Stonewool, also commonly referred to as rockwool, is a man-made growing medium. It’s made by melting basalt rock (a volcanic rock) with limestone and then spinning the resulting lava into fibers. After the fibers are made, a binder is added. These fibers are then compressed into a mat that is cut into cubes, slabs, plugs, croutons, blocks and granulate.

When it was originally created, stonewool was used as an insulating material, but by changing the manufacturing process just a bit, it turned out to be a highly effective growing medium that promotes vigorous plant growth.

Today it is used to grow a wide variety of vegetables, herbs and flowers all around the world. Horticultural wool is produced on special equipment using a varied fiber configuration to best suit the plant phase the product will be used for.

Normally, stonewool insulation fibers repel water because of the natural mineral oil added to them. High-quality horticultural stonewool, however, uses a wetting agent that makes the wool water-absorbent, rather than repellant.

This, combined with the plentiful air pockets within the finished product, allows for uniform wetting while still allowing for good drainage. As an added bonus, horticultural stonewool never loses its ability to take up water.

What You Can Grow in Stonewool

Common vegetables grown in stonewool include tomatoes, cucumbers, peppers and leafy greens. Commercially grown lettuce is often started in stonewool cubes before being transplanted into an NFT system in the hydroponic greenhouse. Eggplants, melons and various herbs also perform well when grown in stonewool, and many cut flowers are grown in stonewool, the top two being roses and gerberas.

General Tips For Using Stonewool

Reusing rock – There are those who dislike stonewool because it is not biodegradable, as it is essentially a mineral. However, others like it for this very reason. Because it breaks down into smaller and smaller particles very slowly over a long period of time, it can be re-used over and over.

Before reusing stonewool, there are some things to consider. Hot water will not work to sterilize used stonewool. The regular user has two choices: crop rotation or leaving the stonewool to get bone dry before reuse. Having stonewool become bone dry will help kill off any potential fungi or pest problems.

However, having roots left inside the media means your second crop will grow in a media that is no longer 100% inert. Just like any other media, the risk is too high for commercial growers to attempt to grow a second crop in the same stonewool because the used stonewool will have a massive root structure growing through the media, so it’s typically recycled rather than reused. Home growers need to decide for themselves if it is better to reuse or purchase new stonewool.

Health concerns – There are some health concerns related to stonewool. The perception is that the rock dust can be inhaled and cause irritation. Any time you mix a dry, loose-fill growing media, a dust mask is recommended for comfort. A Material Safety Data Sheet classifies stonewool as bio-soluble and in its dry form it can cause a superficial mechanical irritation (non-allergenic). To avoid any discomfort, simply wet the blocks and cubes during the first step of handling.

Preventing algae – As with any medium with a moist surface containing nutrient solution, stonewool is a great place for algae to grow when it is wet and a lot of light is present. This can also occur in your nutrient tanks, input and output pipes and other wet areas in your hydroponic system. Algae growth can be prevented by keeping light from reaching the stonewool (and other wet areas) by covering it with plastic sheeting, shade covers or numerous other materials.

Balancing pH – The limestone in stonewool creates a naturally high pH that most plants do not thrive in, although stonewool itself actually has a neutral pH. The pH problem can be fixed by soaking new stonewool for 30 minutes or so for the larger units, and about a minute for small to medium units just before use. This will involve a little more time than other growing mediums, but many growers feel this extra effort is worth it because stonewool offers a higher yield than many growing mediums at a lower cost.

Quick pH balancing tip:

  • SOAK with pH 5.5 until fully saturated
  • FLUSH with your balanced nutrient solution
  • PLANT unit into your growing system

How to Grow Using Stonewool

Typically, growers use 1.5-in. cubes to start seeds. For vegetable plants, it’s possible to plant two seeds per stonewool cube. When planting herbs, use no more than four seeds per cube. After a few days, you will see the plants starting to sprout. Once those sprouts reach 2 to 3 in., they can be transplanted into a 4-in. cube, a slab or even into soil.

Cuttings can also be successfully grown in stonewool cubes. Just take a cutting that has been dipped into rooting hormone and plant the end into a 1.5-in. stonewool cube. Cubes can be placed directly into a growing tray and then watered.

Cover the tray with a clear lid and place it on a warming pad set at 80°F. As roots start to appear in about one to two weeks, vent gradually over the course of a few days. Afterwards the lid can be removed and the cuttings transplanted.

Transplanting should be done when you see roots coming out of the cube. When transplanting smaller cubes into 4-in. cubes, make a hole just large enough to drop the smaller cube into it. The larger cubes are then put into the hydroponic system until roots start to show again. Then these cubes are ready to be transplanted.

Generally, once seeds are planted and the dome is on, one does not need to water for at least a week, during which time you do not remove the cover. Once the stonewool is pre-soaked, in most cases the seeds will have enough moisture to keep them happy for roughly 10 days.

If they appear dry at the end of one week, only lightly mist them once, and re-cover. Watering every other day is overkill and will most likely cause unnecessary problems. You should only lightly water the young plants when cubes start to get dry.

Stonewool works well in recirculating systems or single pass systems, though a drain-to-waste system is most commonly used. This is mainly because it is easier to monitor the exact nutrient balance of the solution when it is not being reused.

The downside is that as much as 75% of nutrients and water are wasted through runoff. When a recirculating system is used, it is typical for growers to sterilize the solution before running it back through using ozone or UV sterilization methods.

The vast majority of North American commercial salad vegetable growers use stonewool / rockwool as their preferred growing medium, which is a true testament to its effectiveness. But, after determining your growing needs, it is you who ultimately has to decide what growing medium will fit your garden the best.