Growing Microgreens On Rockwool – Guide

16 August 2020

Growing microgreens hydroponically is a very fun process.

Hydroponics is a method of growing plants without soil, on an inert substrate, using a regularly supplied balanced nutrient solution.

There are many substrates for hydroponics, but based on experience in greenhouse vegetable growing, we will choose special rock wool for growing harvest microgreens as the most sterile, inert, and mechanically resistant substrate.

The cultivation of plants on rock wool began in Denmark in the 70s of the XX century.

Many countries produced this substrate.

This substrate is easy to use due to its low bulk density – 80 kg / m3.

Its high porosity (95-97%) allows it to have enough air in the root zone, even with significant moisture content (80%).

Forms Of Rockwool

Rockwool is a dense mat made of long strands of organic fibers that resemble steel wool in texture.

It is produced in the form of standard products: corks, cubes, and mats.

Rockwool is an inert substrate, i.e., it does not bind to the solution and does not release any substances into it.

Therefore, one must be very careful about plant nutrition.

However, because also of their inert nature, growers are also able to quickly alter the conditions in the root zone to suit the needs of plants.

The main requirements are no watering with clean water and top dressing with dry fertilizers.

The nutrient solution supplied to plants must contain all the necessary macro- and microelements.

Solution recipe (unit of measure – mg / l):

Some might argue that different plants need a different ratios of nutrients. Right.

This is used on an industrial scale, but for several dozen plants, it is inconvenient to calculate and dilute a dozen different solutions.

Still, if desired, this can also be done.

However, nutritional recommendations for different plants should be clear, not “more nitrogen, less potassium”, but specific values.

Advantages of Rockwool

Because of their advantageous structure, rockwool cubes are popular with both hydroponics and conventional growers.

Spinning the fibers results in a structure that is ideal for holding water while also holding more oxygen than typical soil mediums.

Starting seeds and rooting propagation cuttings benefit greatly from the increased root zone oxygenation and water-holding capacity.

Rockwool cubes have another advantage in that, despite being manufactured, they are viewed as natural products.

This is due to the fact that they are made of natural substances such as basaltic rock and chalk.

They are more widely used in organic farming methods because they are accepted as natural products.

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Growth Solution

All flowers and vegetables are grown in the same solution; there is no deficiency or excess of elements. This can happen if there is an unfavorable pH in the solution, even if it contains all the elements.

Therefore, if you decide to grow a perennial on mineral wool and do not have a pH meter and conductometer, it makes sense to use a ready-made solution (our advice is to buy fertilizers from the GHE series).

Microgreens can be grown with a pick – this allows you to reduce the cost of heating and lighting. Besides, a small sprout does not need a substrate volume of 0.5 liters (cube).

Before sowing, the plugs must be fed with a solution; for this, it is advisable to use a container of appropriate dimensions, into which the entire cassette can be immersed.

The corks are well saturated, a couple of minutes underwater is enough. The use of a container will lead to a slightly higher consumption of the solution than with regular watering.

Still, later this will facilitate the maintenance of the water regime of the plants.


Unfortunately, plugs made of domestic cotton wool do not yet have a hole for the seed, so they must be made manually.

It should be conical, no deeper than 2-3 mm. I don’t prepare seeds for sowing; I sow immediately.

Having placed the seed in the recess, sprinkle it with vermiculite or perlite (you can use organic materials – peat, sawdust – but where is the guarantee of their sterility?

However, sprinkle only relatively large seeds.

Too small seeds are not sprinkled after sowing; The bags of these seeds typically state that the crops must be covered with film or glass after moistening.

The same is done with the sprinkled seeds; the film is removed when 50% of the seedlings appear. Temperature and humidity conditions are individual for each plant species.

Well, if they can be observed, the seedlings will be more friendly. I have all the crops in one place.

Approximate temperature conditions – 18-19 ° at night and 20-21 ° during the day; in winter, plants are illuminated with Reflax lamps for 20 hours.

To control the moisture content of the cork, you can use the following technique:


Seedlings dive when they begin to compete for light (leaves of neighboring plants close up). If there is an additional cassette, you can place the plugs wider. You can continue further cultivating the plants in normal soil.

The transition from rock wool to the soil is almost 100% successful for the roots. The cubes must also be saturated with a solution before picking seedlings.

Complete submersion in the solution will hasten the soaking of a rock wool cube. The corks in the cassette are gently pushed from below and transferred to the cube.

The resulting voids between the walls of the cube and the cork are covered with loose material, carefully watered with a solution, or moistened with it from a spray bottle.

The cubes are placed on a smooth and even surface, or on a film (to improve illumination – on a white one). Germination occurs in the cube the same way it does when using corks for direct sowing.

Over time, the top layer of mineral wool “turns green”.

The development of algae on a damp, illuminated surface is normal; it serves as an indicator that there are no substances toxic to plants in the substrate. The correct watering regime is the most important thing in this technique.

It is very important not to over-moisten the substrate – if there is a lack of oxygen, the roots die.


Developing Seedlings

Typical recommendations: the moisture content of the cube should not fall below 50%; the weighing method is used for control.

Weigh the cube immediately after picking.

Wait until it has lost half of its weight, pour 50-70 g of the solution, and then maintain the weight within these limits.

But these are only general recommendations. If the plant withers, you dry out the cube. If, after watering, a solution accumulates on the film, water it.

It is advisable to maintain a water regime appropriate for the type of plant.

For example, a gerbera needs drying between waterings, so you shouldn’t keep even constant humidity in the cube (but be sure to water it in time!).

An important point is the temperature of the root environment; it should not fall below 16 °. Otherwise, it will lead to a delay in root growth, a decrease in their absorption capacity (it can cause chlorosis or even wilting), to the development of pathogens.

If growing annuals, you can plant cubes in the ground. You only need to remove the film from them; the whole cube is buried in the substrate.

When transplanted into the ground, rock wool quickly loses moisture (because there is less moisture in the soil).

Therefore, while the plants have not taken root, you need to water them often – you can use plain water.

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