The Grodan Hugo is the largest rockwool block; 6 x 6 x 6 inch Growing blocks with 1-1/2 inch hole for smaller cubes. These are used for growing real big plants. These cubes work best with a drip system. In a flood system the top half can dry out quickly and lead to salt buildup so rinse often is necessary. These blocks are individually wrapped in plastic on four sides to hold moisture and stop alga growth. The base of each block is grooved for drainage. Holes in the center for the 1.5 inch cube.