45 x GRODAN 40/40 ROCKWOOL CUBE The MM40, Grodan 40mm Cube, is designed for propagation. It is suitable for both seed raising and cuttings. It is made of Rockwool (spun basalt) and is a perfectly sterile medium. The cubes should be washed with warm pH adjusted water set at 5.5 before use and should be kept moist but not saturated once seeds/cuttings have been planted. Over watering can inhibit root development as well as making the plant more susceptible to root rot.