhouse and start to acclimatise the cuttings so that they don’t suffer when transplanted.

Cuttings in Rockwool cubes
Cuttings in Rockwool cubes

5. How to grow in Rockwool

Rockwool slabs
Rockwool slabs

As previously outlined, the growing medium must be stabilized at pH 5.5 with an initial EC of 0.5-0.6 during this first week of growth, not exceeding two waterings of 1 minute each, with a solution of around 60-70 ml per plant. In this way we will avoid water-logging and facilitate the root growth of plants in this highly absorbent and water-retentive substrate.

To ease the task of watering, using an automatic irrigation system will help enormously and is simple to assemble and use, as you can read in our post on how to install a hydroponic system.

We can maintain this irrigation schedule, varying according to conditions and demand, until the first week of flowering when we will start to gradually increase pH from the initial level of 5.5 , raising it slowly over the course of the week to a level of 5.8-5.9 suitable for this stage of flowering.

Hydroponic crop in rock-wool
Hydroponic crop in Rockwool

From the second until the end of the third week of flowering, we will need to increase the frequency and duration of the irrigations, watering three times a day. The first watering should be of 2 minutes duration when the lights turn on, ensuring the plants start the day with energy. The second watering should be six hours later and 1 minute in duration, with an EC of 0.8-0.9 and a pH of 6.0.

Over the 4th week and until the end of the 5th week of flowering we will again have to increase the frequency of irrigation to 4 waterings at day. The first and the last irrigation should each last 2 minutes, and the remaining two waterings should be 1 minute, distributed evenly through the day. EC levels should remain at 1.0 to 1.2, always taking into account the condition of the plants. We should keep the pH at 6.0 allowing it to fluctuate up to 6.2 to lower it again to 6.0 with  pH down.

During the 6th week of flowering it will once again be necessary to increase the frequency of waterings from 4 to 6 per day. The first and last irrigation should last 2 minutes each and the four remaining waterings will each be 1 minute. The EC should still be at 1.4 and the pH continues fluctuating between 6.0 to 6.2 as in the previous week.

In the 7th week of flowering period we will have to keep the same frequency and duration of irrigations; however, in this week, depending on the condition of the plants, we can increase the EC from 1.4 up to 1.6-1.7 and we can also let the pH fluctuate from 6.0 up to 6.3.

In the 8th week of flowering, plants will receive the same number of irrigations, at the same duration as in the previous week, although we can raise the EC to 1.8 depending on the condition of the plants.

In the 9th and last week of flowering phase we will have to increase the duration of all the waterings to 2 minutes, with the pH adjusted to 6.2 and with EC levels as low as possible. We can take advantage of this last week to apply a flush product to help us wash the roots, leaving the plants' metabolism free of any remaining salts and nutrients and ensuring a far better, cleaner aroma and flavour in the end product.

Hydroponic cannabis grow using Rockwool
Hydroponic cannabis grow using Rockwool

6. Recycling Rockwool

Rockwool can be a complicated product to dispose of, given that it isn’t biodegradable. However, today the technology is available to recycle surplus and used material and help to avoid the potential contamination caused by sending it to landfill sites.


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.

Comments in “Growing cannabis in Rockwool” (11)

avatar

man 2020-11-18
Hi again... This round Im so suprised... I went long veg 4 weeks, I make girls big in veg with only 500ppm... Im in 4 week bloom and Im constatntly fighting with massive deficienties Im 100% sure roots are ok and pH are in range. I was feeding 750ppm 800ppm for first two weeks and in 3 week I just push to the 900ppm 5x daily with 10% runoff ... the runoff was about 750-850 depend on radiation ... Im at 1000ppm now and im wondering how far I can go... I have yellowing every majority fan leaves its typical N def. I guess the underfeeding already push my yield down. Truth is I manage enviroment nicely this time and I make big girls in veg already with huge root systems... I was feeding plant way lower my entire life. My main concern is I wanna avoid damage roots with high PPM. So I maybe stay at 1000ppm no matter what... how much plants can manage? in your guilde you end up with 1.8EC in the end of the round. I have major defs with 1.8EC in early flowering Im confused. Please some advice.

Alchimia Staff

Tim Alchimia 2020-11-19
Hi Man, nice to hear from you again, thanks for your comment. You really shouldn't be seeing deficiencies with that high an EC, it sounds like there's some kind of nutrient lockout going on. Have you been using any Cal/Mag supplement? A Calcium deficiency can cause this kind of serious imbalance or lockout. Another thing... are you checking the pH of your water before or after you add the nutrients? It might sound like a silly question but it would surprise you how many growers only measure the pH before adding the nutrients to the solution! And while I think about it, I'm sure you're on top of this, but have you ensured that your pH and EC meters are correctly calibrated? Inaccurate readings can really cause problems. In summary, there must be something else that's out of balance because your plants should not be having deficiencies with such a high EC. Please let us know how you get along. Best wishes and happy growing!

avatar

noobie 2020-11-01
hi i juste recently got a problem with my rockwool. i give them a nutriment base water set at 5.5ph and they juste keep getting lower when i check on my feed back. (around 5.1) do you guys have a hypothesis on what could cause this? thanks and good growing

Alchimia Staff

Tim Alchimia 2020-11-03
Hi and thanks for your comment and question. Have you checked that your pH meter is properly calibrated? One reason for the lower pH of the runoff could be due to microbial action within the Rockwool cubes, and may not necessarily have a negative effect, but I'd verify the meter is working correctly before jumping to any conclusions. Best wishes and happy growing.

avatar

man 2020-10-30
Hello, I have question about run off PPMs from RW slabs... I water them with automatic irigattion system 6x a day with nutrient solution about 650PPM in. The run off really slightly rise to 500ppm after 8days so it is really difficult feed them properly becouse right now in stretching phase they eat so much that even with 700ppm the run off drops to 450 and they are way too close to show some deficiencies... My next move is set to ppm to 800 and wait for the run off... but after one day the run off moves very slightly even with 800ppm the samples from RW slabs have 460ppm so about 10ppm more ... I guess its really nice, no shock in pH and EC stability whatsoever in the rootzone but rising the PPMs seems little difficult right now... Its really neccesary to hit them around 1000ppm ?? The run off will be way lower than the original intake. Please send me some advice. Thank you.

Alchimia Staff

Tim Alchimia 2020-11-03
Hi and thanks for your comment and question. How are the plants doing? If they look happy and you're not seeing any signs of nutrient deficiencies then There's no need to raise the ppm of your irrigation solution. If at any point you do notice a deficiency then it will be pretty easy to remedy and your plants will bounce right back really quickly. It sounds to me like you have things nicely under control there. Best wishes and happy growing!

avatar

Slavica 2020-05-28
Hallo I'm researching the methods for microbiological analysis of rockwool (sampling, sample preparing and the microbiological analysis) that will be used in medicinal cannabis industry. Can you give me some information or advice ? Thank you

Alchimia Staff

Tim Alchimia 2020-06-02
Hi Slavica, thanks for your comment. I don't know much about the subject but there have been a few studies of microbial populations in Rockwool that you can find very easily with an internet search. The one I looked at (Microbial community responses associated with the development of oomycete plant pathogens on tomato roots in soilless growing systems - L.A. Calvo-Bado, G. Petch, N.R. Parsons, J.A.W. Morgan, T.R. Pettitt and J.M. Whipps) has a fairly detailed explanation of the sampling protocol, the sample prep and the methods of analysis. I'd give that a try for a start. I hope that helps, best wishes and happy researching.

avatar

Man 2019-11-13
Hello Im really curious about irigation style... Becouse you dont say if you water until run off or not... I can believe in this system you irigate 6x a day without run off is it right?

Alchimia Staff

Tim Alchimia 2019-11-13
Hey Man, thanks for your comment. Whether you want run-off or not will depend on whether you're recirculating the nutrient solution or not. If you are, then calculate for around 20%. If you're not recirculating, then it's best to set the timer so there's as little run-off as possible and remember to apply enzymes every 10-15 days to keep the root zone healthy. Hope that helps, all the best and happy growing!

avatar

Dayna Sanderson 2019-08-22
I am currently research ways to recycle Rockwool, do you have any suggestions? It doesn't biodegrade so I'd like to find somewhere other than the land fill. Any suggestions would be greatly appreciated.

Alchimia Staff

Tim Alchimia 2019-08-27
Hi Dayna, thanks for your question. I know that Rockwool has a recycling facility at their factory in Wales, where clients, contractors and builders can send Rockwool to be recycled. Apart from that, I've seen that some people will break up the Rockwool and add it to heavy soil to improve drainage and aeration, some add it to their compost, where it apparently speeds up composting due to the extra aeration. It can also be added to normal potting soil with the same aeration/drainage benefits. I hope that's some help, all the best and happy growing!

avatar

ed haskins 2019-02-24
hi my question is can I use rockwool like its in potting soil and water it by hand once or twice daily with my soloution as long as the ph is at 6.5 I was going to use the seedling ones to start them out then transfer them to 6by6 then to slabs s

Alchimia Staff

Tim Alchimia 2019-03-06
Hi ed, thanks for your question. Yes, sure you can hand-water rockwool, you'll just need to be extra vigilant that the plants don't dry out, but if you've got the time to dedicate to them then you'll be fine! All the best and happy growing!

avatar

Mark stowers 2018-08-30
With the watering schedules you show how do you get your runoff to help with nutrient build up? Do you recommend any flushing at all?

Alchimia Staff

Tim Alchimia 2018-09-05
Hi Mark, thanks for your question. I'm strictly a soil grower, so I consulted our resident hydro expert, here are his answers: Flushing is necessary with all hydroponics substrates. You can even flush your plants from time to time (every 2-3 weeks), especially if you're using high EC values. Here we use an EC of around 1,5 and don't normally flush until the last 2 weeks If you want to get some runoff water to check the EC, just wait until the cubes are irrigated and collect it from the base of the cube (squeezing it a bit if necessary) By testing it you'll know the difference between the nutrient solution and the runoff water, which should be similar to each other. I hope that answers your question, if you've any doubts just let us know. All the best and happy growing!

avatar

matthew 2018-07-12
im a newbie at growing pot would love to even talk over the phone about it or something. need some help...

Alchimia Staff

Tim Alchimia 2018-07-16
Hi Matthew, we're happy you've decided to start growing! We can answer any questions you may have right here on the blog, so feel free to ask us but I'm afraid we don't offer a phone-consultation service unless you're enquiring about a specific product. All the best and happy growing!

avatar

Mike Saulmon 2017-11-02
Will the Rockwool cubes restrict root growth if left on when planted in outdoor gardens?

Alchimia Staff

Dani Alchimia 2017-11-03
Hi Mike, As long as you cover the entire cube with soil, roots will grow without a problem. Any part of the cube left in contact with air will restrict root growth. Best!

avatar

larry kirk 2017-07-08
would like to try the rock wool

ATTENTION!! Queries about shipping and payment

For doubts relating to deliveries and payments you can check the sections shipping cost and payment methods.

Do you want to give your opinion on "Growing cannabis in Rockwool" or ask a question abut this post?

Watch out, it will be published!

Make sure that it is a valid email address. It will not be published

About this Cannabis Blog

This is the official blog of Alchimia Grow Shop. This blog is intended exclusively for the use of adults over the age of 18 years.

To buy equipment for growing cannabis at home you can consult our catalogue of cannabis seeds, grow shop and paraphernalia


Subscibe to the blog

Do you want to receive all the latest developments, news and curiosities from the world of cultivation?

error_outline Use of cookies
We use own and third-party cookies to improve the browsing experience and to offer contents of interest. By continuing navigation we understand that you accept our cookies policy.

keyboard_arrow_up