Hydroton grow rocks are a special kiln-fired, lightweight aggregate that is super-heated and formed into pebbles 8-16 mm in size. The ceramic shell and inner pore structure retains moisture and releases it back to your plant’s roots as needed. Grow rocks will not compact, ensuring excellent aeration for the life of your plant. It is reusable and lasts indefinitely, making it a great value. Grow rocks are excellent in net pots for ebb & flow and drip systems. Measures 16.8-inch length by 12.7-inch width by 3.3-inch height.