Grodan RWDM65 Delta 6.5 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 4 x 4 x 2.5-inch block. The Delta 6.5 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 six and labeled with instructions.