Pioneering Hydroponics

Since 1976

How to Plant Seeds in Rockwool

Rockwool cubes are chosen by many growers for grow rooms, as they provide a great medium to plant seeds in. With very high density, Grodan cubes provide great structural integrity for plants. 

This ensures both increased and longer oxygen-holding abilities in addition to better insulation. Germination and rooting of plant cuttings need this increased insulation to improve the dispersion and wicking of nutrients and moisture.

Top Tips to Using Rockwool 

  • Algae prevention. Keep an eye out for algae growing on Rockwool cubes, as the constant moisture of the medium and the constant light make it a great medium for algae. Other areas of the hydroponic system can also be conducive for algae to grow, particularly in output and input pipes and nutrient tanks. Cover Rockwool with either shade covers or plastic sheeting to prevent light from reaching it.

  • Reusing Rockwool. Although Rockwool isn’t biodegradable, it can be reused multiple times. It breaks down slowly over a long period of time into smaller particles. But before reusing the medium, you need to keep in mind that you can’t sterilise it with hot water. You can either wait for Rockwool to completely dry off or rotate your crops. Keep in mind, if you decide to reuse it, that it might no longer be 100% if it has roots left inside.

  • pH balance. Rockwool contains limestone, which naturally creates a high pH level. Most plants need a more neutral pH and you can balance Rockwool cubes’ pH for optimum use in hydroponics. The medium offers a cost-effective way of growing higher yields, which makes many growers prefer it.


How to Plant Seeds in Rockwool

You’ll need:

Step #1: Balancing Rockwool’s pH Level

The first step in growing seeds with Rockwool is to bring down the cubes’ pH level. Make sure to never squeeze Rockwool, as this can decrease the airflow to the roots and cause root rot. 

Get a container with water and your pH meter so you know when you’ve reached the ideal pH. Plants usually love a pH level of around 5.5 and not the approximately 7.8 level that Rockwool cubes have.

Water typically has an alkaline level around 7.4, which you also need to take into account when balancing out the cubes’ pH. Measure the pH level of the water and add pH down until you achieve a level of 5.5-6. USe pH up if you happen to make the water too acidic.

The Rockwool cubes can now be hydrated and stabilised for about an hour before you take them out. Save the water for a later step.

Step #2: Planting the Seeds

Rockwool cubes tend to come with pre-made holes, however, if yours don’t, you can easily create a hole around a quarter-inch, or 0.75 cm, deep. Place only one or two seeds in each hole, making sure to push them down to the bottom.

Make sure that a piece of Rockwool covers the hole so that the seeds have a dark and moist environment to grow in.

Step #3: Sealing in Humidity and Heat

The X-Stream propagator provides the ideal environment for seed germination. The combination of humidity and heat retention of this spacious propagator, with space for up to 450 seeds or cuttings, it has an extremely accurate thermostatically-controlled heated based.

As plants grow, you can open the vents in the lid to decrease humidity and slowly harden off the plants.

Step #4: Transplanting 

Around two to three weeks after germination, plants are ready to be moved into a hydroponic system. Make sure that you transplant them right after the first roots start to poke out of the cubes. If you wait much longer, the roots will become entangled around the Rockwool.