Seeding in cubes of rockwool in hydroponics

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Seeding in cubes of rockwool in hydroponics

Rock wool is one of the most widely used substrates in the world of hydroponics. It is light, has very good water retention and offers good oxygenation. You still have to be careful that it does not dry out, without saturating it with water.

In this article, we will explain how to sow your seeds in a rockwool cube to give your plants the best start. But first, let’s see what this substrate is.

What is Rockwool?

Stone wool is primarily known as a thermal and sound insulator, but it is also widely used in horticulture. It is not a natural product. It is the fusion of a natural material, basalt, a volcanic magmatic rock, which produces a fiber. It is then shaped into bars, granules or cubes.
To purchase rockwool cubes, click here .

Stone wool cubes are very popular with horticulturists. They have the advantage of being compatible with all hydroponic systems. However, you have to know how to use them. Far too many beginners end up with underdeveloped plants or even drowned in the worst case. Here’s how to give your crops the best start. And it starts with germination in cubes.
Also discover our article on sowing in soilless culture .

How to sow seeds in rockwool cubes?

Humidifying Rockwool Cubes

First, moisten your cubes. Just immerse them in a bowl filled with water. Dip each side of the cube to speed up the absorption process. Once the cubes are saturated, they no longer absorb liquid.

It is not recommended to leave them in a bottom of water afterwards. The seeds would be drowned and the future roots totally smothered. You can therefore remove what has not been absorbed.

Looking more closely, we notice a small hole above the rock wool cubes, this is where we place the seeds. We advise you to sow 3 or 4 small seeds per cube or a single large one, this will optimize your chances of success. A selection of the best plant is therefore to be expected.

Seedlings of basil, chilli and parsley seeds in rockwool cubes

Seed germination

Once the seeds have been sown, place the rock wool cubes in a humid and warm place, but also aerated, otherwise the risk of mold development is very high.

After 3 days of germination, the basil is the first to emerge.

The best solution is to use a germination greenhouse which will control all the important parameters for you . All you have to do is carefully observe the arrival of the young seedlings.

You can also try placing them in an airtight box. In this case, pierce the lid of the box to let the air circulate. Otherwise, you have to be rigorous and open the box at least once a day so that the seedlings can breathe.

Then, to increase the temperature in your germination box, we advise you to place it on a heating mat .

Normally, in our experience, it is not necessary to re-wet the rockwool cubes with this method. In addition, thanks to the heat input, germination is done very quickly. Basil, for example, took only 3 days to reveal its cotyledons.

Transplanting in a hydroponic system

When the cotyledons, that is to say the first “leaves”, appear, you must immediately transplant your young seedlings. Indeed, rock wool is a completely inert substrate, without nutrients. And young plants need nutrients to grow without deficiencies. We therefore recommend that you place them directly in your hydroponic systems with a nutrient solution suitable for seedlings:

  • The chosen system does not use a substrate : place the rockwool cube in a hydroponic basket. Then add a few clay balls to stabilize everything. You can pre-cut the rockwool cube so that it fits into the basket.
  • The chosen system uses a substrate : insert the rock wool cube directly into the substrate.

For your new seedlings, prepare a very light nutrient solution with an EC of 0.6 to 0.8 and a pH of 5.5 to 6. These baby plants don’t need a lot of nutrients at first. But it will be necessary to gradually increase the amount of fertilizer as they develop.

  • Cutting a cube of rockwool
  • Installation of basil seedlings in the hydroponic basket.

Thinning of seedlings

Once established in the hydroponic system, the young seedlings grow rapidly. It is therefore time to choose the best plant based on selection criteria such as robustness and vigor, in the same way as we do a thinning in the vegetable garden. Only one should be kept here per cube.

Selection of the best basil plant

In the end, germinating seeds in rockwool cubes is therefore very easy. These cubes can then be placed in any soilless growing system. No wonder they are so popular with amateur and experienced horticulturists.

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