For marijuana growers wanting total control over the nutrient elements and compounds their plants absorb through their root zone, rockwool has long been the reliable choice.
Recent breakthroughs in hydroponics nutrients and rockwool technology make it useful to provide an update on the latest innovations in rockwool marijuana growing.
Let’s first understand that rockwool is made from basalt, a volcanic rock composed mostly of silica, with small percentages of titanium, iron and other elements.
Basalt starts out as lava; rockwool manufacturers turn it back into lava so they can spin it at high speeds to create a porous, wool-like material that has the texture of cotton candy.
Horticultural rockwool is an extremely versatile root-zone material because it comes either in slabs, cubes or loose chunks. Each of these rockwool forms has specific, ideal uses. The smallest rockwool cubes are the go-to choice for professional marijuana cloning or seed germination.
Larger rockwool cubes that are 2×2-inch or 4×4-inch squares are useful for cloning, early grow phase, and for platform drip irrigation systems in which the cubes sit on top of rockwool slabs, which are long, thick cuts of rockwool configured for use in ebb-and-flow systems.
Rockwool is 100 percent sterile and inert, which means none of its elements or materials transfer into your cannabis plants. This inertness is one reason rockwool is so useful for hydroponics marijuana growers who want total feed control.
So unlike soil, soilless mix and coco coir, which may transfer some on-board nutrient elements into plants, thus complicating control over what your plants imbibe, rockwool is an inert blank slate, a neutral carrier material that holds water, the nutrients you put in the water, and oxygen.
When you use rockwool in your root zone, along with reverse osmosis water and high-quality hydroponics nutrients, your marijuana plants get exactly what you want them to through their roots, and nothing else.
Understanding The Rockwool Presoak
Rockwool is more difficult to prepare than soil, soilless mix and coco coir, which is one reason it isn’t as popular today as it was 30 years ago.
The difficulty comes from the fact that you can’t just take rockwool out of its package and use it immediately. It has an initially wonky internal pH and must be presoaked with solution that has a pH of 5.5–5.7.
Rockwool manufacturers tell growers to do the presoak with reverse osmosis water adjusted to pH 5.5, but if you’ve ever tried to get accurate pH readings and adjustments in zero-parts-per-million reverse osmosis water, you know it’s not easy to achieve pH consistency.
The advent of pH Perfect hydroponics base nutrients makes rockwool pretreatment much easier. My presoak is comprised of 1–2mls of Connoisseur grow-phase hydroponics base nutrients per liter of reverse osmosis water. The base nutrients automatically adjust water to 5.6–5.7 pH. But be aware that extremely low concentrations of pH Perfect base nutrients — or if you’re using non-reverse osmosis water — can sometimes partially decrease the many benefits that pH Perfect base nutrients provide when you dose them at or near full strength. High-strength pH Perfect balances pH to 5.6–5.7, no matter how bad your originating water is.
Now, here’s an important caveat that only applies to presoaking rockwool cubes for seed germination or for placing fresh cuttings: Don’t use nutrients in your presoak water. Even 1ml per liter of hydroponics nutrients could be too intense for seeds or fresh cuttings.
Because zero-parts-per-million water is tricky to pH adjust, I use beneficial microbe supplements, such as Voodoo Juice, Piranha and Tarantula, added to my presoak water for rockwool that will host new cuttings or marijuana seeds that have enough PPM for accurate pH metering. Along with these three supplements that guard roots and help them function better, I often also add House & Garden Roots Excelurator. This combination provides enough PPM so you can successfully set your presoak water pH to 5.5.
In all situations, if I find my rockwool presoak water is higher than 5.6–5.7 pH, I use pH-Down to lower it.
Rockwool Presoaking Drainage & Moisture Logistics
Soaking rockwool cubes and slabs is easier than soaking loose rockwool.
With cubes and slabs, you put them into horticultural trays or your ebb-and-flow system and either pour the presoak onto them or run it through your ebb-and-flow irrigation.
You want the material to be thoroughly soaking wet so that water is running out the bottom of the rockwool.
When using loose rockwool (which is sold as large shredded pieces), I first place the material into a five-gallon bucket and pour in the presoak. After the rockwool is drenched, I put it into a five-gallon bucket that has holes in the bottom, and place it in the bathtub so water can drain and not create a mess.
In all cases, before you put seeds, clones or plants into presoaked rockwool, you want it to dry out from its pre-treatment phase so it’s moderately moist but not soaking wet.
Attention to the moistness of rockwool is crucial after presoaking and during your marijuana growing season. Overwatering is one of the most common problems for novice rockwool users. Although new types of high-quality horticultural rockwool are naturally porous and aerated so they have great drainage characteristics, if you water too often and too much, or if your grow-op drainage system isn’t set up for proper drainage, then your rockwool may become waterlogged.
When your irrigation system is on, or if you hand-water, always irrigate from above. Rockwool isn’t good at absorbing water from bottom irrigation to transfer to the top of the root zone.
The irrigation rate, duration, and amount of water should be enough to send H2O out the bottom of the rockwool, but not a lot. If a constant stream is pouring from your rockwool while you irrigate, you’re using too much water.
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 Rockwool Management Technology
Professional marijuana growers use moisture meters that measure root-zone moisture content far more accurately than can be done merely by looking at the material.
These growers also use even more sophisticated tools such as a sensor system made by Grodan, the world leader in horticultural rockwool and rockwool management technology.
Yes, there are several companies making horticultural rockwool, but Grodan has them beat.
Grodan is based in the Netherlands and has in the past few years upped its game when it comes to rockwool. The company’s products are researched and tested in the Dutch nation’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, with superior customer and technical support. I’ve always been very pleased with the level of detailed tech support I get from Grodan when I’ve asked for help choosing and using the right rockwool product.
Grodan scientists and product support advisers are especially keen on helping you enjoy one of the main benefits of rockwool: The ability to deliver nutrients into plants more efficiently so you save money on water and nutrients.
Grodan rockwool is unique and superior among solid root-zone media in its ability to optimally release, store and transfer nutrient elements into your plants. The practical consequence is you should slightly underdose your nutrients. For example, if the manufacturer calls for 4mls per liter, go with 2–3mls and see how it works.
Another benefit of rockwool is that it’s much easier to flush than coco coir, soil or soilless mix. This is a great advantage because instead of losing 4–8 days at the end of bloom phase while you flush, you only need 2–3 days flushing, as long as you use a hydroponics marijuana flusher like Flawless Finish. The rapidity with which nutrients salts clear out of rockwool is also great if you need to flush during grow or bloom phase because of overfeeding.
In the world of hydroponics marijuana growing, the purest methods are equally deep water culture and aeroponics, with rockwool in second place. If you use Grodan products, you get a clean, safe, inert, aerated grow media engineered for plant roots. If you’re looking for a beneficial change in your marijuana growing materials, consider using rockwool next season.