Grodan Rockwool Cubes 4 x 4 x 2.5 are cubes of stonewool (rockwool) that have a grooved base. Promoting drainage to avoid root rot. Grodan Rockwool is pH neutral. Making it easy for you to maintain the ideal hydroponics pH value of 5.5 – 6.5. Each gro-block is wrapped in white plastic, For shape stability. Rockwool Cubes have evenly distributed air pores that encourage your plants root growth throughout the entire block.