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Grow Mediums – Rockwool

what is rockwool?

Rockwool (also known as mineral wool and stone wool) is an efficient way to grow marijuana and all kinds of plants, particularly for hydroponic systems. It is a substance that has been melted from basalt rock, into a lava, which is then spun, making a cotton-like stringy fibre. This is then made into blocks or cubes, that come in a range of sizes.

It can be a great medium to start off your young cannabis seedlings as the material means it does not disrupt the root system. Rockwool is also used in the construction industry for insulation.

Rockwool is a popular choice amongst growers and has been used for over 40 years, particularly in hydroponics. It is highly absorbent and has great water retention. 

rockwool blocks

preparing rockwool

Before you begin to use rockwool, you need to prepare it first. Naturally, it is too alkaline for cannabis (pH 7.0), resulting in poor nutrient uptake. You will need the adjust the pH level. Before using, you should soak your rockwool in water around 5.8 to 6.3pH level for 24 hours.

When the water or nutrient solution is running off at around 5.5pH, then it is ready to use. If not, soak it for a few more hours and test the pH level again. Without squeezing the remaining solution, let the rockwool air dry, allowing for the air into the rockwool block. 

rockwool in hydroponics

Rockwool is an extremely popular choice for hydroponic growers and can work well with NFT, flood and drain and drip systems. When plants are sitting in a rockwool slab, the top of the slab is usually drier, giving the plants more room for aeration and oxygen intake, which is great for respiration and root development.

The bottom of the rockwool slab will be drenched in water, allowing good moisture for your plants. Th rockwool ideally would work best if placed in a position where only the bottom half is sitting in water, as in the NFT system. 

cannabis germination

transplanting seedlings to larger cubes

With rockwool, its easy to transplant your seedling once it has outgrown the starter cube. Simply buy a larger rockwool cube and cut a hole in it, large enough for the smaller cube to fit into. Then place the cube with the seedling in it, into the new larger cube. The roots should then develop and spread out into the new cube. 

maintaining your plants in rockwool

Be sure to fertilise every time you irrigate, allowing for up to 30% of the fertiliser to be washed away as run off. Do not let more than 30% run off, as this could cause the rockwool to dry up and therefore cause stress to your plant. Rockwool can hold a lot of water, so try not to overwater your plants.

As long as the plant is healthy and the pH level is around 5 to 7.5, your plants should do well, but do monitor the pH levels to check everything is as it should be. Using rockwool allows you to have complete control over the nutrients going into your water solution. 

rockwool cubes

a disadvantage of rockwool

One disadvantage of using rockwool it that it is not re-useable and will not biodegrade. Re using rockwool can cause problems with fluctuating pH levels. It is best to start with a new cube of rockwool for each grow.