Rockwool Information

An Introduction to Stonewool

Stonewool (or rockwool) is a popular and easy-to-use hydroponic growing medium. It provides a superior air-to-water ratio and clean start for cultivation as well as strong structural support for most types of plants.

Just What is Stonewool?

Quality horticultural stonewool from manufacturers like Grodan starts as volcanic basaltic rock which is composed roughly of the following elements: 46% silica, 16% calcium, 14% aluminum, 1% magnesium, 8% iron, 2% sodium, 1% titanium, 1% potassium and 1% manganese. In addition, a biologically-derived binder and limestone are often used in the production process.

This mixture of all-natural elements is melted at extremely high temperatures (almost 3,000 degrees Fahrenheit!) and transformed into chemically and biologically inert lava. The lava then goes through a spinning chamber where it is pulled into long, hair-like fibers. These fibers are bagged loosely as stonewool granulate or packed together in varying densities and cut into different shapes and sizes to form finished stonewool products like plugs, cubes, blocks and slabs.

Using and Reusing Stonewool

Pre-formed stonewool wares can be used individually according to your particular growing needs; however they are actually designed to work together throughout a plant’s growth cycles. Most plugs come with either a hole or v-cut slit to facilitate easy propagation of seeds and cuttings. Once plants establish a healthy root system, the plugs can be transferred directly into soil or "blocked-on" to a larger stonewool cube or block. Most of these have larger pre-formed holes which will accommodate the entire plug. For the final stage of planting, you can "block-on" again to the biggest form of stonewool, the slab. After a harvest, you can even recycle your stonewool by chopping it up into quarter-inch cubes and amending the soil used for potted plants, thereby aerating the soil and improving its water-retaining ability.

pH and Stonewool

It is a common misunderstanding that Grodan stonewool is alkaline and continuously requires pH adjustment, when in fact it is pH-neutral. The issue stems from the residual lime that occurs during the manufacturing process. As such, Grodan recommends the following:

Saturate the stonewool in pH 5.5 half an hour before use. There will be a rise in pH of about 1.0 from the residual lime dissolving. The final step involves flushing the “dissolved lime” from the stonewool with nutrient solution just prior to planting or sowing. From this point on, the pH-neutral Grodan stonewool will not contribute to any changes in the pH of the solution. You’ll also want to maintain pH values at 5.5 – 6.5 for optimal plant development and retention of substrate integrity.

Alternately, you may also choose a product like Europonic Rockwool Conditioner or Sunleaves Rockwool Formula One that will make the adjustment for you.

Take Care

Finally, it is worth noting that some people may experience irritated, itchy skin when working with stonewool, particularly when it’s dry. Protective gear like a long-sleeved shirt and gloves can afford some extra protection.

Stonewool comparison table