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.

December 12, 2016 | Indoor marijuana growing
9 Comments


9 comments on “Growing cannabis in Rockwool

  1. larry kirk

    would like to try the rock wool

  2. Mike Saulmon

    Will the Rockwool cubes restrict root growth if left on when planted in outdoor gardens?

    1. Dani Alchimia

      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!

  3. matthew

    im a newbie at growing pot would love to even talk over the phone about it or something. need some help…

    1. Tim Alchimia

      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!

  4. Mark stowers

    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?

    1. Tim Alchimia

      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!

  5. ed haskins

    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

    1. Tim Alchimia

      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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