Rockwool is probably the most popular growing medium on earth. Rockwool was originally used as insulation and was called "Mineral Insulation". It was developed for gardening in Denmark and is used extensively around the world for "Drip-Style" hydroponic systems. |
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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, which makes rockwool one of the most versatile growing mediums.
The advantages to rockwool are many, however there are several disadvantages to this type of growing medium as well. The pros and cons are listed below.
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RETAINS WATER
- Rockwool holds an incredible amount of water which gives you a
"buffer" against power outages and pump (or timer) failure. |
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HOLDS AIR - Rockwool
holds at least 18 % air at all times (unless it is sitting directly in water), which
supplies the root zone with plenty of oxygen. This means that it is practically impossible
to over-water rockwool. |
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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 dozens of shapes and sizes making it a versatile growing medium.
Rockwool also comes "Loose" so you can fill pots or containers of any size. |
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CLEAN AND CONVENIENT - Rockwool
holds together very well so it can't spill. Rockwool also comes wrapped in plastic,
which makes it easy to handle and keeps evaporation to a minimum. |
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NOT
ENVIRONMENTALLY FRIENDLY - Rockwool is hard to dispose of, if buried it will last
indefinitely. |
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DUST AND
FIBERS ARE A HEALTH RISK - The fibers and dust from the rockwool are bad for your
lungs. Wear a dust mask when handling to prevent problems. |
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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 a bit more routine
maintenance to keep the pH levels correct. |
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LONG PRE-SOAK PERIOD - Rockwool must be pre-soaked for 24 hours before use. Most other growing medium only needs to be well watered. |
Before you use rockwool you must first soak it in water adjusted to a pH of 4.5 to 5.0. You should soak the rockwool for about 24 hours. To soak rockwool cubes use a bucket or other water tight container, just put the cubes in the water and let them float around. To soak the rockwool slabs cut a hole in the bag around the slab and pour in pH adjusted water until the slab is totally saturated, let soak for 24 hours Then cut drainage slits in the bottom.
Rockwool cubes come in many different sizes. There are two sizes of "starter cubes" that are designed for propagation. The 1" cubes are not wrapped in plastic and are normally used for starting seeds. The 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. With the hole you can easily transplant the 1" starter cube into the larger cube simply by inserting it into the hole.
Using SlabsSlabs 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 (see above) and then transplant them to the slab. To transplant a rockwool cube to the slab you 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) . |
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There 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.
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