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growing cannabis seeds in rockwool

Growing cannabis seeds in rockwool
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.

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.

Growing cannabis seeds in rockwool
The marijuana seed needs to sit in the bottom of the hole in your rockwool cube. Drop seed in hold and very gently push down with a toothpick or wooden skewer. Be gentle!

Weed seeds in rockwool: how to grow marijuana seeds in rockwool cubes

It’s easy to learn how to grow marijuana seeds in rockwool cubes in a few simple steps. Here’s how you do it.

Growing marijuana seeds in rockwool cubes is an easy and effective way to propagate cannabis seeds. Rockwool is a reliable growing medium for pot seeds, and it’s suitable for all types of growing including hydroponics and coco coir.

In this post we’ll show you how to germinate a weed seed in a rockwool cube. We’ll discuss how to prepare the cubes, proper watering (watch that PH!), proper lighting, and how long your seed will take to sprout (usually within a few days).

How to germinate weed seeds in rockwool cubes:

  1. Soak rockwool cube in one pint of water that is PH’d down to 5.5 (no extensive soaking is required). Include 2ml Canna Rhizotonic per pint (or use a similar rooting product) to encourage root growth.
  2. Remove from water and shake gently to release excess water.
  3. Add marijuana seed to the hole in the rockwool and push to the bottom gently with a toothpick.
  4. Place under a light in a hum >Most rockwool cubes made for growing marijuana seeds come with a hole in them.

Preparing your rockwool cube is fairly easy. Most rockwool cubes made for seed propogation have tiny holes in them. If yours doesn’t, bore one out by gently twisting a wooden skewer about 3/4 down. You want your seed to sit in the bottom of the hole.

Your next step is to soak the cube in a light nutrient solution. I like to use Canna Rhizotonic to stimulate rooting, it’s worked well. I’ll also add a very light mix of Canna Coco A & B, the same nute line I will run once the seeds grow roots. If using tap water let it sit out 24 hours so the chlorine can dissipate (you can use an air stone to speed up this process).

Since rockwool cubes have a very high PH, you need to offset this by lowering the PH of your watering solution to offset it. This is the most critical part of growing pot seeds in rockwool.

What is the proper PH when watering rockwool cubes for cannabis seeds?

The proper PH for watering rockwool cubes when trying to germinate cannabis seeds is 5.5. Try to keep the PH of your nutrient solution as close to that as you can. Use PH down (or substitute lemon or lime juice) to lower the PH once you’ve added your nutrients.

If mixing a small amount of nutrient solution, use a dropper to apply one drop at a time. Mix gently and test with a PH meter. If you go too far, add a drop of PH up until you hit the 5.5 mark.

Soak your cube in the 5.5 PH nutrient solution for 15 minutes – overnight. Remove from the water, gently shake. Now you’re ready to add your seed.

Where should weed seed be placed in rockwool cube?

The marijuana seed needs to sit in the bottom of the hole in your rockwool cube. Drop seed in hold and very gently push down with a toothpick or wooden skewer. Be gentle!

How long does a marijuana seed take to grow in rockwool?

Marijuana seeds grown in rockwool cubes usually germinate within a few days. If they still haven’t germinated after a week under proper lighting, watering, and humidity conditions then your seed might be a dud.

Some growers use 2 seeds per rockwool cube, letting each sprout for a few days. They’ll then thin out the weaker seed by snipping it’s stem at the base of the cube (don’t pull out, it may damage the other seed’s roots). The benefit is that the grower is left with the strongest, most robust plants.This method is great if you have a lot of seeds, but for the average small home garden it might be a bit wasteful.

What type of lighting should be used?

Seeds like warm temperatures. Using a clear plastic humidity dome provides an optimal environment. I use a large CFL lighting fixture designed for growing seeds and clones. You can also use flourescent tube lights, LEDs, low wattage HID lighting, or even a sunny window. Avoid high-powered HID lighting such as 1000W Metal Halide or High Pressure Sodium lights, they are too intense for young seedlings.

Tips for germinating marijuana seeds in rockwool

Rockwool is very alkaline so you need to go 5.5 with the PH to balance it out. You’ll notice a few leaves emerge and will start to see roots poking out the sides. At this point your marijuana seed has sucessfully rooted in the rockwool cube. Monitor the rockwool for dryness, and water with additional PH 5.5 water, Rhizotonic, and a very light nutrient mix when needed.

Avoid compressing the rockwool cube after soaking in water in step 1. You want to keep the fibers loose to allow the roots to easily penetrate them.

Make sure to monitor your rooted weed seeds closely. Look for signs of nutrient deficiencies in the new leaves, which could be due to either too light nutrient mix or potentially nutrient lockout caused by improper PH. PH issues can be addressed by soaking the cube in 1 pint PH 5.5 water with light nutrients and Rhizotonic. Adding the plant to the permanent medium will allow you to introduce a properly PH balanced environment for the roots to expand outside of the rockwool cube.

When should a weed seedling be transplanted from a rockwool cube?

As your new marijuana plant sprouts from the rockwool cube it will begin growing roots. These roots will begin to poke out of the sides of the cube soon after the seedling emerges. At this point it is ready to transplant.

One advantage to growing weed seeds in rockwool is that they can be easily transplanted across most common growing mediums, such as soil, coco coir, and hydro methods such as Deep Water Culture (DWC). If you’re growing in coco you can learn how to transplant seeds in coco coir here.

What is rockwool?

Rockwool is made by melting a mixture of Chalk, rock, and sand at very high temperatures. Air is blown through the molten mixture similar to making cotton candy, resulting in a mineral wool with long fibers that are capable of holding a lot of liquid. This makes rockwool a great medium for germinating cannabis seeds and growing hydroponically.

Rockwool is a man-made substance and it doesn’t break down, so it’s not the best for the environment. Avoid breaking up the cubes and breathing in the small fibers, they’ll damage your lungs.


Growing weed seeds in rockwool cubes is an easy way to start your cannabis plants. Marijuana seeds typically germinate within a few days in rockwool cubes. It’s important to properly pH the nutrient solution to 5.5 to balance out the higher pH of the rockwool, this provides the slightly acidic conditions that marijuana thrives in.

My weed seeds rooted in rockwool – what next?

Once your weed seeds have rooted in your rockwool cube you’re ready to plant them permanently. Learn how to choose the best growing medium for your weed plants.

If you’re new to growing and are looking for an easy medium to grow in, consider using coco coir. It provides the quick, robust growth of hydroponics but while being somewhat forgiving.