germinating seeds in rockwool cubes

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.

If you're looking for a soilless substrate for seed starting, stem rooting or hydroponics, consider using rockwool growing medium. A wool-like material, rockwool for plants is easy to use and available in cubes and blocks. Learn about rockwool in this article.

Starting Seeds In Rockwool – Rockwool Grow Guide

What is Rockwool, and why are home gardeners turning to it for starting seeds?

I knew very little about Rockwool other than it’s a substrate common in commercial hydroponic growing systems. But after trying Rockwell out, I now know the benefits of using this unusual growing medium for starting seeds at home and want to share this information with you.

In this article, I explain the origins of Rockwool and the advantages of Rockwool versus soil. I also discuss how to care for and transplant Rockwool seedlings, and tips for cloning plants in Rockwool.

Keep reading to understand why with a little practice, many gardeners prefer using Rockwool for starting seeds.

  • What Is Rockwool
  • Rockwool VS Soil
  • How To Start Seeds In Rockwool
  • How To Care For Seedlings In Rockwool
  • When To Transplant Rockwool Seedlings
  • Rockwool Cloning Tips
    • Preparation
    • Gather clone cuttings
    • Condensation and venting
  • In Summary

What Is Rockwool

Rockwool ingredients consist of molten basalt rock with a limestone additive. The liquid rock and limestone mixture goes into a machine that spins the material into superfine fibers that are both sterile and inert. One cubic foot of rock makes 37 cubic-feet of Rockwool.

Rockwool, also known as mineral wool or stone wool, came into use in the 1930s as an insulating material during the construction of homes and businesses throughout Europe.

1.5″ Rockwool Starter Plugs | Editor Recommended

Top Features

    Made from molten rock Includes Twin Canaries Chart Absorbs nutrient solution while retaining oxygen for rapid plant growth

Rockwool is now readily available in convenient shapes likes cubes, blocks, and slabs ideal for any gardener to grow plants from seeds.

Back in the late 1960s, Denmark scientists thought that Rockwool’s stable moisture and aeration levels could work for seedlings in hydroponic growing systems. They began tests to see how plants they grew in Rockwool faired.

The success of these tests has led to further improvements in a Rockwool formulation explicitly made for starting plants in hydroponic growing systems.

Rockwool VS Soil

For centuries, soil has been the tried-and-true medium for starting seeds. While soil has its benefits, Rockwool offers properties that can improve seedling success rates. Let’s take a look and the pros and cons of Rockwool versus soil.

Rockwool pros:

  • Stable retention of air/water
  • Reusable through several crops
  • Sterile and compostable material
  • Convenient to use
  • Ideal for hydroponic growing systems
  • Easy to monitor/control nutrient levels

Soil pros:

  • Easy to use/safe to handle
  • Affordable
  • Can help buffer excessive use of fertilizer
  • Adjustable for texture/moisture/nutrients

Rockwool cons:

  • Fibers could irritate eyes, lungs, and skin
  • Takes practice to perfect seed germination technique
  • Naturally high pH level needs adjustment
  • Initial investment costly

Soil cons:

  • Hard to regulate nutrient/moisture/aeration levels
  • Pests/Diseases/mold thrive in soil
  • Messy to handle

The most significant benefit to Rockwool over potting soil is the ability to grow more in less space. Plants in soil have to expend energy to grow roots long enough to reach the nutrients they need to survive.

Plant roots in Rockwool have instant access to water and nutrients. Therefore, the plant expends its energy on growing taller and stronger, instead of root development.

How To Start Seeds In Rockwool

I use Rockwool for growing vegetable and flower seeds before transplanting them into my garden, but it’s a staple in hydroponic gardening systems.

Here are the steps to start seeds using Rockwool.

  1. Fill a container with water set at a pH level of 5.5.
  2. If your Rockwool has a plastic wrap, poke small holes at the base of the cube so excess water can drain.
  3. Submerge the Rockwool in the water for at least one to two hours so it can fully absorb the liquid.
  4. Remove and shake the Rockwool to release excess moisture. Don’t squeeze the material, as this compresses the fibers and destroys its natural aeration properties.
  5. Most Rockwool comes with holes on the top. Set one seed in each opening and gently push it down.
  6. Place the Rockwool in a tray with good drainage. Investing in Rockwool cube and tray sets takes the guesswork out of this step. Place the tray in a room between 70 to 80-degree Fahrenheit. Use grow lights or set the tray in natural daylight.
  7. Always keep the Rockwool moist by misting it and placing a plastic dome over the tray.

How To Care For Seedlings In Rockwool

Rockwool has no natural nutrients for your seedlings. This feature is great for allowing gardeners to precisely dose their plants with the specific nutrients they need for optimal plant growth.

I add fertilizer to my misting water when caring for my seedlings in Rockwool. Since the plants uptake the nutrients easily, I find I don’t need to use as much as I would when I plant my seedlings in soil.

Watching the moisture level in the Rockwool seems daunting at first since you can’t let the material dry out and kill off your seedlings.

The plastic tray dome helps regulate the humidity and temperature levels of the Rockwool. Removing the lid during watering or for a short time during the day allows for beneficial air circulation.

Over time, you learn how the material reacts in your home’s environment. I find my trays need misting every other day, while other people may need to water twice a day.

I keep the dome on until my seedlings are tall enough to push against the lid.

When To Transplant Rockwool Seedlings

When you plant seeds in Rockwool, cut them apart after the first set of leaves fully open. You can transplant these seedlings, while still inside the Rockwool medium, into individual pots you fill with standard potting soil.

You also know it’s time to transplant Rockwool seedlings when the root system begins to bust out of the starter block.

You can transplant these blocks directly into your garden if outdoor weather conditions permit or place them in containers with soil until the season is ready.

You also can place the seedling into a larger block of Rockwool to continue growing the plant hydroponically. Cut a hole the size of your seedling block into a four-inch Rockwool block and place the seedling inside. It’s that simple!

I find my seedlings fair best when I grow them one more time through a four-inch Rockwool block. I then transplant them directly into my garden.

Rockwool Cloning Tips

Cloning plants works fantastic in Rockwool. Here are some tips to help make the cloning process more successful.


Disinfect your tray and prepare a room where the temperature can stay around 75-degrees with about 50-percent humidity. Make sure you have a barrier, like a sheet of cardboard between the tray and table surface to keep any cold surface from harming the clone rooting process.

For best results, set a T5 24-watt grow light an inch or two above your tray (with the dome on).

Soak your Rockwool cubes as you would for normal seed germination, remove excess water, and set them in the tray.

Gather clone cuttings

Cut clones from new growth. These can be suckers growing from the plant’s base or stems near the top of the plant. Cut the clone at the stalk snipping it at a 45° angle. Remove all leaves but the top two, which promotes new root growth.

Dip the cut end into a rooting hormone powder.

Condensation and venting

After you push the clone stems into the Rockwool put the dome on the tray. Keep the vents closed to create condensation inside the tray by pulling the moisture slowly from the Rockwool. This process triggers the clone to start growing roots as the plant tries to locate water.

Remove the lid daily to shake off excess moisture and allow air circulation. If you don’t see condensation, your tray is either too cold or set in direct sunlight which can evaporate the liquid. Move the tray to a better location before the clones wither.

Once you see roots growing through the bottom of the Rockwool, open the vents on the dome, making them larger each day. After the vents are fully open, remove the cover a couple of hours each day. Extend the removal period until the cover is always off.

At this point, you can transplant the clones as they now should be healthy and have active root development.

In Summary

Once you get comfortable using Rockwool as a growing medium, you’ll find it to be a pretty forgiving substrate. I like how the material allows plants to grow strong and fast by delivering water and nutrients directly to plant roots without much fuss.

While great for full-hydroponic home gardening systems, Rockwool is beneficial for growers like me who use it to start seeds for transplant into outdoor gardens.

I hope you find this guide to starting seeds using Rockwool to be helpful. Consider experimenting with Rockwool versus soil in your next seedling crop and see if you too become a Rockwool convert!

Learn what Rockwool is, the advantages of Rockwool vs Soil, and how to start seeds. Use these hydroponics tips for care, transplanting, and cloning your plants.