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Gardening with Rockwool

By Sunny Datko

About Rockwool

rockwool tomato garden by grodanRockwool has long been among the most popular hydroponic growing medium on earth. Horticultural rockwool has been used for hydroponic growing in Europe since 1969 and produces 50% of their greenhouse grown vegetables. Originally used as insulation, it was later developed for gardening in Denmark.  The main distributer of Rockwool is a company called Grodan, which means, “to grow the Danish way”.

Rockwool is made from basaltic rock and limestone.  These two items are heated to temperatures as high as 3000°F, at which point they become lava. They are then put into a spinning chamber and are spun together into fibers that resemble the texture of insulation or cotton candy. Immediately following spinning, a binder is added to the fibers and they are compressed and cured into large slabs. By adjusting the amount of pressure, the density of the media is adjusted. The large slabs can be cut into smaller slabs and propagation blocks for easy handling. The spun fibers are also formed into a granulated product, which can be handled in a manner similar to bales of peat.

Rockwool can be used as a growing medium from the time of propagation right through to harvest. Small rockwool cubes are typically used for the propagation of hydroponic plants.  Seeds can be planted directly into the cubes. Once the plants reach the proper level of maturity, the cubes can be transplanted to larger cubes, slabs or containers. The roots of the plant can then grow down into the new rockwool. A standard rockwool slab is big enough to support three or four plants. Smaller cubes, chunks, and loose fibers from bales can be used as filler for buckets, pots and net cups.


Conditioning Rockwool

When rockwool is new it contains some residual lime from production. This has led to a mistaken belief that rockwool is alkaline and that one has to continuously adjust pH.  In fact, once the lime is flushed out, rockwool is pH neutral. Immediately before use, soak the rockwool with a pH 5.5 solution then flush it out. This is done to flush out the dissolved lime. The lime will make the pH value rise to 6.0. From this point onwards rockwool does not change the pH in any way.  Most rockwool cubes will have presoak instructions on the packaging.

europonic rockwool conditioner by hydrodynamics internationalYou can also use Europonic Rockwool Conditioner, which will make the presoak process much more efficient by adjusting and stabilizingrockwool for maximum nutrient uptake.  A unique blend of pH controls and minerals, use it for conditioning your rockwool before starting seed, clones or transplants.  Use three ounces of Europonic Rockwool Conditioning Solution per gallon of water and mix thoroughly. Saturate dry rockwoolwith this mixture and let soak overnight.


Using Granulate & Grow Cubes

Within the past few years, loose rockwool and more recently granulated rockwool have been promoted as components for soilless mixes. Loose rockwool comes in rather coarse wads that resemble the formulation used for insulation, while the granulated formulation is much finer with a crumb-like appearance. The granulated product was made to facilitate easier blending with other components. 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.

Grow cubes are small cubes, like loose substrate, made of rockwool. Grow cubes provide a perfect water buffer, with a very high water holding capacity. The water content in a pot of Grow cubes is at a maximum of 85%. They also provide large airspaces, since there are always open spaces around the cubes. Grow cubes maintain their shape throughout cultivation. Due to these unique features the substrate typically has an air-space volume of 30%, depending on the size and shape of the pot.  This means that long irrigation cycles are possible without causing lack of oxygen to the roots.

There are also Grow Chunks available, which are quite similar to grow cubes.  Their larger size allows Grow Chunks hold onto moisture and nutrients longer than the Grow Cubes, providing you with a little more time between watering.  Like other loose media, Grow Chunks can be mixed with other media and used in any system.


Advantages of Rockwool

  • Retains Water – One quality of rockwool that has found great favor among commercial growers is its water-giving abilities. Since rockwool will easily give up water to the roots, even when it is almost dry, growers can allow more of the pore space in rockwool for air, while still maintaining a satisfactory supply of nutrient solution to the roots.
  • 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.
  • 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.


Disadvantages of Rockwool

  • Pre-Soak Period – Rockwool contains limestone which gives it an initially high pH, meaning it must be pre-soaked for several hours before use. Most other growing mediums only needs to be well-watered.
  • Lung & Skin Irritation – When dealing with dry rockwool, it is essential to avoid handling it with bare hands. Also, be sure not to inhale any rockwool fibers. Although it is non-toxic, rockwool can cause skin irritation. Always wear a dust mask and gloves when handling dry rockwool. If skin irritation occurs, rinse the area with water.
  • Algae Growth – Algae flourishes in wet, well-lit locations, and since rockwool stays moist, it can become an excellent algae breeding ground.  The best way to handle algae is to prevent it from occurring. Algae need light to grow, so cover the rockwoolwith a dark plastic to prevent light from reaching it.  You can also use block covers, keep the black side down and the white side up.  Shading the nutrient tanks, input and output pipes, and other “wet” equipment will inhibit algae growth.

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