Grow InfoHydroponics

Learn how to grow marijuana in rock wool

Learn how to grow marijuana in rock wool. Rockwool is a substrate which is a great alternative medium for germinating and growing marijuana or any plant in general.  It is a substrate for hydroponic cultivation that has all the main advantages of any other hydroponic medium and also some other similarities with the soil or coconut coir.

Learn how to grow marijuana in rock wool
Learn how to grow marijuana in rock wool

Rock wool is a natural product that belongs to the family of mineral wool. It was discovered for the first time in Hawaii in the early years of the twentieth century.  The results of the natural action of the active volcanoes of this archipelago. Later, the way to manufacture this material artificially from the basaltic volcanic rock was discovered.

The process involves melting the rock at a temperature of more than 1600 ° C to return it to its initial state of lava. The lava is poured into large wheels that rotate at very high revolutions and through centrifugal force, this lava is transformed into fibers. An organic binder is also added to gather the fibers before compressing them lightly.

The composition of the rock wool is approximately 98% volcanic rock and 2% organic binder. One of its main uses is thermal insulation, acoustic and passive protection against fire. Its fibrous and multidirectional structure allows it to accommodate a large amount of air. This is something that also makes it suitable for hydroponics cultivation of all types of plants. It is classified in the European Waste Catalog as Insulation material that does not contain asbestos or dangerous substances.

Rock wool

Its great advantages when it comes to growing cannabis in this type of substrate are Its porous structure and a great capacity to retain air and liquids are the most important.

Also, Rockwool blocks are cheap and they can be used without pots or containers, as it is covered with a plastic or lining that will prevent the light from damaging the roots and its structure.

An important step the first time we are going to use rock wool plugs, is to wash them to eliminate any possible residue. They also have a neutral pH, of around 7, which is quite alkaline for growing marijuana. Once we have soaked them well, we can pass them to a bucket or tub with water with pH 4.5-5 and a low EC, after a slight draining, we are ready to use.

what size Rockwool cubes to use.

We started talking about the small square Rockwool blocks which are usually 3.5cm or 5cm. They are perfect for both germinating seeds and for rooting cuttings. The blocks are also perfect for continuing the cultivation in rock wool and are 100% hydroponic.

Unlike pressed peat jiffy, rock wool never sheds waste, which is important in automatic irrigation systems used in hydroponics which can be easily blocked.

An added benefit is that It simply transplants into other mediums like jiffys without affecting the behavior of any substrate.

how to grow marijuana in rock wool
how to grow marijuana in rock wool

Learn how to grow marijuana in rock wool

For cuttings, just make a hole in the block with a toothpick, introduce the stem, and press to eliminate excess water and at the same time ensure that the cutting is not loose and moving.

starting seeds in rockwool

For seeds, we can germinate them first in tissue and introduce them in the block as soon as they have a small root of approximately 1cm, or germinate directly in the Rockwool block, after which it should be placed in a dark warm place until the small seedlings appear.

Rock wool can become dehydrated very quickly, so this should be monitored carefully, especially during the first days. The blocks we can be held in a simple tray.

Rock wool is an inert substrate, so in each irrigation, we must provide the necessary nutrients. You can use any quality line of fertilizers for hydroponics and follow the manufacturer’s recommendations regarding dosage and basic additives. In the growth phase, it is not advisable to exceed an EC of 0.60-0.70, in flowering 1.40 in the last weeks.

what size Rockwool cubes to use

When the time comes to transplant, we will opt for larger square blocks or slabs. The larger blocks usually come with a hole to introduce the sowing block, while in the slabs you can simply put the block on it, and in a few hours the roots will fix the plant, so it is an operation that will take a few seconds.

Cultivation in rock wool

how to water Rockwool cubes

From here, it is convenient to have a cultivation table where to place the blocks or slabs, since it will help irrigation and drainage. Although this task can be done by hand throughout the crop watering little by little, we install a simple automatic irrigation system so that watering can be done in two or three daily intervals, keeping the plants perfectly hydrated and nourished.

Irrigation needs will always depend on several factors, so it is impossible to determine the exact amount. Good results are obtained with 4-6 daily irrigations during the flowering phase, with durations of 1-2 minutes, during the 12 hours of lights on photoperiod.

In the last week of cultivation, it is enough to irrigate only with water and slightly smaller intervals to eliminate the residual salts that the Rockwool accumulates.

it is not necessary to do large root flushings since it does not retain many nutrients and if we have followed good Irrigation habits we will not have large accumulations and it will suffice to change the water in the tank and adjust the pH.


Cannabis Sativa, Cannabis Indica, and Cannabis Ruderalis: Thank you for being the “Gateway drug” to perpetual inspiration, compassion, benevolence, and medicinal miracles: Cannabis grower, photographer with a long experience in cannabis cultivation. His articles are journalistic reports of places where cannabis is already legally cultivated and owned. They are intended to give an impression of the wide range of cannabis cultivation. These reports are intended to help identify the truth about cannabis and reduce prejudice.

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