growing cannabis in rockwool cubes

Rockwool For Hydroponic Marijuana Growing

Rockwool is a great medium for growing marijuana. You can take your strain of choice through the whole cannabis life cycle with rockwool. Growers of all levels, using a variety of hydroponic setups are harvesting heavier yields a whole lot sooner. It’s about time you tried rockwool too.


Well, originally this wonderful medium was exclusively used as insulation. Mineral wool, stone wool, or rockwool, it’s all the same. Interestingly, this material just so happened to have all the properties essential to a thriving hydroponic root zone by accident, rather than by design. Of course, modern horticulture-grade rockwool substrates have been refined and packaged specifically for cannabis cultivation. Nevertheless, rockwool is still a cotton candy-like fibre spun from basalt rock. Blocks, cubes, and slabs of rockwool come in a variety of shapes and sizes. These products are staples of any decent hydroponics store.


Before you can get growing in rockwool, you must first adjust the pH. Rockwool is naturally alkaline. Too alkaline for cannabis. It’s best to soak your cube or block in 5.8–6.3pH water for 24 hours in advance. Going forward, you will need to invest in either a pH pen or pH-perfect nutrients to maintain optimal pH levels.


Clones and seedlings will readily root in rockwool cubes. You can get a tray of 12–24 small rooting cubes cut to size for about €5. Tiny rockwool blocks are great for getting started, especially for those new to hydroponics. Seedlings and clones can tell you it’s time to transplant when roots start to poke out. All you really need to watch out for is algae. Keep the cubes covered with some plastic if they start turning green. Without light, the algae will die.


Transplanting couldn’t be simpler with rockwool. Simply cut a hole to size in the slab or larger block you plan to use. Then, sit the smaller block in place and let the roots do the rest. This is really handy if you are tight on space and/or working with large numbers of seedlings or clones. If a plant should ever get root bound, you can always get a bigger block of rockwool. Heavier mediums like soil can be awkward, and you risk doing more harm than good stressing the plants with transplant shock.


Rockwool is commonly associated with advanced hydroponics like flood and drain, NFT, and DWC. However, it’s a pretty versatile substrate and works well with top feeding from drip irrigation to hand-watering. Rockwool slabs are preferred by high-volume cultivators, but you can also grow a monster plant in an oversized 50l+ rockwool block.


Brand name, cannabis-specific hydroponic nutrients, ideally formulated in a pH perfect solution are the smart choice. As rockwool is inert, the grower is in complete control of plant nutrition. This can be a double-edged sword. You can potentially pump-up bud production to the max with a high-quality chemical fertiliser regime. At the same time, it’s all too easy to over-fertilise. Feeding must be controlled. Unfortunately, most organic nutrients are too thick and often can become difficult to flush from the medium. Organic fertilisers are also notorious for causing blockages in water lines.


Take care to maintain optimal pH with each and every watering. The biggest downside of using rockwool is also its unique selling point: total control over nutrients. Unlike soil, this medium is sterile and very unforgiving. Being in charge of plant nutrition is a full-time job for the duration of the grow. Soil acts more like a buffer and doesn’t require the same kind of precision.

Organic growers will have the hardest time with rockwool. But to be honest, if you really want to grow organic cannabis, you should just use soil.

Rockwool is not really a natural product, so it won’t biodegrade. When you’re done with rockwool, it’s garbage. Old soil you can dispose of in the back garden or reuse. We can’t recommend either for this hydroponic substrate. Reused rockwool has a tendency to have a shifting pH that requires extra monitoring to keep in check. Play it safe and buy some fresh, clean media for the next crop.

Hydroponics is all about getting bigger, more potent buds, faster than a typical soil grow. We take a closer look at cannabis cultivation with rockwool.