With such a wide variety of hydroponic growing mediums on the market, choosing the right one can be overwhelming. Each medium has different positive and negative attributes, and of course, their suitability depends on what plants you wish to grow.

Rockwool is one of the most versatile and popular growing mediums among hydroponic enthusiasts. Available in a range of sizes from 1” cubes right up to huge slabs, it is equally ideal for seedlings right through to whole crops of fully mature plants.

What Is Rockwool?

Although it sometimes occurs naturally in volcanoes, Rockwool is predominantly a man-made substance. It is formed when igneous basalt stone is melted with chalk and sand, before being spun into a cotton candy type material. Horticultural rockwool rose to popularity in the 1960s and has been used widely ever since.

What Are the Advantages of Rockwool In Hydroponics?

  • Rockwool is extremely resilient material that doesn’t fall apart or break down easily
  • Rockwool is available in all shapes and sizes, making it ideal for seedlings right through to large plants with extensive root systems. A one for all solution!
  • Rockwool has a large water retention capacity. This is a brilliant feature which protects the plants in the event of a pump failure
  • Rockwool also retains up to 18% air, meaning better oxygenation of the plant roots and less chance of overwatering

Cube, Slab or Loose?

Rockwool comes in cubes, slabs and in loose form. Smaller cubes are ideal for starting out seedlings. These cubes can then be inserted into larger sections of rockwool to continue growing.

Bigger cubes can be used as the primary growing medium or as a temporary halfway house for bigger plants which are then transplanted into other growing mediums.

Slabs can be used to grow plants of any sizes. Transplantation from smaller rockwool cubes is easy- you simply cut a cube sized hole in the slab and slot the smaller piece in. Lazy but effective!

Loose rockwool is convenient as it doesn’t have the restrictions of rigid cubes and slabs. It can be placed in varying amounts in pots, allowing the user to tailor the rate of water retention. However, as loose rockwool releases more particles than cubes and slabs, care must be taken to prevent inhalation.

Some Points to Consider

Unfortunately, the durability of rockwool also means that it is not very environmentally friendly. On top of this, the rockwool fibres are hazardous to the lungs, so special precautions should be taken when using it. Nothing a simple dust mask won’t fix, but still something to bear in mind.

Rockwool has a high pH, so nutrient solutions need to be adjusted to neutralise the levels. pH shifts are also common with rockwool use, so systems need to be monitored regularly to prevent any unwanted elevations.

Pre-soaking is required when using rockwool. Cubes must be immersed overnight in a water solution with a pH of 4.5. In the morning, the rockwool should be removed and the pH of the water tested. The level should be around 5.5. If it is higher, the soaking process must be repeated.

Using rockwool does entail some extra thought and effort, but the versatility of the medium along with the benefits of water and air retention far outweigh these minor inconveniences. Happy planting!