If you’re looking for a soilless substrate for seed starting, stem rooting or hydroponics, consider using rockwool growing medium. This wool-like material is made by melting basaltic rock and spinning it into fine fibers. Rockwool for plants is then formed into easy-to-use cubes and blocks. But is rockwool safe to use for the production of food? Formed from natural materials, Rockwool contains no harmful chemicals. It's safe to use as a rooting medium and substrate material for plants. Due its physical structure, rockwool drains excess water quickly. Yet, it retains small amounts of water near the bottom of the cube.