Looking for a sterile, easy-to-use root zone media your marijuana roots will love in indoor grow ops? Then you need to look at rockwool.

Rockwool is one of the most useful hydroponics growing media ever invented. It’s made from basalt, a volcanic rock composed mostly of silica, and also contains smaller amounts of titanium, iron and other elements. Basalt starts out as lava, then hardens into rock.

To make rockwool (which is sometimes called “stonewool”), manufacturers temporarily turn basalt back into lava and spin it at high speeds to create an end-product of porous, wool-like material with the texture of cotton candy.

The only rockwool I recommend is made by the Grodan horticultural products company, which is based in The Netherlands. Grodan is by far the world leader in quality horticultural rockwool.

Rockwool is a manufactured commodity, and like cheese or bread, it can be fabricated into a variety of shapes and sizes. Grodan rockwool comes in slabs, cubes, chunks and loose material.

Be aware that you can get confused by Grodan product terminology because there are two types of Grodan cubes: one type are various sizes of cubes used to germinate seedlings and/or root clones, as well as to provide a platform for plants sitting on rockwool slabs. The other types are little cubes, a quarter inch square, used to fill pots in root zones.

Explained another way, the very smallest rockwool cubes are used as loose root zone material. The next largest cube sizes are used for professional marijuana cloning or seed germination, cloning, early grow phase, and for platform drip irrigation systems in which the cubes sit on top of rockwool slabs, which are long, thick pieces of rockwool for use in ebb-and-flow systems.

You see Grodan rockwool systems in the largest commercial greenhouses and glasshouses in The Netherlands where a significant portion of Europe’s tomatoes, peppers, lettuce and other vegetables are grown. You also see rockwool in plenty of cannabis gardens worldwide.

Grodan rockwool comes in specialized types including Grotop Master, Grotop Master dry, and Grotop Expert, all of which have slightly different properties and horticultural uses. Grotop Master Dry, for example, maintains a slightly drier root zone and is used by growers in high-humidity areas. Grotop Expert is designed for rapid root growth and development.

Rockwool = Total Control Over Hydroponics Nutrients & PH

Why is rockwool so useful for marijuana growers? It gives you total control over nutrient elements and other compounds plants absorb through their root zone. Rockwool is 100% sterile and inert—it has no nutritional component unless the grower adds nutrition to the media.

That makes it an inert blank slate, a neutral carrier material that holds water, nutrients put in the water, and oxygen. Cannabis roots must have easy access to water, oxygen and nutrients, and rockwool provides this very efficiently, especially compared to soil and coco coir.

When growers use rockwool along with reverse osmosis water and high-quality hydroponics nutrients, plants get exactly what they need, and nothing else. This gives the grower power to control exactly what plants feed on and how much of it. It also allows precision control of root zone pH, which is a crucial factor in roots’ nutrients absorption. These benefits increase growth rate, harvest weight and potency.

Contrast this with soil and coco coir, which contain on-board nutrient elements that complicate and interfere with your feed program. Coco coir and soil can play havoc with root zone pH as well.

Pre-Soak Rockwool Before Using

Growers must adjust rockwool pH before they put plants in it. This is done via a pre-soaking process. You can’t just take rockwool out of its package and use it immediately. In its native state, it has a glitchy pH and must be presoaked with solution that has a pH of 5.5. That’s why you do a presoak with reverse osmosis water adjusted to pH 5.5. The grower has to have pH adjusting solution and a reliable pH meter such as the Bluelab series of meters to make this happen.

When I pre-soak rockwool, I place it in a clean bathtub filled with reverse osmosis water adjusted to 5.5 and allow it to soak overnight. Then I drain the water and place the rockwool on racks over a drain so excess water can leak out. I capture the excess water and measure its pH.  If the pH of the excess water is 5.5-6.2, I’m satisfied that the pre-soak has worked well enough. If it’s not within that range, I contact Grodan and ask for their advice.

Some growers pre-soak rockwool cubes and slabs by putting them into horticultural trays and using an ebb-and-flow hydroponics to circulate treatment water. When using loose rockwool (which is sold as shredded loose pieces), growers place the material into a large bucket and pour in the pre-soak solution. After the rockwool is drenched and soaked for several hours, the grower transfers it to a bucket that has holes in the bottom, and place that bucket in a place where excess water can drain and not create a mess.

Before I put seeds, clones or older plants into presoaked rockwool, I dry it out a little so it’s moderately moist but not soaking wet. Overwatering is one of the most common problems for novice rockwool users. Although some types of high-quality horticultural rockwool are made porous and aerated so they have unusually efficient drainage, if you water too often and too much, or if your grow-op drainage system isn’t set up for proper drainage, the rockwool may become waterlogged.

On the other hand, growers with extremely low humidity in their grow ops might find that Grodan rockwool dries out too fast, so you have to send a tremendous amount of nutrients water through your root zone. Grodan has professional, timely technical support. I suggest you contact them to tell them what type of garden you run and your environmental factors (especially humidity), before you purchase specific Grodan products.

Growers must also note that rockwool should be watered from above. Rockwool isn’t good at wicking water from bottom irrigation to transfer up into the  root zone.

In the best rockwool marijuana growing systems I’ve seen, the grower is using drip irrigation from a reservoir with a pump timer that delivers very small amounts of nutrients water on an hourly basis.

Professional growers use sophisticated rockwool sensor system made by Grodan, the world leader in horticultural rockwool and rockwool management technology. Grodan has had an edge in developing horticultural rockwool because it has been able to test its products in Holland’s thriving indoor hydroponics agriculture industry.

This has helped Grodan develop and improve many types of rockwool to suit a wide variety of grow conditions. The company provides excellent technical support. I’ve always been very pleased with detailed tech support I get from Grodan when I’ve asked for help choosing and using rockwool products.

Nutrients Dosing in Rockwool

Because Grodan rockwool is so efficient in its ability to optimally release, store and transfer water, oxygen and nutrient elements into plants, growers save money on nutrients by under-dosing. For example, if the nutrients manufacturer calls for 4ml per litre, a grower would use 3ml per litre or even less, depending on how well the plants are handling the nutrients load.

Rockwool is much easier to flush than coco coir, soil or soilless mix, Flushing is useful because it clears plants of nutrients elements, producing cleaner buds that burn and taste better.

The rapidity with which nutrients salts can be totally purged out of rockwool is a benefit at the end of bloom phase, and any time during the growing season if the root zone has been overfertilized.

The main thing to remember is that in the world of hydroponics marijuana growing, one of the purest, most productive gardening methods is to use rockwool and synthetic hydroponics nutrients. Only deep water culture and aeroponics provide better nutrients control and more oxygenation in the root zone.

Please note: I don’t recommend using organic nutrients in rockwool, as organics are likely to clog the medium and foul it, creating habitat for harmful fungus gnats and root rot.

Also note: a two cubic-foot bag of Grodan cubes or chunks has instructions printed on it claiming that one bag will fill 23 1-gallon pots. It won’t. I can only fill between 12-14 gallons of pot space total from two cubic feet of cubes or chunks.

For growers who’ve been using soil, coco coir, lava rocks, or soilless mix and have been experiencing problems with nutrients and/or root zone pH, Grodan rockwool is a great alternative. For anyone with more questions about whether rockwool is right for their garden, contact Grodan directly.