How to grow marijuana in rock wool

Using rockwool to grow cannabis is a decades-old practice. It is an inert substrate for the indoor cannabis grower that provides an excellent environment for root development. Complete control over the rhizosphere promotes optimal growth of your marijuana plant.

Rockwool is the trade name for mineral wool. It is an inorganic fibrous material made from molten minerals (basalt in horticulture) or slag. A high-pressure jet of air is blown through a molten stream of source material, or the same material is spun at high speeds through rotating cylinders, just like cotton candy.

The result is similar to Pele's Hair, which are fine mineral fibers caused by winds blowing through lava eruptions. These fibers are then pressed or rolled into sheets of desired thickness.


Mineral wool was first manufactured in the late-19th century, used predominantly as a thermal insulator in buildings—either directly as insulation in roofs and walls, or as thermal jackets and sleeves for pipes and other dangerously hot infrastructure.

However, the rockwool used for horticulture varies from this original product, which does not hold water and can be toxic. Be sure to use purpose-made rockwool for growing cannabis, and not insulation.

In Denmark in 1971, initial horticultural experiments began utilizing rockwool. Plants grew and yielded very well, and it has been a mainstay of hydroponics ever since. It is also a favorite of the indoor marijuana grower. Today, rockwool cubes, blocks, and sheets are specifically made for the horticulture industry.


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Rockwool can be hand-watered just like any other growing substrate. The important thing is the EC of the runoff water. Plants uptake nutrients and water at different rates, which can alter the EC. Regularly check the EC or PPM of the runoff, making adjustments if necessary.

Rockwool has quite a high pH (around 7.0, which is too alkaline for cannabis) and needs to be treated before use. Soak the rockwool in a nutrient solution with a pH of 4.5 and EC levels at 0.4–0.5. This is ideal to start growing seeds without having to worry about nutrient or pH issues.

Once soaked, test the block by running pH-balanced water of 5.5 through the block. If the runoff is still too high, soak again for a few hours and repeat the process.


Irrigation systems work particularly well with rockwool. Drippers at each plant should deliver 4x1-minute irrigations per light cycle for smaller plants, and 6x1-minute for larger plants. This rate allows for enough runoff to maintain correct EC and maximize aeration in the root zone.

The irrigation rate will alter as plants grow. The larger the plant, the more water and nutrients required. A hygrometer is an essential grower’s tool and should be used often. A simple manual test is to press the bottom of the block.

If it is still damp, then there is plenty of moisture for plants to continue healthy growth. Each setup is different, so there are no strict rules. Temperature, light intensity, and humidity in particular can all affect irrigation rates.


Moisture and drainage

Make sure there is plenty of drainage when using rockwool. If the blocks do not drain properly, the nutrients won’t be pushed down and flushed out. This can create salt buildup from unused nutes and alter the EC and pH of the solution. Allow at least 15–30% of the solution to drain out the bottom. If there is more runoff than 30%, then overwatering may occur, which can give rise to algae buildup.

Up to 85% of the moisture held by rockwool is water available to plants as they grow. The rest of the volume is air and the medium itself. Rockwool can dry down substantially (10% moisture) before plants become water-stressed. It is best to water before this for a very practical reason. As the blocks dry out, they can become too light, causing larger plants to tip over.

Rockwool is manufactured with a built-in moisture gradient, but because of the limited root zone per plant, it is best to water with less, but more often. This keeps the substrate in the “field zone.” This is enough water to drain away, but enough left to feed the plant until the next watering.


Always fertilize when using rockwool. The medium is entirely inert and nutrients must be supplied by the grower at every watering to maintain plant vigor.


  • Rockwool does not lock out any fertilizer. What is present in the block at any time is available to plant. This simply means that any nutrients or additives provided will have an immediate effect. Mistakes can be corrected in the short-term, allowing for optimal nutrient use.
  • Rockwool provides very little resistance to roots, so they penetrate throughout the medium very easily. This quickly increases root surface area, nutrient uptake, and plant development.
  • Because rockwool is made from minerals, the blocks or sheets can be broken up and composted. They may also be chopped up and used as an effective soil amendment.
  • It is possible to germinate directly into rockwool cubes, then transfer the cubes into bigger blocks as the plants grow. This means less stress on the plants at each upscaling, and uninterrupted growth.
  • When the correct moisture levels are not exceeded, rockwool offers excellent air penetration into the rhizosphere (root zone).


  • It is not very environmentally friendly. It takes substantial energy to produce and is not recommended to be used more than once.
  • It is relatively expensive.
  • Overwatering can cause algae buildup. Make sure there is plenty of air circulation over the grow blocks to aid in evaporation.
  • Rockwool fibers can be harmful if they become airborne. Wear a dust mask when handling in the dry stage. Do not remove the plastic coatings.
  • Rockwool can be susceptible to pH swings. Continually monitor the runoff and adjust nutrient solutions to suit.