Grodan Stonewool Gro-Blocks Delta 6.5 - W/ Hole - 4" x 4" x 2.5" - Seeds or cuttings are started in these larger Grodan Gro-Blocks and, once rooted, the blocks are transferred to a larger stonewool slab. Gro-Blocks can also be planted directly into containers for soil gardening. This is a strip of six 4" x 4" x 2.5" Gro-Blocks, each with pre-formed holes. Grodan stonewool is known for its quality, consistency and performance. Holes are perfect size for any 1.5 starter plug.