Rockwool is a sterile growing medium made from spun, molten rock. Because rockwool is able to hold a lot of water and air, it is a popular medium in hydroponics. To avoid damping off, the fungus that attacks seedlings germinated in soil, many dirt gardeners choose to start their seeds in rockwool cubes. Rockwool is sold in blocks or cubes.
Add water to the shallow tray until it is half full. Dip the rockwool cubes into the water until the bottom half of the cubes is soaked. Do not allow the water to moisten the upper half of the cubes. Squeeze the cubes to remove excess water.
Place the seeds into the slots on top of the rockwood cubes. Use a toothpick or other small stick to gently push them to the bottom of the opening. Cover the seeds with perlite.
Place the rockwool cubes in a shallow tray and set it on a heat mat to the temperature required for your specific seeds.
Mist the cubes periodically to keep them moist. Do not allow the cubes to dry out.
Cut the cubes apart when the first sets of leaves have fully opened. Plant each cube in an individual pot containing potting soil.