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. In France, the age of majority is set at 18 years old. Consult your country's legislation and obey it. Alchimiaweb, SL is a Spanish company governed by Spanish law and can not, in any case, be held responsible for any misuse or non-compliance with the laws and regulations specific to each country, in relation to the articles published on its Blog. We remind you that the cultivation of cannabis seeds on French territory is strongly prohibited by law. We do not encourage our customers to break the law in any way and are not responsible for their use.

Comments in “Growing cannabis in Rockwool” (8)

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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

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.

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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?

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!

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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.

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!

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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

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!

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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?

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!

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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...

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!

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

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!

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

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