Rockwool is a widely used soilless media for germinating seeds. It consists of spun volcanic rock, which makes it look very much like fiberglass. The Rockwool slabs are cubes measuring 4″ x 4″ x 1″, or 10cm x 10cm x 2.5cm.
Rockwool holds onto more nutrients than most soilless media. The added support of the cube shape makes it easier to handle and prevents root damage when transplanting seedlings into their new home. Rockwool is generally pH neutral, which helps prevent a nutrient lockout. Rockwool also has good capillary action, which means it wicks moisture to the roots.
Rockwool cubes come in a variety of different pH levels (which can affect your growth rates and your nutrient uptake), and also with or without starter fertilizer inside the cube. Make sure the type you purchase is meant for germination purposes.
Once you’ve got your Rockwool, soak it in water for about an hour. While that’s soaking, fill some tray or container (or a seed starter kit!) with your desired growing medium. I like to use Vermiculite mixed with perlite and worm castings. Make sure the medium is moist but not sopping wet.
When you’re ready to plant, layout your Rockwool cube on top of the medium surface. Make sure to leave room for the root growth (leave at least 1″ or 2.5cm around all sides).
Sow two or three seeds per cube, and cover with a thin layer of growing medium. Spray the cubes with water until they’re nice and moist.
If you want to speed up the germination process, put your cubes under a light inside of a humidity dome or in plastic wrap with tin foil on top. This will ensure that your seedlings are nice and warm, which is optimal for growth rates.
After about 3 days you should have some tiny little seedlings! At this point remove the humidity dome or plastic wrap, and make sure to move them under your lights.
After about 10 days you can transplant your seedlings into their new homes. Be careful when handling the cube, as it’s likely glued in multiple spots (these are great for holding rooting in place, but not so much when moving plants). Leave around 1″ to 2.5cm of space between seedlings for root growth.
Plant Rockwool cubes
If you want to transplant your newly germinated seedlings without damaging them, try using a pencil or a dowel rod. Push it through the cube at an angle so the tip sticks out the top of the cube. Then be very careful as you lift it straight up and gently place it down in its new home.
The ideal temperature range for germination is about 70-80 degrees Fahrenheit (20 to 27 degrees Celsius). If you find your seeds are taking too long, or not germinating at all it may be due to heat. Try creating a heating mat using an aquarium heater and warmer water.
If there’s too much heat in the Rockwool cubes, your seeds will not germinate. Try lowering the temperature by moving your lights further away, or getting some airflow. If that doesn’t work you may need to get rid of your heating mat!
If you’re putting 3 or more seeds in each cube, you’ll see two seedlings close together after about 10 days. At this point remove all but the strongest seedling, and wait about a week to transplant the second one. You may also like to read this article germinating seeds in paper towel in House I love.
