Stonewool is recognized the world over as the ideal hydroponics growing substrate. Seeds or cuttings are started in these Grodan Gro-Blocks and, once rooted, the blocks are transferred to a larger stonewool slab. Gro-Blocks can be planted directly into containers for soil gardening as well. This is a strip of eight 3"x3" x2.5" Gro-Blocks, each with pre-formed holes. Grodan stonewool is known for its quality, consistency and performance. The Delta 4 is also available without a preformed hole.