Preparing Rockwool Cubes—The Right Way!

There are many various methods of preparing rockwool / stonewool cubes promoted by growers. Here's a simple two-step process that works really well for consistent germination and great results.

Growers use rockwool cubes to start their seedlings and also to root their cuttings. Rockwool is a fantastic hydroponic growth media offering excellent water and air holding capacities—but it does not come ready to use.

Rockwool has a high pH of around 8 caused by lime forming on the fibers during manufacture. This lime can be removed through pre-soaking the rockwool in pH adjusted water. It’s an easy two-step process—here’s how it’s done:

We’re going to start seeds from a delicious sweet pepper variety called Yolo Wonder.

First, take a clean bucket and fill it about two-thirds with luke-warm water—the purer, the better—but tap water will usually work fine if that’s all you have. Next, measure the pH of the water with a calibrated digital pH meter. Gradually add pH down adjuster fluid, drop by drop, stirring and re-testing, until the pH meter shows a reading of 5.5. Next, submerge the rockwool cubes and leave to soak for between fifteen minutes and two hours.

The second part of the process is to prepare a mild, vegetative hydroponic nutrient solution at around 560 parts per million. Remember, peppers are fairly light feeders and we don’t need much nutrient to start seedlings anyway. Here we’re using an easy-to-use, single part nutrient called Pure Blend, manufactured by Botanicare. We’re also adding some Rapid Start by General Hydroponics to speed germination and early growth. Use a conductivity meter to check the nutrient solution strength. Adjust the pH to 6.0 using diluted pH down adjuster. Pour away the water from the rockwool cubes and replace with the nutrient solution. Now your rockwool cubes are pH adjusted and charged with a mild dose of nutrients perfect for seedling development.

Now the rockwool cubes are almost ready for seeds. Give each cube a firm shake to remove excess moisture. Don’t squeeze them or you will damage the cubes’ internal structure, decreasing their ability to hold air and moisture at the right ratios.

Carefully place one seed in the hole in each cube. Don’t place them too deep. Just a fraction below the surface is fine. Place in a propagator and place under a T5 fluorescent grow light for 18 to 24 hours a day. The ideal temperature range for germination is 70 to 80 degrees Fahrenheit.

We’ll keep you posted on progress throughout the season.