Using Rockwool

Rockwool has long been among the most popular growing medium on earth. Originally used as insulation it was called “Mineral Insulation” and was later developed for gardening in Denmark. 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. The process is very similar to making cotton candy. The shapes vary from 1″x1″x1″ starter cubes up to 3″x12″x36″ slabs, with many sizes in between. While versatility and ease have contributed to its popularity, there are several disadvantages to this type of growing medium which should be considered along with the pros before deciding on whether or not you want to use it.

Advantages of Rockwool

RETAINS WATER – Rockwool holds an incredible amount of water which gives you a buffer against power outages and pump or timer failure.

HOLDS AIR – Rockwool holds at least 18 % air at all times (unless it is sitting directly in water). This supplies the root zone with plenty of oxygen, making overwatering less likely.

COMES IN A VARIETY OF SIZES AND SHAPES From 1″ cubes designed for use in propagation, to 3″x12″x36″ slabs capable of holding the root systems of huge plants, Rockwool comes in many sizes, including loose so you can fill pots or containers of any size.

CLEAN AND CONVENIENT – Rockwool holds together very well so it can’t spill. It also comes wrapped in plastic, making it easy to handle and keeping evaporation to a minimum.

Disadvantages to Rockwool

NOT ENVIRONMENTALLY FRIENDLY- Rockwool is hard to dispose of. If it is buried, it will last indefinitely.

DUST AND FIBERS ARE A HEALTH RISK – The fibers and dust from the Rockwool are bad for your lungs. It is strongly advised that you wear a dust mask when handling it to prevent problems.

pH PROBLEMS – Rockwool has a high pH which means you have to adjust your nutrient solution low so that the root zone is neutral. Rockwool is also susceptible to pH shifts meaning more routine maintenance to keep the pH levels correct.

Getting Started with Rockwool

1) Adjust the pH of water to 5.5. (NO LOWER).

2) If soaking A-Ok plugs, Macroplugs, or Miniblocks, mix in a little bit of plant food (no more than ¼ strength). Then just dip the starter in this pH 5.5 mix and it’s ready for the seed or cutting.


3) For all other Grodan products- SOAK, FLUSH, and then PLANT! Soak in plain water (pH 5.5) for 1/2 an hour (longer if soaking granulate).



They are designed for the perfect air/water ratio. When you squeeze the product you may ruin the good structure.*DO NOT SQUEEZE the cubes or blocks.
4) Flush through with pH 5.5 water mixed with some plant food. Make sure the flushed water goes to waste, then plant into your prepared Grodan block, slab,   growcubes™, or granulate!

*Thanks to Grodan for use of above material on pre-soaking.

Using Cubes

Rockwool cubes come in many different sizes. There are two sizes of starter cubes that are designed for propagation. The 1″ x 1″ x 1 1/2″ and the 2″ x 2″ x 1 1/2″ cubes are not wrapped in plastic and are normally used for starting seeds. The 1 1/2″ x 1 1/2″ x 1 1/2″ cubes are wrapped on four sides with plastic to slow evaporation and are used primarily for taking cuttings.

The 3″ and 4″ cubes can be used as the primary growing medium or in conjunction with other growing mediums. For small plants a large cube may be all the growing medium that you need. For larger plants these cubes are used as an intermediate medium that gets transplanted into a different type of growing medium as the plants grow.

The larger cubes come with or without a hole that is designed to fit the 1″ cube. This enables you to easily transplant the 1″ starter cube into the larger cube simply by inserting it into the hole.

Using Slabs

Slabs come in 6, 8, 10 and 12 inch widths. All are 3 inches deep and 36 inches long. The 6″ and the 8″ are by far the most popular sizes and are large enough to grow just about anything.

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. (See drawing below).

Using “loose” Rockwool

here are basically three kinds of loose Rockwool; absorbent, repellant and Hortiwool. Using loose or granular Rockwool enables you to fill pots or other containers with the growing medium so that you aren’t locked into the preset sizes of the cubes and slabs mentioned above.

Another benefit of using the loose Rockwool is that you can custom tune your medium to retain just the right amount of water for your particular plant.

Care must be taken when handling Rockwool, especially the loose varieties. You should wear a particle mask because the fibers of the Rockwool are bad for your lungs.