Rockwool (Stonewool)

Definition - What does Rockwool (Stonewool) mean?

Rockwool is a type of soilless horticultural grow medium that is used in hydroponic systems for all types of crops like flowers, tomatoes, and lettuce. It is also especially good for growing cannabis plants.

Rockwool, sometimes referred to as stonewool, is finely spun volcanic rock (basalt) that resembles fiberglass insulation. Another way to look at rockwool is like cotton candy, except it’s spun from basalt rather than sugar.

Rockwool grow media is available in several shapes and sizes, including cubes and slabs.

Hydrolife explains Rockwool (Stonewool)

Rockwool is a popular growing medium for several reasons. It has good water-retention capabilities, meaning that if it is allowed free drainage, it holds just the right amount of water for optimal plant growth.

Rockwool provides a sturdy anchor for plant roots in various types of systems, and can be reused over and over again if it is sterilized in between each growth cycle.

Starting out, rockwool is slightly alkaline, with a pH of around 7.7, so growers often soak it first in a solution of phosphoric acid (pH down) to make it slightly more acidic, closer to a pH of 5.5. The soaking stage also allows any air bubbles to escape before planting, which optimizes this grow medium just that much more.

When it is dry, the fibers in rockwool are abrasive for some growers, and can release small particles that can irritate or itch when in contact with the skin.

The major manufacturer of rockwool in the horticulture industry, both for cannabis plants and greenhouse produce, is a company called Grodan. Grodan is considered to be the pioneers of this grow medium.

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