Stonewool, sometimes referred to as rockwool, is a growing medium made of basalt melted back into lava and poured into a spinner in a process similar to making cotton candy.

Basalt is arguably one of the most abundant materials on Earth and a large percentage of these rocks are above sea level. Most rocks and soil above sea level originate from a combination of basalt and granite, which has a strikingly similar chemical composition to basalt.

As a growing medium, stonewool has been in use for more than 40 years and is one of the most commonly used substrates in commercial hydroponic productions across the world thanks to its inert properties, sterile start and unique water-holding capacity.

The best growing performances can be achieved with stonewool, but like with most high-performance things, a few details have to be right for maximum effect. The following guidelines contain some of the most common do’s and don’ts to help you make the most of your stonewool.

Do: Pre-soak Your Stonewool

Like most growing media, stonewool is shipped dry. There is 0% water in it when you buy it from the store. The initial wetting of stonewool, and actually all other media as well, is critical. Ideally, stonewool should be immersed in a nutrient solution adjusted to pH 5.5 until all the rising bubbles are gone. This will allow the water to infiltrate the tiny pores in the product and prepare the entire volume to be colonized by roots. The immersion time depends on the size of the block in question, from just a few seconds for tiny seed plugs, to several minutes for the largest products. In all cases, let the stonewool drain freely until water is no longer running out of it. Air will re-enter the largest pores, achieving the perfect balance between air and water in the medium. No squeezing or wringing is required.

Do: Keep the Wrap On Stonewool Cubes

Most stonewool products are wrapped in an opaque plastic foil. This wrap has the same function as the walls of a plastic pot—it keeps the light out and the roots in. By covering the wool, it also prevents the growth of unsightly, yet harmless, algae. In the case of stonewool slabs, where the wrap encases the product completely, it also aids the initial pre-soaking, as most slabs will not fit in buckets or tubs. Nutrient solution fills the wrap like a bag until the slab is thoroughly wet. Only after that stage should you cut a drainage slit in the slab’s wrapping.

Do: Always Fertilize Your Stonewool Garden

Stonewool does not lock out any fertilizer. All the nutrients present in the block will be in the solution available to the plant or ready to drain out the bottom. This means whatever nutrients you use will have an immediate effect on the plant, and any feeding mistake can be immediately corrected, allowing you to optimize your fertilizer use. If you irrigate with just water, the fertilizer concentration in the block will drop and may cause a slight shock to the plant. Stonewool has a tremendous capacity to retain water, so it is better to irrigate less often with a well-balanced nutrient mix than to water more frequently without the nutrients.

Do: Allow for Sufficient Run-off

Plants always take up water at a higher rate than nutrients, so expect some increase in the concentration of fertilizer salts inside the medium between waterings. Furthermore, not all nutrients are taken up by the plant at the same rate, which creates nutrient imbalances and modifies the pH over time.

When the root zone is irrigated again, the water will push those nutrients down and replace them with the fresh nutrients it carries. If you never allow the substrate to drain sufficiently, the nutrients that were not consumed will accumulate in the medium, which may be unhealthy for your plants.

Allow 15-30% of the volume of liquid going in to drain out the bottom of the medium to maintain optimal nutritional conditions in the root zone. Stonewool is the fastest and easiest substrate for this, but this principle applies to all media.

Do: Reuse and Recycle Stonewool

Although it is true that stonewool was originally manufactured as building insulation, horticultural stonewool shares few properties with the stuff that keeps your house warm in winter and cool in summer. Plants won’t grow in the insulation material because that material does not absorb water at all.

The fact that the two materials look similar has given rise to urban legends regarding reuse, toxicity and disposal. At the end of the plant life cycle, horticultural stonewool can be reused to grow different plants, or you can shred it to be composted and then reused in potting mixes or in garden beds.

Remember, stonewool is basalt, which is an excellent amendment for potted plants as crushed rock. Avoid using previously used stonewool to start plants, and also avoid reusing any growing media repeatedly to grow the same crop or crops within the same botanical family.

To prevent issues due to the presence of rotting roots, you can simply treat the used stonewool with enzymes. Your local hydroponics shop employees can help you with this.

Don’t: Overwater Plants in Stonewool

Always fertilize the stonewool when you irrigate and allow for some drainage every time to wash off any excess fertilizer. The appropriate level of runoff should not exceed 30%, which means very little water should come out the bottom of a stonewool block.

If you are watering and the amount of drainage exceeds 30%, you are probably overwatering. This may lead to algae growth. Most of the water volume in stonewool—as much as 85%—is available water for the plant, and the block can dry down to very dry conditions before the plant finds itself in water stress (as low as 10% in stonewool). The plant can literally tip over because the block is too light before you need to water again.

Don’t: Squeeze Your Stonewool!

A common myth is that excess water in stonewool should be removed by squeezing the block or plugs. Do not do this! The structure of the fibers in stonewool contain beneficial water retention and air porosity properties that allow root systems to develop. Crushing the wool damages that structure and the air pockets that were initially present will never be restored. If stonewool is squeezed too much, the material will become a wet, soggy mess with no aeration whatsoever.

Don’t: Over-stack

Consider the structure and porosity of stonewool to determine how tall a stonewool container can be. Problems arise when irrigation is attempted from the bottom. Water is subject to gravity and will not sufficiently wick up more than 5-6-in. above the water surface, which actually applies to most media, leaving the top of the container dry and the bottom soaking wet.

All of the roots will be at the bottom of the block looking for the water and most of the media volume at the top of the container will be devoid of roots. Stacking stonewool products higher in ebb and flow systems is not a problem as long as irrigation is applied from the top. Water must travel through the network of fibers that constitutes the wool, so the container can be as tall as needed to get enough root volume to sustain a plant.

Don’t: Over-think

Growers have an emotional attachment to their plants. They love, baby, spoil, water and fertilize them. Some religiously measure electrical conductivity and pH daily, and obsessively check for growth. Well, don’t…or at least back down. As mentioned earlier, stonewool has a huge water-holding capacity and all the fertilizer applied is available to the plant so it can often go for a long time without too much attention in the irrigation department.

Don’t believe the myth that stonewool affects the pH of the nutrient solution. It is actually the natural process of the plant removing nutrients that raises the pH. If the plants are growing actively and are healthy, don’t obsess about the pH. It’s easy to correct it by changing the feed solution, but the pH can fluctuate from 5-7.5 without any ill effect on the plants.

If you follow the guidelines in this article, you will find that stonewool is actually one of the easiest growing mediums to use. For more information, talk to the professionals at your local hydroponics shop. Happy growing!