Rockwool, sometimes referred to as stonewool, 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, rockwool 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 rockwool is produced on special equipment using a varied fiber configuration to best suit the plant phase the product will be used for.

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

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 rockwool never loses its ability to take up water.

What You Can Grow in Rockwool

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

General Tips For Using Rockwool

Reusing rockwool – There are those who dislike rockwool 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 rockwool, there are some things to consider. Hot water will not work to sterilize used rockwool. The regular user has two choices: crop rotation or leaving the rockwool to get bone dry before reuse. Having rockwool 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 rockwool because the used rockwool 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 rockwool.

Read More: Can You Re-use Your Grow Media?

Health concerns – There are some health concerns related to rockwool. 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 rockwool 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, rockwool 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 rockwool (and other wet areas) by covering it with plastic sheeting, shade covers or numerous other materials. (Read more: Algae: Friend of Foe?)

Balancing pH – The limestone in rockwool creates a naturally high pH that most plants do not thrive in, although rockwool itself actually has a neutral pH. The pH problem can be fixed by soaking new rockwool 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 rockwool offers a higher yield than many growing mediums at a lower cost.

Read More: Balancing pH in a Hydroponics System

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 Rockwool

Typically, growers use 1.5-in. cubes to start seeds. For vegetable plants, it’s possible to plant two seeds per rockwool 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 rockwool cubes. Just take a cutting that has been dipped into rooting hormone and plant the end into a 1.5-in. rockwool 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 rockwool 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.

Rockwool 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 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.