Rockwool Hydroponics For Beginners 1

Rockwool Hydroponics For Beginners

A simple guide for beginners venturing into rockwool hydroponics!


Rockwool (stonewool) is an inorganic insulator formed from stone, made by blowing a jet of steam through molten rocks or slag.

This process as a more advanced one, involves using high-speed spinning heads to spin molten rock.

It is often used as a soil less growing medium for plants and it also comes in various forms and sizes to suit various types of gardens.

They include: rockwool slabs, cubes and also loose chunks of rockwool.

Rockwool Hydroponics rockwool cubes

Asides being used as a medium for growth, rockwool is widely used in a large number of applications and products.

This is as a result of its high melting point which makes it an excellent insulator.

It could be used in fireproofing and other heat sensitive applications.

It is necessary to do some research before sourcing rockwool, so as not to end up with horticultural grade rockwool.


Rockwool is a very popular medium for gardening and hydroponics such as fruits, vegetables and herbs.

Not just because it holds water well, but also because it allows air to circulate through the plant’s roots.

Thus, allowing for exceptional plant growth.

Rockwool hydroponics competes with other types of growth medium such as grow stones, clay pebbles, coco air, floral foam, gravel, perlite and vermiculite.

Rockwool can be reused as it is lightweight.

It provides plants with a sterile growing environment.

While it usually starts with a naturally high PH with proper conditioning it can be suitable for plant growth by stabilizing its pH level.

The fibers in rockwool are abrasive and they release micro particles, which can irritate or itch when it is exposed to the skin.

This is why growers before planting often soak it.

Its fibers absorb moisture making it a suitable medium for mold growth, which is why it shouldn’t be used in high residential areas.


Grodan rockwool is made from molten basalt rocks and chalk. Which is melted at 1600 Celsius then spun into cotton candy like fibers and compressed after into slabs, slabs or blocs.

How It's Made - ROCKWOOL Stone Wool Insulation

Grodan has the highest rockwool density on the market. Which translates into greater structural integrity allowed for increased and longer oxygen holding capacity.

Grodan rockwool is not just a single product and is highly advanced. Growers have a number of grodan rockwool choices which they can select from.

These grodan rockwool choices also include; “Grotop Master”, “Groptop Master Dry” and “Grotop Experts, which all have varying properties and uses.

For instance, “Grotop Master Dry” is used by tomato growers to stop crops from overly growing and it also maintains a slightly dry root.

“Grodan Classic” is designed for multi-year use and “Grotop Expert” is designed for ultra quick development as well as root growth.


The way its molten rock fibers are stacked as well as the density of the rockwool product determine its properties as a growing media.

These properties often include;

Moisture Gradient;
After irrigation, standard rockwool products drain freely and will typically contain about 80% of its nutrient, 15% pore space and 5% rockwool fibers.

These ratios however differ slightly depending on the product.

For instance, a typical rockwool slab after draining contains about 80% of its nutrient solution regardless of its drainage holes.

Which allow for the draining of excess solution.

Plants can still extract water for growth even with very low water in the media.

This means that even if the rockwool has just recently been irrigated and is saturated.

After it loses about 70-80% of its moisture content and it has dried down.

It still has enough water that can be used for plant growth.

This moisture gradient is a very important characteristic of rockwool.

Which makes it a hydroponic substrate but at the same time also poses the risk of over irrigation by growers.

Irrigation and EC management
Rockwool is designed to have just the right degree of moisture gradient which allows for limited root zone for fairly large plants.

Hence this makes it necessary to be irrigated with frequent applications of nutrient solutions.

This irrigation has to be just enough for the rockwool to reach its field capacity.

The point where the substrate has drained totally, but still has enough moisture for the plant roots to access until next irrigation.

Draining about 10-35% of the nutrient solution fed to the plants from the rockwool is optimal.

This amount of drainage solution causes fresh nutrient solution to flush through the rockwool slab and keeps the EC stable.

The EC in the root zone should be check frequently.

Although rockwool doesn’t contain natural minerals or salts that will affect it EC levels, the EC levels still change.

Owing to plants extracting different ratios of water and nutrients from the root zone.

As such, it is necessary to maintain and control the EC and pH in the rockwool hydroponics like in any other growing media.

Under hotter temperatures, plants extract higher levels of water from a nutrient solution. Causing the EC to increase faster and requiring the addition of top-up water.

The EC may drop in cooler temperatures as plants do not often require as much water but takes up its nutrients.

Hence, the EC levels should be properly maintained.

Microbial Characteristic
Rockwool is a sterile product which does not contain natural microbial beneficial populations when first planted.

However, these microbial organisms develop in the same way as they do in organic mediums.

The buildup of microbial populations is however slower as at the beginning there are limited carbon sources for these microbes to feed on.

This is because as the root systems develop, microbial life gradually builds within the rockwool.

As a faster solution, rockwool can also be inoculated with these microbial products.

To assist this process and help in developing plants root zone, increased oxygenation in the rockwool hydroponics system allows for the establishment and further multiplication of these beneficial microbe populations.

Re-usage Characteristics
Rockwool does not decompose, fracture or breakdown over time, hence it can be used for successive crops by growers.


Rockwool is easily over watered. It goes from being wet to being barely moist before it is watered again.

Rockwool hold about 80% water and about 20% air which makes it necessary to dry it out when over saturated before being watered again.

Ideally, rockwool should be watered once a day at the vegetative growing straight and twice a day at the bloom stage.

It is very important that rockwool be barely moist, not completely dry before it is watered again.

Also do not squeeze the tabs together as this will decrease its absorption of water.


Rockwood is a very versatile growing medium that can either be used in standalone product or in a variety of hydroponic systems.

It is an excellent option as a soil less material for both commercial farmers and hobby gardeners.

Here are a few tips on how to use rockwool properly in hydroponic gardening.

Purchase the Product:
The first step to using rockwool hydroponics is to purchase the product.

Rockwool Hydroponics For Beginners 2Rockwool Hydroponics For Beginners 3

Buy a pack of 1” or 1 1/2″ rockwool cubes for seed propagation. These packs are sold in convenient packs that fit into the standard 10” x 20” plastic tray.
Adjusting the pH level:
The rules to adjusting the pH level of rockwool are largely dependent on the brand being purchased. Adjust the pH of the water using pH down or lemon water to 5.5. Then using a pH kit or meter, test and adjust pH level as needed. Soak the rockwool until it is fully saturated, making sure to not squeeze the block as it has been designed for optimal air/water ration to be used for horticulture purposes.
Plant the seeds
Seeds in rockwool should be planted in each cube. However depending on the type of seeds, you can also plant multiple seeds in each cube. Information on how to plant the seeds should be researched from your seed supplier. I like to use sterile water and a toothpick. Dip the toothpick in the water. This makes it easy to place directly in the cubes.

rock wool hydroponics preparing seeds

Hot Tip: Once the seedlings begin to emerge from the cubes. It is vital to their growth that you place them under grow lights for 18 hours daily. Generally, as soon as 3 day’s!

Add the Nutrient solution
As needed, use a diluted nutrient solution to water your plants as needed. This is typically 1/8 to 1/4 of the nutrients recommended for the nutrient solution. The nutrient solution can be increased as the plant grows into full maturity; however take care to not over water as this can slow down root development.

Transplanting into a New Medium
Once the taproot is visible at the bottom of the cube, then the rockwool cubes are ready to be transplanted. Once this happens, you can choose where you would like to transfer your plants, whether to a large rockwool cube, a hydroponic system or a planter with loose materials.

hydroponic plants under grow lights

Ensure to properly presoak the new media in nutrient solution or run the transplanting properly to reduce the risk of transplanting shock.

Transfer the rockwool to a new soil less media carefully. Because the new media can suck away water and dry the rockwool cubes. Even before the roots can reach the medium.

As such, before transplanting, ensure that the plant’s root system is established.

Despite being a popular medium used by hydroponic growers in recent times. As well as the large number of benefits in using rockwool.

Before choosing to use rockwool in hydroponics, there are certain factors that should be considered, these factors include:

Environmental Concerns;
Rockwool hydroponics is totally not a natural material despite a similar substance is naturally made by volcanoes.

Rockwool is in fact made by heating chalk and rock to 3000 degrees Fahrenheit.

It is then spun and cooled which turns it to the materials sold as rockwool hydroponics online and in stores.

Rockwood is not a biodegradable material as it is not natural, which means that disposed rockwool will sit indefinitely in a landfill.

If these environmental concerns bother you, but you still want the benefits of rockwool.

You should rinse and reuse old rockwool cubes rather than buying a new one every growing season.

Not only is this better for the environment, but it also saves a great deal of money.

Health Concerns
Rockwood has a number of health concerns which you should be careful of when handling rockwool.

It can irritate the skin, eyes and lungs and too much exposure to the skin might have serious long term effects.

The skin irritation isn’t caused by a chemical danger in rockwool.

But rather its physical makeup in the same way that grass or wheat might cause irritation when in contact with the skin.

To reduce all risks of health complications, use a dusk mask, goggles and gloves when handling rockwool.

Ph Problems
Rockwool is more alkaline than other medium which means that you have to personally adjust your pH nutrient solution.

To create an ideal pH thus allowing for root growth.

This might be tiring especially if you are using multiple mediums.

It might require that you create one pH balanced nutrient solution for one media.

As well as a different one for your rockwool hydroponics grown plants.

As rockwool shifts its pH balance rapidly, you must be extra vigilant about regularly checking your pH balance and making necessary adjustments.

Before using rockwool, it needs to be treated.

It is necessary that you soak it in pH treated water for at least 6 hours before transferring your plants into it.

As a hydroponic medium, Rockwool has a number of advantages, which include;
⦁ Rockwool is lightweight and easy to handle, after irrigation; it becomes heavy and provides stability for crops.
⦁ Rockwool maintains its physical properties over time and with successive crops.
⦁ Rockwool is made from molten rock. The plastic wrapping which the product comes in ensures that it is sterile and free from pests, weed seeds and pathogens.
⦁ Rockwool being a manufactured product are consistent; they do not decompose or break up over time as other natural products do.
⦁ Rockwool hydroponics is reusable
However there are disadvantages that come with using rockwool as a hydroponic product, these include:
⦁ Rockwool contains no natural occurring substances. Which makes plants highly dependent on a well balanced, complete nutrient solution at each stage of growth.
⦁ Often times, it might be a bit difficult to determine the right frequency and amount of irrigation which rockwool hydroponics systems require.
⦁ Rockwool is not biodegradable, thus making disposal difficult despite its reuse properties.
⦁ Rockwool fibers are irritable to the skin.
⦁ Despite being lightweight, rockwool is bulky to transport and store

Planting Seeds in Rockwool for Hydroponic Growing

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