Rockwool Growing Medium




Rockwool Growing Medium can hold large quantities of water and air that aid root growth and nutrient uptake in hydroponics; the fibrous nature also provides a good mechanical structure to hold the plant stable. It is used primarily for drip hydroponic systems.

Rockwool is made by melting a combination of rock and sand and then spinning the mixture to make fibers which are formed into different shapes and sizes.

Rockwool  has a pH of roughly 7.8. Please see ‘Preparation’ further down…



As can been in the diagram above, rooted Rockwool Plugs can be placed into Rockwool Cubes and the rooted cubes can then be placed on top of Rockwool Slabs. The roots will then grow into the slab over time.


Square Rockwool Plugs: (36 mm x 36 mm (Top) x 40 mm (H)) – sold individually.

77-Plug Tray: (Tray Dimensions: 53 cm x 31 cm x 3 cm) filled with 77 x Plugs.

Rockwool Cubes:
Smaller sized cubes that can be placed on top of a Rockwool slab: 100 mm x 100mm x 65 mm (H).
Larger, higher cubes (A.K.A. Hugo Blocks): 150 mm x 150 mm x 150 mm

Rockwool Slab: 100 cm x 20 cm x 7.5 cm.

Rockwool Slab NARROW : 100 cm x 15 cm x 7.5 cm.


Disclaimer: Brand of horticultural rockwool may differ from time to time, depending on availability.

Rockwool Growing Medium Preparation

Items you will need:

Rockwool Cubes have a pH of roughly 7.8. This is pretty alkaline, yet our plants prefer to grow in a slightly more acidic environment (between 5.5 – 6.5). In order to prepare our Rockwell cubes for the seeds, we need to soak them in some pH adjusted water, that way they have everything the seeds need to germinate and sprout; water and a slightly acidic environment.

Hydrate And Stabilize The Rockwool Cubes

Get a bowl or some other container that is big enough to fill with water and have room left for your Rockwool cubes.

Fill the container/bowl with water from your tap.

Using either a pH test kit or a pH meter, determine the pH of the water. Water comes out pretty alkaline, usually around 7.4, so you will need to acidify it a little bit to bring that pH down to the desired level. Aim for as close to a pH of 5.5-6 as you can get.

To accomplish this, use pH down. Add to the water in small increments (a drop), and test the water to see where the pH is. Continue doing this until you have a pH of 5.5-6.

Important: Do not let the pH of the water go below 5.  A pH this low will damage the fibers of the Rockwool Cube

Now that we have the pH adjusted water, it’s time to stabilize and hydrate the Rockwool cubes in it. Insert the Rockwool Cubes into your container and let them soak for roughly 1 hour. Once the hour is up, the cubes will be big and fat with water.


Rockwool Cubes are designed to maintain the correct water to air ratio and squeezing them may damage their structure.

Using Slabs

The slabs are wrapped and sealed in plastic. Most people will start their plants in Rockwool cubes and then transplant them to the slab. To transplant a Rockwool cube to the slab, simply cut an X that is the same size as the cube, in the plastic on top of the slab. You then lift the plastic tabs and set the cube on top of the slab. You may want to pin the cube temporarily with a plastic spike until the roots grow down into the slab.