Grodan Stonewool Expert Gro-Slab - 15/75 - 6 x 3 x 36 inches. Growing slabs provide the final stage for growing a spectacularly large plant. Typically, seedlings are first nurtured in grow blocks. The blocks are then placed onto slabs, and the grow slabs special structure provides a medium that allows uniform water and nutrient distribution which enables plants to root rapidly. These slabs come in clearly marked UV resistant sleeves for comfortable handling and algae prevention. Because they are so convenient Grodan rockwool slabs are by far the most popular for the hydroponic hobbyist market.