How To Plant Weed Seeds In Rockwool Cubes

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In this post we focus on explain the functioning of a hydroponic crop with rock-wool, pointing out the different steps to follow to get a successful c SSSC’s Cannabis Seeds Germination Guide: In this article we will try and give you an overview over the different germination methods used for cannabis seeds and then tell you our preferred way of

Growing cannabis in Rockwool

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 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

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.

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Cuttings in Rockwool cubes

5. How to grow in Rockwool

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 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

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.

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SSSC’s Cannabis Seeds Germination Guide:

In this article we will try and give you an overview over the different germination methods used for cannabis seeds and then tell you our preferred way of starting SSSC-seeds. In general cannabis seeds germinate best at a warm temperature of around 21-26 degrees Celsius.
We have personally tested all the described methods in a side-by-side germination test already years ago and found them to be almost equally effective, if done correctly.
It strongly depends on the growers personal preferences (e.g. organic-grower, soil-grower, hydro-grower etc.), which method of germination is best suited for him individually. Somebody who wants to grow purely organic, will not want to use rockwool-plugs as starting media out of principle. A hydro-grower on the other hand will not want to start the seeds directly in soil, when he later continues his grow on rockwool-cubes.

The best way to germinate cannabis seeds:
a) directly in soil
b) in rockwool cubes
c) on wet paper towel

a) Germination directly in soil:

In nature a seed will start its germination on or in the ground as soon as it comes in contact with enough water. The water will permeate into the seed-shell and start the germination process inside, the small plant-embryo inside the seed will then start to grow. If it stays wet enough and does not dry out during the germination process, the plant will grow into a small seedling. As growers we want to re-create this natural process.

Therefore, the first thing to do is putting the seeds into a small glass of water and let them soak there for 24 hours.
There is special planting soil for seedlings (which is not fertilized) you can buy at any gardening shop, or you can simply use a light-mix (=soil with perlite) for cannabis (which is only slightly fertilized).
It is very important to put the seed NOT too deep into the soil when you plant it! The deeper you put it into the soil, the more the seedling has to fight to reach the surface.
The maximum depth a seed should be buried is about two times its own diameter, therefore with a cannabis seed that is about 5 mm deep!
The seed is planted with the TIP OF THE SEED POINTING DOWNWARDS into a small hole in the center of the pot and then covered with soil.
The WHITE TAP-ROOT that will grow out of the seed, WILL ALWAYS GROW OUT OF THE TIP OF SEED and then search its way downwards into the soil. Therefore it is important to put the seed into the soil in the correct position, this will ensure that your roots don’t get tangled and your plant can develop quickly.
The small pots are put under a grow-lamp (e.g. T5, LED, HPS, MH) and after 24 to 48 hours in the soil the stem of the seedling should start to show and poke out of the soil. The first set of round leaves, the so called cotyledons, will open up and the seedling can start photosynthesis and start growing.

b) Germination on rockwool cubes:

Rockwool Cubes are especially designed and produced as a sterile growing medium for plants and come in different standard sizes for easy re-planting. The smallest cube-size (25-36 mm side-length) is used for the germination of seeds.
First the rockwool cubes MUST BE soaked in ph-adjusted water (ph=5,5) for 2 hours in order to get their ph-value to 6,0 (they are ALWAYS produced with a ph-value of 7,5, which is much too high for cannabis and MUST be neutralized first!) and then the excess water is drained.
Now the rockwool cubes are ready for use. They can be either be soaked again, this time in a mild nutrient solution (EC=0,3) with a ph of 6,0 or used directly.
The rockwool cubes are put into special trays, which hold 77 to 125 little cubes.
Each cube has a tiny hole on the top, which is made to hold the seed.
The seed is carefully (with a pincet) put into the tiny hole with THE TIP POINTING DOWNWARDS, in order to make sure the root grows downwards.
The tray holding the rockwool-cubes has to stay moist at all times during the germination process.
Now it takes appr. 48-72 hours under a grow-light for the seedling to appear and grow out of the top of the rockwool cube.

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c) Germination on wet paper towels:

As alternative to starting your seeds inside the growing-medium (e.g. soil-, peat, rockwool) you can pre-germinate them on wet paper towels and let the white tap-root grow out of the seed-shell before you transplant the seeds into your preferred growing medium.
The seeds are put on one half of a wet paper towel, which is then folded over to cover the seeds from both sides. After all air-bubbles are carefully pressed out of the folded wet paper-towel and insuring that the seeds are completely encased inside, the paper-towel is put inside a zip-loc plastic baggy. The bag is closed and then hung up at a dark place (e.g taped to the inside of a closet-door). By hanging the seeds inside the bag instead of laying them on a plate, you ensure that the white tap-root can grow out of the seed-shell STRAIGHT and does not curl up in a circle. This will later enable you to transplant the germinated seed easier into the growing medium. After 48-72 hours inside the plastic bag and encased by the wet paper-towel the seeds will have opened up and the white tap-root should have grown out of the seedling for about 5-10 mm. Now they can be taken off the wet paper-towel with a pincet (VERY CAREFULLY without hurting the fragile white tap-root!) and planted into the growing medium with the WHITE ROOT POINTING DOWNWARDS! Since the seed has started its germination already, it does not need to be buried deep. The best way is to put it into a small hole in the soil with the seed-shell still peaking out of the soil a little bit. This way it can start to grow upwards right away and you will have a tiny plant already 24 hours after transplanting the pre-germinated seed into the soil and putting it under a grow-lamp.

Here at the SSSC we prefer to grow organic (see Karel’s ‚Bio Grow-Book‘) and therefore we prefer to germinate in soil.

Here is our little step-by-step guide to germination success in soil:

1.) Soak the seeds for 24 hours in a small glass with distilled water
2.) Fill a small pot with potting soil or light-mix
3.) Water the soil well with ph-neutral (ph=6,0) water, but make it not too wet (not soggy!)
4.) Make a small hole in the center of your pot
5.) The hole should be appr. 5 mm deep and wide (e.g. you can use your small finger)
6.) Place the seed in the center of the hole with the TIP OF THE SEED POINTING DOWNWARDS!
7.) Cover the hole with soil and carefully push the soil from all sides inwards to make sure the seed is well enclosed inside the soil
8.) Push down and compress the soil at the outside, directly at the walls of the pot, in order to make a little ring around the whole pot. This ring is for drainage while watering. It makes sure the seed does not get flushed out of the soil at the beginning and later prevents the stem from standing in water and starting to rot.
9.) Water the little pot from time to time, by pouring a little water into the ring on the outside, before the soil gets too dry. The water will be sucked towards the center of the soil right away. Try to avoid pouring water directly into the center of the pot, in order to avoid flushing out the seed per accident!

We hope you find the germination method that fits best to your growing style and wish you lots of success with your SSSC-seeds!

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