This inert mineral is sterile, porous, non-degradable and offers plant roots firm support and good oxygen access! It also is lightweight and ideal for hydroponic growing – where the grower has complete control of pH levels and nutrients at all times.

Horticultural mineral wool has been used for hydroponic growing in Europe since 1969 and produces 50% of all their greenhouse grown vegetables. Yields have been recorded of up to 160 tons per acre, crop after crop, with greenhouse tomatoes grown in rockwool.

Rockwool is an inorganic mineral product that was first discovered on the islands of Hawaii in 1840. These mineral formations were created as a natural by-product of volcanic activity. For today’s horticultural industry it is either produced from rock alone, or from a combination of rock, limestone and coke. The components are melted at temperatures exceeding 2,500 degrees F. This molten solution is poured over a spinning cylinder. As the molten solution flys off the cylinder, it elongates and cools to form fibers. These resultant fibers (rock wool) are then pressed into sheets, cubes, blocks and/or are ranulated. Granulated mineral wool can be used as a soil mix amendment or placed in plastic bags for bag culture.

Like all inert (with few or no active properties) media, the mineral wool acts only as a temporary reservoir for nutrients supplied through the irrigation water. This allows the grower a tremendous amount of control over plant growth through nutrition. Also, rockwool’s fibrous structure contains a high percentage of air space; approximately 20% even when it is fully wet. There is little danger of over- watering plants! Since the fibers cannot bind nutrients or water (like peat media can), all nutrients and water contained within the fibers are available to the plant. pH is also controlled by the irrigation water and may be easily changed.

The basis of the nutrient solution used in a rockwool system begins with the chemical composition of the existing irrigation water. Once the original water has been analyzed, fertilizer rates and pH additives are determined. A complete water breakdown (specific levels of chemicals) is necessary to make these determinations. Because the existing irrigation water forms the basis of the ultimate nutrient/pH solution, any changes in water quality during the crop cycle may require nutrient and pH adjustments. Contact your local agricultural extension agency and they will inform you where to send your water to get it tested. They are familiar with the water in your area and will give advice if there are any specific problems. Testing is a must if you are taking water from a well.

It is recommended that you keep a daily record of your growing environment, including the nutrient solution quality, for your first crop using the mineral wool system. Monitor the air temperature, humidity and carbon dioxide levels. In the nutrient solution , monitor temperature, pH, dissolved salts and fluid depletion levels. This will give you a “feel” for the needs of your plants and will enable you to optimize conditions. An inert media cannot buffer a growing mistake – errors will be magnified and will appear much sooner than they would if the crop were grown in peat or soil; growing in rockwool is much more precise.

High temperatures during the manufacturing process make mineral wool products disease, weed and insect free. The process also creates a non-degradable, long-lasting media. Barring disease problems, rockwool can be used for several successive crops. However, residual roots from the previous crops decompose and can harbor pathogens. In commercial vegetable production, rockwool is usually discarded or steam sterilized after two or three uses.

The key to growing record yields in rockwool is a matter of combining its remarkable areation and nutrient holding capabilities with the proper nutrients, temperature, lighting and CO2 levels to obtain the ultimate growing environment for the crop.


Prepare rockwool by thoroughly soaking it to capacity with the proper nutrient solution. Always apply nutrients to the rockwool before handling the product. If handling rockwool in a dry state, and working in confined spaces, use a nuisance mask. (These precautions should also be applied to perlite and vermiculite horticultural products.) Keep the rockwool growing medium moist – do not let the media dry out! The soluble salt levels will be thrown out of balance if the moisture levels in rockwool are too low. A good rule of thumb is to keep the rockwool wet enough that nutrients will squeeze out easily. Irrigate 20% to 30% more than needed to leach out excess soluble salts. If using a floor or capillary type irrigation system; the medium should be thoroughly leached every two weeks – or sooner if salt build-up appears to be causing problems with growth.

Soluble salts should be measured after the nutrient solution is added to the rockwool media. The propagation stage should be maintained within the range of 450 to 650 P.P.M. (Parts Per Million) and the growth stage should be kept between 1100 to 1425 P.P.M. The flowering (or fruit production) stage of plant growth requires that enough nutrients will be available at the onset to produce maximum yields. Soluble salt levels should be between 1175 to 1300 P.P.M. at the start of the bloom stage, tapering off on the concentration of the nutrient solution gradually (over a few weeks) to a level of 975 P.P.M. at the end of the flowering period.

The pH levels of the nutrient solution should be maintained at 5.3 to 5.7 pH in the reservoir. When applied to the rockwool the pH will raise to the range of 6.0 to 6.5, which is the ideal level for plant growth throughout the cycle.

Temperatures need to be monitored in several different places to accurately determine what is happening in the growing environment.

Rockwool and air temperatures need to be in the 60 to 85 degree F range, with the lower temperature at night (dark cycle).

If carbon dioxide and light levels are high; temperatures can be increased to 85 degrees F during the day with a ten degree drop in temperature at night. These are guidelines and specifics will depend mainly on the type of crop. Temperatures for plant propagation are generally maintained in the 70 to 75 degree F range until rooted. Seedlings are very specific and should be treated accordingly.

Containers can be of almost any size or shape. A good container for loose rockwool would be one of the new root pruning pots, in the one gallon round or the three gallon square sizes. Rockwool slabs come wrapped in plastic and can be used as is.

After rooting the seedlings in a small cube, cut out a square of plastic the same size as the cube and place the seedling on that spot. Its roots will grow down into the slab. A slab 3 feet long will support 3 or 4 mature clones (plants started from cuttings).

Horticultural tools to assist you in adequately monitoring your hydroponic garden include a good pH meter (or paper strips), PPM gauge (E.C. meter or dissolved salts meter), light meter, microscope or a hand magnifying lens and a quality sprayer.

When applying insecticides and fungicides directly to the rockwool, cut down the rate to 15% – 20% because of the lack of buffer activity (no interaction with applied chemicals).

For hydroponic nutrients, a good hydroponic GROW and BLOOM formulation is recommended. Use these formulas at 1/2 strength for the propagation and seedling stages of growth and the full strength nutrient solution during the growth and bloom stages.

Click on the Rockwool link to see our recommended supplier.

(The foregoing is for educational purposes only. The author in no way endorses nor condones the cultivation of controlled substances. He does, however, condemn narrow-minded legislators and law enforcement agencies who continue to punish those who do. From his own experience, alchohol and tobacco use are inherently more harmful and dangerous than is the occasional recreational use of naturally-occuring psychoactive plants.)

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