What is Rockwool & Rockwool Cubes [Using Rockwool in Hydroponics]

One of the biggest questions that all hydroponic gardeners have in their minds is related to the right growing medium for their hydroponic setups!

If this question bothers you as well, then this is just the article you have been looking for. Since there are several options to choose from, confusion is only natural. Here, we are going to make an honest attempt to clear your confusion by explaining rockwool.

Rockwool is a versatile, lightweight growing medium that people have been using for over 40 years. In this guide, we are going to delve deep into knowing how you can get the best results out of this growing medium.

So, without any further ado, let us get right into it!

Rockwool Explained

Hydroponics system, planting vegetables and herbs without using soil for health, modern food and agricultural design concepts.

Rockwool is an extremely lightweight and versatile growing medium; it is able to hold both moisture and oxygen pretty well. This special hydroponic substrate is acquired by melting basaltic rock and chalk into very thin wool-like fibers.

These thin wool-like fibers are then converted into blocks, cubes, and other powdered products. Years ago, it was used as an insulation medium for construction projects, and in 2022, it is the most sought-after growing medium for hydroponic gardening.

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What Vegetables Can Be Grown Using Rockwool?

There are several vegetables and plants that can be grown with the help of rockwool. The most widely grown vegetable is lettuce. However, it can also be used to grow tomatoes, herbs, leafy greens, pepper, and cucumbers. Rockwool blocks are perfect for cloning new plants and germinating seeds.

The rich properties of rockwool make it perfect for growing all sorts of vegetables and plants. Apart from retaining water, it is also good at holding oxygen. This ensures that the root of the plants gets enough water and oxygen for growth. All these factors make rockwool the perfect growing medium.

Advantages and Disadvantages Of Using Rockwool As A Growing Medium

In this section, we are going to take a look at the various advantages and disadvantages of using rockwool as a growing medium.


  • It is compatible with plants of various shapes and sizes.
  • It has the ability to hold good amounts of moisture.
  • As a growing medium, it is inert, which makes it safe for all kinds of plants.
  • It can hold up to 18% of oxygen, which makes it extremely crucial for the plant roots, and also, it puts a stop to overwatering.
  • It is an economical growing medium.
  • It can be reused several times as long as it is properly fed with hydrogen peroxide and enzymes.
  • Being a very strong growing medium, it does not disintegrate mid-season.


  • Many hydroponic setups are not suitable with it.
  • It often dries out quickly, which leads to plants becoming heavier on the top, so they may require extra support for growing.
  • One needs to wear proper protection while using it as it is harmful to the skin, lungs, and eyes.
  • It is inert, which means it does not have any nutrients present.
  • Moist rockwool cubes are vulnerable to algae.

What Are Rockwool Cubes?

Rockwool cubes are a popular growing medium for hydroponic gardening. They are derived by melting rockwool with basalt rock at a temperature of 3000 degrees Fahrenheit, which creates lava.

The lava is then spun to create fibers that look like cotton candy. After the complete process of spinning, they are blended with a sticky agent and then compressed into mats. These mats are further cut into various cube-like structures, and they are sold to gardeners as a growing medium for hydroponic gardening.

How To Prepare Rockwool Cubes For Hydroponic Use?

There are basically two things you need to do to prepare rockwool cubes for hydroponic use.

  • First, hydrate it; you can do it by drenching it into a large bucket of water.
  • Then, you have to ensure that the rockwool cubes have optimum pH level since rockwool has a pH level of 8.0, which is pretty high.

Drench your rockwool in a nutrient solution water and keep the pH level to 5.5; this will ensure all the lime that its fibers contain is dissolved away.

While doing this, keep in mind that the pH level of the nutrient solution does not drop below 5.0; anything below 5.0 means too acidic.

If the rockwool cubes are large, soak them for at least a couple of hours and if they are small, soak them for a few minutes. All the tiny openings of rockwool cubes need to be drenched in water. Once you see there are no bubbles rising from the water container, it means these cubes are well saturated.

Leave the rockwool cubes to dry until there is no water coming out. We strongly advise you against squeezing the cubes as the internal construction of the rockwool can get damaged if too much pressure is applied.

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Employing Rockwool In Hydroponics

Now that you have learned how to prepare rockwool cubes, it is time to learn how to employ them for hydroponic purposes.

Employing Rockwool In Ebb And Flow Systems

This special method is for gardeners that are keen on growing plants for both wet and dry periods.

For doing this, you need to place the cubes into a flow table; ensure that each flooding soaks the plants, the roots, and the rockwell as well. Then drain the rockwool out and then again flood it.

Even though this method is very effective, it is a bit difficult to perform. It needs several trials and errors to achieve the ideal conditions.

Employing Rockwool in Drip Systems

This is the most common way of employing rockwool in hydroponics. If you are new to hydroponic gardening, we suggest you follow this method.

Rockwool is best suited for this; all you need to do is take care that each rockwool cube carries a drip strike. The size of the rockwool cube does not affect anything; just a single drip strike would be more than enough. The drip strikes will ensure that the cubes remain moist without being waterlogged.

How To Start Seeds With Rockwool?

Starting seeds can be a little tricky, as in this process, keeping the seeds alive is the biggest challenge. You have to ensure that the right amount of moisture reaches the seeds for them to thrive.

This is where rockwool cubes shine as a starting medium for seeds. The rockwool cubes make sure that seeds are not oversaturated, and at the same time, they do not let the seeds completely dry down either.

This ensures that the moisture is perfectly balanced, and hence it enhances the germination process as well as wholesome growth.

Employing Rockwool Cubes For Starting Seeds

Follow the process mentioned down below to successfully start seeds with rockwool cubes.

  • Insert two seeds into the holes of each rockwool cube.
  • Use a toothpick to ensure that the seeds sit deep within the cubes and then close the top of the holes by softly pressing them.
  • Cover them with a plastic dome to maintain humidity between 70% and 80%. Keeping the whole thing moistened is the goal so spray water if the humidity level starts to drop.
  • Keep checking them every day.
  • Remove the plastic dome when the seeds start to germinate.
  • Wait for them to grow until they are big enough to transplant into hydroponic setups.
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How To Propagate Plants With Rockwool?

Propagation means growing a new plant with the help of a cutting from a cultivated plant. It is one of the easiest methods of producing new plants. Propagation offers a few perks like it is very cheap and also very fast, it is much quicker than growing plants from seeds.

In order to propagate plants successfully, the cuttings should never be left to dry out, and this is where rockwool comes into play. Since rockwool is inert, there is practically no scope for it to contain any bacteria or fungus that may impede the process of propagation.

Steps To Use Rockwool For Propagating Plants

Follow these steps to ensure you are able to successfully probate plants using rockwool cubes.

  • Immerse the plant cutting into a tub containing rooting hormone and then insert it into a cube.
  • Fill a tray half with perlite, and place the cube and cut on it.
  • To maintain 80% humidity, cover it with a plastic dome, and spray water if the cube comes dry.
  • When roots start to come out, keep the plastic dome moderately open.
  • Remove the plastic dome for a couple of days when you first notice the roots.
  • Once you see roots coming out of the bottom of the cube, it is time for it to be transplanted.

How To Transplant Plants Grown In Rockwool Cubes?

Once you see roots coming out from the side of the rockwool cubes, it means it is time to transplant them. You can do this by directly placing the cube in a large slab, or you can move it to a totally different hydroponic system.

But before doing that, ensure that the new growing medium has sufficient nutrients in it to avoid the plants from getting a transplant shock, which can be fatal.

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Things To Keep In Mind While Using Nutrients With Rockwool

As we have already mentioned before, rockwool is inert, which means it does not come with any specific kinds of nutrients. This allows you to adjust the nutrients you want the plants to benefit from.

Different plants use different kinds of nutrients, so the chances of nutrient imbalances are very prominent. In order to reduce this, you need to make sure that you let the rockwell drain completely between each session of feeding and watering. Your plants should drain out around 30% of the nutrient-rich water you feed them from the base of the rockwool.

Another important thing that you must take care of is the fluctuations of the pH levels while using rockwool. Even though it is a natural phenomenon, you need to re-stabilize the pH levels to keep your plants safe. You can do so by adjusting the nutrient solution that is being fed to the plants.

What kind of nutrient solution you use largely depends on the kind of plants you are growing; it should be ideal for both the plant as well as the rockwool. Do not worry, as most plants are capable of tolerating slight pH fluctuations, but extreme pH fluctuations should be kept in check.

Watering And Feeding Rockwool

For most hydroponic setups, feeding and watering once daily is sufficient. Do keep in mind, though, the tinier the rockwool cubes are, the quicker they dry out. So, keep the feeding and watering plan accordingly.

Grown plants and those in warmer temperatures will require to be watered more often than newer plants as they require more nutrients and water to grow. When the time comes for your plant to produce a crop or flower, you will need to increase the feeding and water to twice daily or more.

Safety Concerns Regarding Rockwool Use

These are some of the safety concerns regarding the use of rockwool in hydroponics. We strongly recommend that you read this section very carefully to keep yourself safe while using rockwool.

  1. Is Rockwool Harmful For Your Health?

Yes, Rockwool fibers can prove to be very dangerous for your health, they can severely affect lungs, eyes, and skin. Put on some protective gear while working with rockwool. Wearing a dust mask is a must; it will prevent you from breathing in the toxic fibers.

We also recommend eye goggles for further protection. Rockwool’s texture is an extremely high irritant to the human body.

  1. Is Rockwool Harmful For The Environment?

Even though Rockwool is derived from natural materials, the way it is produced and the end product is not considered to be safe for the environment. The fine fibers of rockwool are made by melting rock and chalk at extremely high temperatures.

And since it is a hybrid material, it can never be broken back to its original form. This feature makes rockwool to be reused each season as long as it is handled properly.

  1. Is Rockwool’s High pH Level Harmful For Plants?

Rockwool’s pH level can harm your plants, but only if you do not soak it well enough in a slightly acidic nutrient solution before using it. Lowering the pH level is essential for creating a suitable environment for your plant.

However, keep in mind that you still have to keep a constant check on the fluctuating pH levels if it is something that your plants cannot tolerate.

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Things To Do To Ensure Better Use Of Rockwool

In this part, we take a brief look at some of the things you need to do to get the best results while using rockwool as an effective growing medium for hydroponics.

  1. Do Not Forget To Soak It

You must soak the rockwool cubes in water, containing a pH value of 5.5, before using it. To ensure that it is completely soaked, keep it drenched in the water solution until no bubbles are coming out from the cubes.

After performing this initial soaking process, you need to let the rockwool cubes drain. After the water comes out from its pores, the cubes will become moistened and make space for oxygen inside. This will ensure that your rockwool cubes have the perfect water to oxygen ratio.

Before using your rockwool cubes in hydroponics, make sure you wet them with all the necessary nutrients.

  1. Do Not Uncover The Cubes

We strongly advise against taking off the plastic covering of the rockwool cubes; they come packed in a plastic foil to prevent the pruning of air by blocking the light out. Also, the plastic coating helps in keeping the algae away.

  1. Know When To Reuse Rockwool

Do not reuse rockwool to start your plants. You can, however, use rockwool blocks as many times as you want for potted plants; just make sure no roots are leaching onto them. The roots tend to deteriorate the rockwool quality, but you can purchase enzymes that will do away with them. And, once you do that, you can use them again for a separate kind of plant.

  1. Do Not Squeeze Rockwool

Since rockwool blocks contain fluids, many new gardeners tend to squeeze them to eliminate the excess water. Doing this damages not only the interior structure of the block but also removes too much water.

We strongly recommend not to squeeze or compress them as the structure in which the rockwool block comes is ideal for the growth of plants. The blocks need just a little soaking and natural draining before use.

  1. Do Not Let Algae Grow On Rockwool

Since rockwool is moist, it becomes a place for algae to grow. Even though it is not a big problem, it deteriorates the visual appeal. To prevent this from happening, you may want to cover your rockwool blocks/cubes with dark plastic to prevent the light from reaching its surface.

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Frequently Asked Questions (FAQ)

Below are some frequently asked questions and answers regarding rockwool and hydroponics that you may want to take a look at to have all doubts cleared.

  1. What is the origin of Rockwool?

In the beginning, rockwool was used as insulation material by construction companies. Its light but fresh feature helped in keeping buildings warm. In 1960, a group of Danish people proposed its use as a growing medium in hydroponics.

They conducted trials for this, and since then, rockwool has been used as one of the most popular growing mediums for hydroponic plants.

  1. Hydroponic or soil – which is better?

Hydroponic is far better than soil as it provides cleaner results, and it also gives a lot of flexibility to the gardener. He can make the required customization to the nutrient solution for effective results.

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  1. What are the problems of over-stacking pots with rockwool?

If the hydroponic system that you are actively using does the irrigation from the bottom end, keep the size of the pots in mind. Water will never wick more than six inches, so the top of the pot should not be more than six inches.

  1. Where can I buy Rockwool from?

You can buy rockwool from your local gardening store, ask them which brand is in demand, and only buy that particular brand. They are also available online, but we strongly recommend buying them from the local gardening store to avoid problems.

  1. How long should I soak Rockwool cubes?

You should soak your rockwool cubes in a pH solution of 5.5 for fifteen minutes to half an hour. If you are soaking blocks, then soak them for about an hour to two hours.

  1. Can I water plants daily in Rockwool?

Yes, you can water plants daily in rockwool; however, ensure that you do not overwater them. Overwatering leads to the growth of algae. Even though algae is not a problem in itself, it can seriously ruin the visual appearance of your plant.

Vegetable sprouts in development by hydroponics technique. Sao paulo state, brazil

Final Thoughts

So, that was it! That was pretty much everything you needed to know about rockwool and its uses in hydroponics. Rockwool is extremely cost-effective, making it the number one choice of growing medium for professional and amateur gardeners alike.

What makes rockwool such a special growing medium is its adaptability. Besides retaining moisture and oxygen, it plays an outstanding role in feeding the plants with all the required nutrients.

Just keep in mind the dos and don’ts while using it. We hope you found this article informative. With that, we wish to take your leave.

Until next time!

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