The Do’s and Don’ts of Rockwool

By Armando Suarez
Published: September 1, 2016 | Last updated: January 20, 2022 08:02:02
Key Takeaways

One of the most popular hydroponic growing mediums, rockwool has been used by growers for more than 40 years. Despite this lengthy history, there are still a few misconceptions as to how to use this substrate to maximum effect. Follow these guidelines to make the most of your rockwool.

Source: Bobkov Evgeniy / Shutterstock

Rockwool, sometimes referred to as stonewool, 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, rockwool 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 rockwool, 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 rockwool.

Seed plugs made of rockwool.Seed plugs made of rockwool. - Ferdiansyah Didi / Shutterstock


The Do's of Rockwool

Do: Pre-soak Your Rockwool

Like most growing media, rockwool is shipped dry. There is 0% water in it when you buy it from the store. The initial wetting of rockwool, and actually all other media as well, is critical. Ideally, rockwool 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 rockwool 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 Rockwool Cubes

Rockwool cubes, like most rockwool 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 rockwool 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.

Rockwool cubesIt's best to keep the plastic wrapping on your rockwool cubes. - AJCespedes / Shutterstock


Do: Always Fertilize Your Rockwool

Rockwool 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. Rockwool 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.

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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. Rockwool is the fastest and easiest substrate for this, but this principle applies to all media.

Grower showing off cannabis root growth in a rockwool cube.Grower displaying roots of a cannabis plant grown in rockwool. - OpenRangeStock / Shutterstock

Do: Reuse and Recycle Rockwool

Although it is true that rockwool was originally manufactured as building insulation, horticultural rockwool 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 rockwool 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, rockwool is basalt, which is an excellent amendment for potted plants as crushed rock. Avoid using previously used rockwool 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 rockwool with enzymes. Your local hydroponics shop employees can help you with this.

A man holding a rockwool with hydroponic plants.Be careful not to squeeze your rockwool when handling. - Rendi Brahma / Shutterstock

The Don'ts of Rockwool

Don’t: Overwater Plants in Rockwool

Always fertilize the rockwool 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 rockwool 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 rockwool—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 rockwool). The plant can literally tip over because the block is too light before you need to water again.

Don’t: Squeeze Your Rockwool

A common myth is that excess water in rockwool should be removed by squeezing the block or plugs. Do not do this! The structure of the fibers in rockwool 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 rockwool is squeezed too much, the material will become a wet, soggy mess with no aeration whatsoever.

Bok choy seedlings at 8 days using rockwool.Bok choy seedlings at 8 days using rockwool. - Hideokun /Shutterstock

Don’t: Over-stack

Consider the structure and porosity of rockwool to determine how tall a rockwool 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 rockwool 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, rockwool 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 rockwool 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 rockwool is actually one of the easiest growing mediums to use. For more information, talk to the professionals at your local hydroponics shop. Happy growing!


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Written by Armando Suarez

Profile Picture of Armando Suarez
Armando has worked with plants since he was a teenager, working in a nursery. He later graduated with a BS in farm management and agricultural engineering. Armando currently works for GRODAN where he is actively involved in helping his clients design and optimize their facilities to grow the best quality crops possible.

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