How To Germinate Cannabis Seeds In Rockwool Cubes

You can germinate cannabis seeds in a variety of different ways, and everyone has their own favourite. For hydroponic growing, the options are more limited than with soil, and often slightly more challenging.

Germinating seeds in Rockwool is one of the easiest and most effective ways to get a hydroponic grow started. In this article, we'll cover how to go about doing this.

Why germinate cannabis seeds in Rockwool cubes?

If your end goal is to grow cannabis plants in a hydroponic setup, then you’ll want to germinate your cannabis seeds in some inert media that’s suitable for hydro—not in soil. For most people, Rockwool (aka stone wool or mineral wool) is the perfect medium.

This material is highly absorbent, which makes it perfect for hydroponic growing, and thanks to the variety of hydro-specific Rockwool products available, it’s possible to transplant seedlings without posing any risk to their roots. The reason for this is that you can germinate seeds in a Rockwool cube, which can then simply be slotted into an allotted space in a larger block of Rockwool. In this way, the roots can continue to grow without disturbance.

Rockwool is a popular material for hydroponic grows as it’s versatile, easy to use, and reliable. But there are two main downsides to it.

First, the production process is not very environmentally friendly, nor is the disposal process. Rockwool does not biodegrade, so it will hang around in the environment for a very long time. Second, the natural pH for Rockwool is around 7.0, which is too alkaline for cannabis plants, especially in hydro, and will inhibit nutrient uptake. To get around this, Rockwool should be soaked in a solution with a pH of 4.5 for around 15 seconds before use. If this is a problem, you can start in Rockwool and then transplant into another hydroponic medium.

The main reason to germinate seeds in Rockwool is that it makes transplanting very easy, and the material is perfect for hydroponic grows.

Note: You can also transplant seedlings started in Rockwool into soil. However, there is little reason to do this, and if you intend to grow your cannabis plants in soil, you might as well start them in soil. Nevertheless, the following guide will also work if you intend to grow in soil.

How to germinate cannabis seeds in Rockwool cubes

Germinating weed seeds directly in Rockwool is pretty straightforward, but you do need to be careful. Plants that grow as part of a hydro setup require a more carefully managed environment, as fluctuations can affect them more rapidly and substantially than in a soil grow.


• Rockwool germination blocks
• Rooting stimulant
• Propagation chamber/humidity dome
• Heat source
• Tweezers
• pH-down solution (optional)
• Skewer (optional)


1. If your Rockwool germination cubes aren’t already adjusted to the correct pH, you’ll need to adjust them yourself. To do this, soak them in a pH solution of around 4.5–5.0 for a mere 15 seconds. This should bring the pH of the block down to around 5.5.

2. Next, soak your cubes in a cannabis-suitable rooting stimulant and nutrient solution. Ensure that this solution has a pH of 5.5, and go very light on the nutrients. The rooting solution will encourage your plants to continue growing once the taproot emerges.

3. Arrange your Rockwool blocks in a propagation chamber or tray.

4. Using clean tweezers, drop your seeds into the holes in the germination cubes. Gently push them to the bottom. If your cubes don’t already have holes, then use a skewer or something similar to make a hole that goes ¾ of the way through the centre of each block.

5. Place the lid or dome over your propagation chamber and place it somewhere warm where the seeds can germinate. Do your best to maintain a temperature of 22ºC to 25ºC and a relative humidity of 70–90%. Placing a heat mat beneath the tray, or using an ambient heater, will work well to maintain these temperatures. Even if you use a heat mat, the ambient temperature should also be warm.

6. Wait 1–21 days for seeds to germinate. All going well, you should see them germinate within a few days.

7. Although you don’t need light to germinate seeds, the seedlings will want light as soon as they emerge, so having something suitable (such as CFL or LED lights) already prepared is advisable.

Hints and tips for germinating cannabis seeds in Rockwool cubes

Much of the following information is embedded in the instructions above. However, here we will give a clear list of tips and considerations that will help you achieve success when germinating cannabis seeds in Rockwool cubes:

• Ensure the Rockwool has a pH of 5.5—adjust if necessary.

• Seeds should be positioned at the bottom of the holes.

• The ideal temperature is 22–25ºC.

• The ideal RH is 70–90%.

• Rockwool cubes should be soaked in a rooting stimulant, as it is an inert material and provides no nutrients unless it is amended.

• Handle cannabis seeds carefully and in a hygienic manner—this helps to keep them from getting infected.

• Avoid compressing Rockwool blocks—there must be space between the fibres so that the roots can easily penetrate the cube.

• Once seeds emerge, keep a careful eye on the seedling’s health, as nutrient-related issues are common with this growing method—and you should respond quickly to avoid lasting damage.

When to transplant Rockwool seedlings

Rockwool is a versatile and effective medium for germinating cannabis seeds. However, it won’t be long until you have to transplant the seedlings to some larger home. You’ll know it’s time when the seedling has two true sets of leaves and/or the roots are emerging from the bottom of the Rockwool cube (which is easy to see—another bonus of using this material).

You’ll then transplant into another inert medium, and this may well be more Rockwool. If not, it might be coco coir, clay pebbles, a vermiculite/perlite mix, or something else. As mentioned, it could even be soil.

Growing cannabis plants in inert media, such as Rockwool, has its own challenges and requires more care and attention compared to soil grows. However, the results can be rewarding, and the process is highly interesting to those who have the time and energy to learn it. Good luck!

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