What is Rockwool & Rockwool Cubes? How to use Rockwool in Hydroponics

First produced back in 1937, rockwool has been a go-to in the construction industry for several decades now as a form of thermal insulation.

However, in the late 1960’s, growers in Denmark realized how productive rockwool hydroponics could be, leading to the material being developed as a growing medium for use all over the world.

What is Rockwool?

Wool-like in texture and appearance, rockwool is a lightweight and extremely versatile growing medium that lends itself perfectly to soil-less growing. Rockwool is highly effective at retaining both moisture and oxygen, which really benefits the root system of a plant.

Although it can form naturally in volcanoes, rockwool is usually manmade with natural materials – basal stone is melted with chalk and then spun in a machine until it forms wooly fibers.

Rockwool comes in a variety of different forms, with the most popular being:

  • Cubes
  • Slabs
  • Loose chunks

This makes rockwool suitable for growing everything from delicate seedlings to large and bushy mature plants.

Pros & Cons of Growing in Rockwool


  • Rockwool fibers can hold 18% oxygen. This not only benefits the roots of plants, but also prevents over-watering.
  • Rockwool is great for moisture retention.
  • Friendly microorganisms thrive in rockwool once plants start growing in it, which is great for promoting a stronger and healthier root system.
  • Rockwool is hardy and resilient, meaning that you can count on it to not fall apart mid-season.
  • Plants grown in rockwool can easily be moved to different locations.
  • It is completely sterile, making it safe for all plants.
  • So long as you treat your rockwool with enzymes, hydrogen peroxide, boiling water, or heat, you will be able to reuse it several times. Once your rockwool starts decomposing, break it up and use it as a soil additive to help with moisture retention, or add it to your compost pile.
  • Rockwool can be used for plants of all sizes.
  • Rockwool is a very cost-effective growing medium.


  • If rockwool dries out, plants can become top-heavy, meaning that additional growing supports may be needed.
  • It can sometimes be difficult to identify when rockwool needs more water.
  • Rockwool doesn’t contain any nutrients at all, but this does also give you full control over the nutrients you provide.
  • If the tops of your rockwool cubes remain moist for long periods of time, they can be prone to algae.
  • Some hydroponic systems aren’t compatible with rockwool.
  • Rockwool needs to be prepared in a certain way before it can be used.
  • Protective gear needs to be worn when dealing with rockwool, as the fibers can be harmful to your lungs, eyes, and skin.

Use of Rockwool Cubes

Rockwool cubes begin as a large sheet of rockwool. This is then cut into cubes, complete with indentations at regular intervals for plants, making for very convenient growing.

Rockwool cubes are available in different sizes – the smallest cubes are great for starting seeds or propagating cuttings, while the larger cubes are perfect for bigger and more mature plants.

Although they may make the growing process easier, rockwool cubes need to be prepared correctly before they’re used.

If your rockwool cubes have a plastic foil wrapping around them, leave this on during the preparation process. Once you’re done, you can then poke some holes into the plastic to allow for adequate drainage.

Preparing Rockwool Cubes Before Use

There are 2 things that you will need to do to prepare your rockwool cubes for use:

  • Soak it to hydrate it, as rockwool comes completely dry.
  • Adjust its pH level, since rockwool has a naturally high pH of around 8.0. This can prevent plants from absorbing certain nutrients, resulting in deficiencies.
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You can do both of these things at the same time by immersing your rockwool in a nutrient solution. The solution you use should be acidic, with a pH of 5.5 – this will dissolve away the lime that rockwool fibers contain, bringing their pH level down.

Make sure that the pH of your solution never drops below 5.0. Anything too acidic will damage your rockwool cubes.

The amount of time you’ll need to soak your rockwool cubes for depends on how large they are. Small cubes will only need a few minutes, whereas larger cubes could need quite a few hours.

Either way, all of those little pores in the rockwool cubes need to be fully saturated with water. You’ll know that this has happened once there are no more bubbles rising from within the solution.

Then, leave your rockwool cubes to drain until no more water is running out. Don’t be tempted to squeeze the cubes to speed up the draining process – the internal structure of rockwool that provides all of those precious air pockets is quite delicate, and this can easily be damaged with excess pressure.

This video’s showing entirely how to do with Rockwool Cubes. Let’s check it out:

How to Use Rockwool in Hydroponics

Rockwool cubes are completely inert, which means that they don’t contain any nutrients. This makes them perfect for hydroponics, as you’ll be able to accurately control the growing conditions you provide to your plants.

Once you have properly prepared your rockwool cubes, you can then use them in a variety of different hydroponic systems. Read on to learn how to use rockwool in hydroponics …

Using Rockwool in Drip Systems

Drip systems are one of the most common ways to use rockwool in hydroponics.

Not only is rockwool perfectly suited to this, but setting up a drip system is also easy, making it perfect for beginners.

All you need to do is ensure that each rockwool cube contains a drip spike – it doesn’t matter how large your cube is, as one drip spike will still suffice. This will keep each cube consistently moist, without leaving it waterlogged.

Using Rockwool in Ebb and Flow Systems

If you are growing plants that enjoy both dry and wet periods, then using rockwool in an ebb and flow system is the way to go.

Directly place your rockwool cubes into your flow table, and make sure that each flooding fully saturates both the roots of your plants and the rockwool itself. You will also need to ensure that the rockwool is allowed to drain out for long enough before it is once again flooded.

The ebb and flow system is very effective, but does require a bit of trial and error before optimum conditions are achieved.

Starting Seeds with Rockwool

Keeping seedlings alive can sometimes be tricky – they need just the right amount of moisture in order to thrive.

This is what makes rockwool cubes such a good seed starting medium – they ensure that seedlings are never sitting in waterlogged conditions, but don’t allow them to dry out either. Providing this perfect balance of moisture promotes germination, as well as healthy growth.

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How to Use Rockwool Cubes to Start Seeds:

  • Place two seeds into each cube hole, and then gently squeeze the tops of the holes to close them.
  • Cover with a dome to maintain humidity at between 70% to 80%. Spray them with water when humidity and moisture levels start to drop.
  • Remove dome once seeds germinate.
  • Cut away the smallest seedling from each hole, and then transplant when seedlings are a few inches tall.

This video is what exactly you are looking for to know using Rockwool Cubes to Start Seeds:

Propagating Plants with Rockwool

Propagation refers to growing a new plant using a cutting taken from an existing plant.

It’s an effective way to produce new plants, and is usually cheaper and faster than growing a plant from seed.

There are different propagation methods out there, and some, such as using rockwool, are more successful than others.

In order for propagation to take place, cuttings can’t ever be allowed to dry out, and this is something that rockwool really helps with. Due to the processing treatments used when rockwool is produced, the material is also completely sterile, meaning that there won’t be any bacteria or fungi present to interfere with propagation.

How to Use Rockwool Cubes for Propagation:

  • Dip your cutting in rooting hormone and place this into a cube.
  • Half fill a tray with perlite, and place your cutting and cube onto this.
  • Cover with a dome to maintain humidity at around 80%, spraying with water if the cube starts to dry out.
  • When roots first appear, leave the dome slightly open.
  • Remove the dome a few days after you first notice roots.
  • Once the roots are trying to break through the bottom of your rockwool cube, it is time for the cutting to be transplanted.

Transplanting Plants Grown in Rockwool

Rockwool does a fantastic job of encouraging growth, meaning that your plants are soon going to outgrow their rockwool bed.

You’ll know that the time has come to transplant when you can see roots poking through the sides of the rockwool, or a taproot making is way out of the bottom.

You can either place the rockwool directly into a larger rockwool cube or slab, or transplant into a different hydroponic system. Either way, make sure that your new growing medium has already been immersed in a nutrient solution – this will help to prevent transplant shock, which can be fatal for some plants.

Using Nutrients with Rockwool

Unlike other growing mediums, rockwool doesn’t bind to any specific nutrients. This means that your plants will benefit from every single nutrient you provide.

However, since plants use each nutrient at different rates, nutrient imbalances can still occur over time. In order to minimize this as much as possible, it’s important to allow your rockwool to fully drain in between each watering/feeding – ideally, around 15% to 30% of the nutrient-rich water you feed your plants should be draining out from the bottom of your rockwool.

You will also notice that the pH can fluctuate quite significantly when using rockwool, even after it has been pre-soaked. This is natural – it happens when the plants absorb more of certain nutrients that would have otherwise stabilized pH level.

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Adjusting the nutrient solution that you’re feeding can help to re-balance the pH level. However, most plants can tolerate minor pH fluctuations, so don’t worry too much unless the pH swings to an extreme.

When it comes to the actual nutrient mix you use, this all depends on the plants that you’re growing. Ideally, choose a feed that’s suited to both your plants as well as rockwool.

How Often Should You Water and Feed Rockwool?

For most growing systems, feeding and watering rockwool once a day is sufficient. However, the smaller your rockwool cubes are, the faster they will dry out, so keep this in mind when setting your feeding schedule.

Mature plants, as well as those in hotter temperatures, will also need to be watered more than younger plants, simply because they use more water to grow. Once your plants start to flower and produce a crop, you may need to increase watering/feeding to twice a day or more.

Is Rockwool Harmful?

There are a few safety concerns to be aware of when considering rockwool as a growing medium, not only in terms of your health, but also when it comes to environmental impact.

Rockwool & the Environment

While rockwool originates from natural raw materials, the way in which it is manufactured and the end result produced would not be considered environmentally friendly:

Melting rock and chalk to such high temperatures is what creates those wool-like fibers, but these are actually considered to be a hybrid material. This means that rockwool will never break back down into its natural form.

While this may be the case, this also means that rockwool can be reused each season, so long as it is treated properly in between.

Rockwool & Your Health

Rockwool fibers can be dangerous health-wise. They can affect the:

  • Eyes
  • Skin
  • Lungs

When working with rockwool, protective gear is a must. A dust mask will help to ensure that you don’t breathe in any of the fibers, while eye goggles and long clothing will provide added protection.

It’s not chemicals that are the problem – it’s simply the fact that the texture of rockwool is an irritant to humans.

Rockwool & High pH Levels

Rockwool’s high pH level can be harmful to your plants, but this is only the case if you don’t prep your rockwool before use.

If you follow the preparation steps mentioned earlier, you will be able to lower the pH of your rockwool, creating an optimum environment for your plants.

Of course, you will still have to deal with fluctuating pH levels throughout the growing process. If this isn’t something that your plants will be able to tolerate, and you’re not going to have the time to constantly monitor pH, then an alternate growing medium may be a better option.


Rockwool is an extremely adaptable growing medium, making it an essential part of hydroponics for many.

Its ability to retain moisture and offer optimum aeration, while efficiently feeding plants with the perfect balance of nutrients provided to it, means that both commercial and small-scale growers choose to use rockwool in their hydroponic systems.

Of course, there are downsides – rockwool hydroponics isn’t as environmentally-friendly as some of the other growing mediums out there, and health issues are a risk.

However, so long as you take the right precautions, understand exactly how to use rockwool in hydroponics, and make the most of the fact that the fibers can be reused indefinitely, rockwool is usually a cost-effective investment in any hydroponics system.

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Jill Sandy

I am a sustainable focus gardener. I love decorating my home backyard with beautiful landscape design and creative garden care techniques I develop myself.