There are two primary reasons that Grodan stone wool, also known as rockwool, is so popular with commercial growers: productivity and consistency. Without this duo front and center, commercial growers don’t stay in business for long! Growers using stone wool consistently achieve higher yields thanks to the medium’s unique physical characteristics—but there’s also some stone wool-specific know-how required to realize all the benefits.
What’s So Special About Stone Wool?
The porous ‘super-material’ created by spinning molten rock into fine fibers, in combination with our Advanced Hydrophilic Binder and Wetting Agent, gives stone wool cubes, blocks, and slabs the ability to hold an incredible volume of nutrient solution while still providing excellent aeration—even when fully saturated. This unique feature makes stone wool stand out from other growing mediums. The fibers are arranged in a way that ensures nutrients are evenly spread throughout the material, making it easy for roots to access moisture without much effort.
Because stone wool has a low cation exchange capacity (CEC), it doesn’t hold onto nutrients tightly, which means they are readily available for plants to absorb. This leads to efficient nutrient uptake and less energy spent by the roots to get what they need.
Additionally, stone wool can quickly rehydrate after drying out, helping growers maintain optimal moisture levels. These combined properties support strong root growth, improve nutrient uptake, and result in healthier plants with minimal water and nutrient waste.
Here, we’ll delve into some of these tips and tricks to help both seasoned growers and enthusiastic amateurs get the most out of their stone wool products.
Proper Initial Saturation and Draining
Before using stone wool, it's crucial to ensure it's properly saturated and drained. This step helps achieve uniform water content and prevents issues like water channeling.
Fully saturate the stone wool blocks with a nutrient solution matching the one you'll use during early plant growth. Allow the blocks to drain thoroughly for at least 15–30 minutes before use to ensure they are evenly moist but not waterlogged.
Optimizing Irrigation Practices
Precision irrigation is key to maximizing the benefits of stone wool. That means shot-sized irrigations, typically every 30 to 60 minutes, during the lights-on period. Unlike soil, stone wool requires more controlled watering to maintain ideal moisture levels and prevent water channelling.
Implement a multi-phase irrigation strategy:
Phase 1 (P1): Start with frequent, small irrigations to build up water content without causing excessive runoff.
Phase 2 (P2): Adjust irrigation frequency based on plant growth and environmental conditions to maintain steady moisture levels.
Phase 3 (P3): Allow for a dry-down period before lights off to encourage root aeration and prevent pathogen growth.
Want to go deeper into timing, shot size, and irrigation phases? The Precision Irrigation chapter in the Grodan Grow Guide covers it all.
Use Drip Irrigation Systems
Drip irrigation is highly recommended for stone wool. This method allows precise control over water and nutrient delivery, ensuring each plant receives the right amount.
Use pressure-compensated drippers to ensure uniform water distribution. Place two drip emitters per stone wool block, positioned at opposite ends to ensure even saturation.
Monitoring and Adjusting EC and pH Levels
Maintaining stable electrical conductivity (EC) and pH levels is vital for nutrient uptake. Stone wool’s inert nature means it won't buffer nutrients, so regular monitoring is crucial.
Invest in reliable EC and pH meters and check your levels daily. Commercial growers can push their plants in stone wool with higher EC nutrient solution.
Steer Your Crop
Crop steering involves manipulating environmental factors to promote either vegetative or generative growth phases. Stone wool’s responsive nature makes it the ideal platform for experimenting with these techniques.
Vegetative Steering: Apply more frequent, smaller irrigations with lower EC to promote leaf and stem growth.
Generative Steering: Reduce irrigation frequency, increase EC slightly, and allow for more significant dry-back periods to encourage flowering and fruiting.
Mitigate Risk of Water Channeling Issues
Water channeling can lead to uneven water distribution, dry spots, and root health problems. Proper irrigation techniques and substrate management can mitigate this issue.
Perform regular weight checks on your stone wool blocks to ensure even water distribution. If you notice significant weight differences, adjust your irrigation strategy or reposition your drippers.
Don’t Transplant Too Early
Some novice stone wool growers make the mistake of transplanting seedlings and cuttings too early. Don’t get over-excited when you see white fuzzy roots pop out of the propagation cubes in record time! It's best to wait a few more days until roots are coming out of the cubes prolifically before making the move.
Hold fire until you see a robust root system emerging from the stone wool cubes before transplanting. During these crucial extra days, keep a close eye on moisture levels to ensure the cubes remain adequately moist without being waterlogged.
Don’t Forget About Beneficial Microbial Products
Some growers mistakenly believe that stone wool isn’t compatible with beneficial microbial products. In fact, the opposite is true. Incorporating microbial and biological additives can improve root health and boost nutrient uptake even further. Early inoculation and regular re-inoculation with beneficial microbes is highly recommended.
