Table of Contents
- 1 Introduction
- 2 What Is Rockwool?
- 3 Rockwool Cubes in Hydroponics Gardening
- 4 Rockwool Cubes for Seed Germination
- 5 Preparation of Rockwool Cube
- 6 Where to Buy Rockwool Cubes?
- 7 FAQ
- 8 Conclusion
- 9 References
Hydroponics does not use soil; rather, growing mediums are used instead to aid in the growth of the plants.
Determining the perfect growing medium is the most challenging part for every hydroponic gardener, as there are many to select from and they each have different benefits.
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.
For more information on soil and other soil amendments, check out our page on soil and fertilizer.
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 temperature.
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 of a hydroponic garden.
Rockwool Cubes in Hydroponics Gardening
Hydroponics is a popular indoor gardening system where hydroponic growers use 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
If you would prefer to grow something other than a vegetative plant, you can also grow flowers. Roses and gerberas grow very well in this medium. 
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.
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’s 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.
Planting Seeds in Rockwool Cubes
When it is treated at 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.
How to Garden and Plant with Rockwool Cubes
Total Time: 10 minutes
Prepare The Rockwool Cube Medium
First, you will have to prepare the rockwool medium. To do this, soak the cubes in the pH-adjusted water. The ideal pH should range between 5.5 and 6.5.
Insert seeds in each hole
There will be holes at the tops of the cubes. Insert two seeds in each hole. Always use a fine object to push the seeds to the bottom.
Cover the Holes and store in Moderate Temperature
Now cover the hole with rockwool pieces and place them in a nursery tray.
Keep these cubes in a humid region and supply them with fresh water to keep them moist. Also, try to maintain a temperature of 70 to 80 degrees Fahrenheit.
Check Every Few Days
Check the moisture and germination once every two days. The cubes must not dry out. Also, make sure that the cubes are not in a waterlogged environment or standing in excess water.
Last step: Transfer the Seedlings
If sprouts emerge successfully, place them under a grow light and feed them with a nutrient solution. Once they are established, you can include drainage holes.
Estimated Cost: 10 USD
- Rockwool cubes
- Spray bottle
- First, you will have to prepare the rockwool medium. To do this, soak the cubes in the pH-adjusted water. The ideal pH should range between 5.5 and 6.5.
- There will be holes at the tops of the cubes. Insert two seeds in each hole. Always use a fine object to push the seeds to the bottom.
- Now cover the hole with rockwool pieces and place them in a nursery tray.
- Keep these cubes in a humid region and supply them with fresh water to keep them moist. Also, try to maintain a temperature of 70 to 80 degrees Fahrenheit.
- 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.
- If sprouts come out successfully, you can transfer them under the growing light and use a nutrient solution.
- Once they are established, you can include drainage holes.
Where to Buy Rockwool Cubes?
You can buy rockwool cubes at any garden center that has hydroponic equipment.
We’ve tried this particular rockwool cube, and found it to be very helpful because it has pre-drilled holes and the packages always came mold-free.
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What is rockwool and Rockwool Cubes?
When it comes to germinating seeds and root cuttings, rockwool cubes, also known as stonewool cubes, are the ideal choice. They are a kind of rockwool that is widely used in hydroponic gardening as a growth medium. The majority of rockwool cubes are one to two inches in size and are packaged in their own wrappers, which should be kept on during the growth phase.
How to use rockwool in hydroponics?
Place the rockwool cubes in a tray and cover them to contain the moisture. Keep the temperature between 70 and 80 degrees Fahrenheit. Conserve water by watering lightly every couple of days or misting with a spray bottle when you notice the rockwool cubes start drying up. As soon as the seeds begin to sprout, remove the seedlings from the covered tray and set them under grow lights.
What can I use instead of rockwool cubes in hydroponics?
When it comes to hydroponics, the most suitable rockwool substitutes include perlite and vermiculite, expanded clay pellets (LECA), coco coir, rice hulls, and other potting or gardening soils, as well as gravel or sand. In order for the hydroponic method to be effective, the growing media must be appropriate for the plants you plan to grow.
Is rockwool biodegradable?
Because rockwool is non-biodegradable, it will survive indefinitely in a landfill, making it a less-than-environmentally friendly product for use in gardening. In terms of environmental impact, rockwool is not a good choice. It is not a naturally occurring substance. Manufacturers employ a combination of chalk and rock, which they heat to over 2,500 degrees Fahrenheit. It is then passed through a stream of air, resulting in incredibly thin threads of the dusty substance in the air.
Can you put rockwool cubes in the soil?
Yes. Seeds can be germinated in rockwool cubes that are 1 to 2 inches in size. Whereas rockwool cubes that are 3 to 4 inches can be used for small plants. Rockwool cubes store a small quantity of water that seedlings and plants absorb from the soil, which helps with root development when both the soil and rockwool cubes are kept damp (not wet).
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.
-  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.
- About/mentions: Rockwool, growing media, soil
Lindsey Hyland grew up in Arizona where she studied at the University of Arizona’s Controlled Environment Agriculture Center. She continued her gardening education by working on organic farms in both rural and urban settings. She started UrbanOrganicYield.com to share gardening tips and tactics. She’s happy to talk about succulents and houseplants or vegetables and herbs – or just about anything in a backyard garden or hydroponics garden.