Grodan RWDM56 Delta 5.6 gro-blocks are the perfect size for you to place a 1.5-inch round starter plug, macro-plug, or mini-block. When seedling roots have grown 1-inch long from the bottom of your starter plug, simply transplant the plug into this 3 x 3 x 4-inch block. The Delta 5.6 block can then be placed on top of a grow slab to provide for a larger growing volume if desired. This is the most uniform block available on the market with its compressed stonewool fibers and improved grooved base to promote superior drainage. Gro-blocks are sold in strips of eight and labeled with instructions.