How to Use Rockwool Cubes in Hydroponics Gardening


Hydroponics do not use soil, rather growing mediums are used instead to aid in the growth of the plants.

Determining a perfect growing medium is the most challenging part for every hydroponic gardener, as there are many to select from and have their different benefits.

One may find a wide range of hydroponic growing media, including coco coir, peat moss, rockwool, sawdust, perlite, vermiculite, pumice, and even gravel.

Related post: Types of Growing Medium to Use in Hydroponics

But choosing the best one among all these substances is quite hard because each of these substances has different characteristics which could impede or aid the gardener, depending on their ability to adapt to the different growing media.

Rockwool is one of the most versatile hydroponic mediums that is widely used by hydroponic farmers.

Gardeners mainly use rockwool for seed germination in their hydroponic systems. This material can retain moisture for a longer duration.

Thus, it is an ideal material for plant cutting propagation and germinating seeds.

What Is Rockwool?


You may know rockwool by another name – stone wool. This material is man-made.

To produce it, old volcanic rock, also known as basaltic rock, must be collected and melted at a high heat limestone.

When the basalt rock reaches the lava stage, it is manufactured into fibers by spinning. At this stage, it looks similar to cotton candy.

The manufacturers add a binder when the fibers are produced and slowly compress the entire substance into mats.

Later, the mats are cut into different sizes and shapes to meet the requirements in a hydroponic garden.

Rockwool Cubes in Hydroponics Gardening


Hydroponic is a popular indoor gardening system, where hydroponic growers use the soilless growing media to grow different plants.

In this system, you need to use nutrient solutions to supply nutrients and water to plant roots.

You can grow many types of vegetables and plants using rockwool.

The best hydroponic system to use this material is considered the nutrient film technique or NFT. Plants that grow best in this substance are:

  • Leafy green vegetables
  • Peppers
  • Herbs
  • Cucumbers
  • Tomatoes

If you would prefer to grow something other than a vegetative plant, you can grow flowers, as well. Roses and Gerberas grow very well in this medium. [1]

Peat moss and coco peat can absorb and retain a large volume of water (more than their own weight). Rockwool has a similar capacity.

It can absorb excess moisture, and, at the same time, it can keep the root zone oxygenated. It is also good to note that rockwool does not dry out completely.

It has been estimated that only 2% of the nutrient solution applied to the organic rockwool compounds is considered unavailable.

Rockwool Cubes for Seed Germination


The cubes are very popular among indoor hydroponic gardeners because of their useful structures.

When the fine fibers are manufactured by spinning, they create a structure that can hold moisture and air simultaneously.

Since it also retains oxygen, you can use these cubes for seed starting or you can propagate the cuttings.

Since it is made of natural materials and volcanic rocks, it is accepted by organic farming standards.

Due to the processing at high temperatures, rockwool cubes become inert, which means they will not undergo decomposition stages.

You may find these cubes in various sizes and shapes. The tiniest ones are considered great for starting seeds or propagating stem cutting. 

Planting Seeds in Rockwool Cubes

When they are treated with high temperatures, rockwool becomes chemically and biologically inert during the manufacturing process.

That’s why you do not have to worry about its degradation. You can follow these steps to plant seeds in the rockwool cubes.

  1. First, you will have to prepare rockwool medium. To do this, soak the cubes in the pH-adjusted water. The ideal pH should range between 5.5 and 6.5.
  2. You will notice holes at the top of the cubes. Insert two seeds in each hole. Always use a fine object to push the seeds to the bottom.
  3. Now cover the hole with rockwool pieces and place them in a nursery tray.
  4. Keep these cubes in a humid region and supply fresh water to keep the cubes moist. Also, try to maintain the temperature between 70- and 80-degrees Fahrenheit.
  5. Check the moisture and germination once every two days. The cubes must not dry. Also, make sure that the cubes are not in a waterlogged environment or standing in excess water.
  6. If sprouts come out successfully, you can transfer them under the growing light and use a nutrient solution.
  7. Once they are established, you can include drainage holes.

Preparation of Rockwool Cube


When rockwool is treated with limestone during the heating process, the pH rises up to 8.

This naturally high pH inhibits the plant uptake of essential nutrients and causes nutrient deficiency.

This is why it is necessary to reduce the pH level of this substance to grow houseplants or flowers.

To do this, the cubes are first soaked in acidic water to lower their alkalinity. The acidic water will neutralize the limestone and reduce the pH.

Once the pH reaches 6, you should wash the cubes properly with distilled water.

Make sure that the pH does not fall below 5 as it can damage the rockwool fibers.

Now soak them in water once again for 24 hours. The entire process is time-consuming. So, you must be careful during the process.

If the pH stays around 5.8, it will indicate that the cubes are ready to use.


What is rockwool and Rockwool Cubes?

Rockwool is a popular hydroponic growing medium with a higher water retention capacity and is ideal for propagation and seed germination.

How to use rockwool in hydroponics?

You can use solely rockwool as the medium or mix it with other minerals or organic materials.

What can I use instead of rockwool cubes in hydroponics?

You can use coco coir, peat moss, perlite, vermiculite, sawdust, rice hulls, and many other types of hydroponic growing media as alternatives.

Is rockwool biodegradable?

Since it is an inert substance, it is not biodegradable.

Does nutrient solution boost plant growth?

Yes, plants growing in nutrient solution will grow faster due to the accessibility of readily available nutrients.

Can you put rockwool cubes in the soil?


By placing small rockwool cubes into the soil, each rockwool cube can hold a little amount of water that the roots of seedlings and plants can draw.

Just ensure you keep the soil and rockwool cubes moist (not drenched) to promote healthy root growth.

For more information on soil and other soil amendments, check out our page on soil and fertilizer.


Since rockwool material retains moisture for a longer duration, roots can absorb water even when the rockwool slab is in high tension.

It ensures a sterile environment for growing plants because granulated rockwool does not contain weed seeds or pathogens.

However, it would be best if you were careful while transferring the seedlings or main stem to other soilless media or growing mediums because new systems can affect new roots to come out. 

Without this, during the preparation of this natural product and its physical properties, many growers may face health concerns, including skin irritation.

It is better to use a dust mask during the preparation process.


[1] Bussel, W.T. and McKennie, S. 2004. Rockwool in horticulture, and its Importance and Sustainable Use in New Zealand. New Zealand Journal of Crop and Horticultural Science, 32(1): 29-37.

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