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How to Grow Plants in Rockwool

For many commercial growers, rockwool, or stonewool, is becoming an increasingly popular growing medium. It is also becoming popular for home growers. With any growing medium, there are advantages and disadvantages to consider, and rockwool is no different. So, is rockwool right for you? Read on to learn what it is, what you can grow with it and how to maximize its potential in your garden.

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.

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.

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.

The Do’s and Don’ts of Rockwool

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.

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. – 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.

It'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 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.

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. – 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!

Growing In Rockwool Cubes – Is Rockwool Safe For Plants

If you’re looking for a soilless substrate for seed starting, stem rooting or hydroponics, consider using rockwool growing medium. This wool-like material is made by melting basaltic rock and spinning it into fine fibers. Rockwool for plants is then formed into easy-to-use cubes and blocks. But is rockwool safe to use for the production of food?

Advantages and Disadvantages of Growing in Rockwool

Safety: Formed from natural materials, rockwool contains no harmful chemicals. It’s safe to use as a rooting medium and substrate material for plants. On the other hand, human exposure to rockwool represents a health issue. Due to its physical properties, rockwool growing medium can cause irritation to skin, eyes and lungs.

Sterile: Since rockwool for plants is a manufactured product, it’s contains no weed seeds, disease pathogens or pests. This also means it contains no nutrients, organic compounds or microbes. Plants growing in rockwool require a balanced and complete hydroponic solution to meet their nutritional needs.

Water Retention: Due its physical structure, rockwool drains excess water quickly. Yet, it retains small amounts of water near the bottom of the cube. This unique property allows plants to attain adequate hydration while allowing more air to circulate and oxygenate the roots. This difference in moisture levels from the top to the bottom of the cube makes rockwool ideal for hydroponics, but it can also make it difficult to determine when to irrigate the plants. This can result in over-watering.

Reusable: As a rock derivative, rockwool doesn’t break down or erode over time, thus, it can be reused many times. Boiling or steaming between uses is recommended to kill pathogens. Being non-biodegradable also means it will last forever in a landfill, making rockwool for plants a not-so environmentally friendly product.

How to Plant in Rockwool

Follow these easy instructions when using rockwool growing medium cubes or blocks:

  • Preparation: Rockwool has a naturally high pH of 7 to 8. Prepare a solution of slightly acidic water (pH 5.5 to 6.5) by adding several drops of lemon juice using pH test strips to attain the correct acidity. Soak the rockwool cubes in this solution for about an hour.
  • Sowing Seed: Place two or three seeds in the hole at the top of the rockwool growing medium. Water using a hydroponic nutrient solution. When the plants are 2 to 3 inches (5 to 7.6 cm.) tall, they can be transplanted into soil or placed in a hydroponic garden.
  • Stem Cuttings: The night before taking the stem cutting, water the mother plant thoroughly. In the morning, remove a 4 inch (10 cm.) cutting from the mother plant. Dip the cut end of the stem in honey or rooting hormone. Place the cutting in the rockwool. Water using hydroponic nutrient solution.

Rockwool is the substrate of choice for many large hydroponic farms. But this clean, pathogen-free product is also readily available in smaller-sized packages specifically marketed for home gardeners. Whether you’re dabbling with cultivating lettuce in a hydroponic jar or you’re setting up a larger system, growing in rockwool gives your plants the advantage of superior root zone technology.