With these reusable plastic trays and accessories you can easily customize your own seed-starting setup. The trays are durable, watertight plastic and measure 11" x 21". 12 inserts with 6 cell trays for 72 individual cells are available to fit the planting tray. A clear plastic greenhouse cover which efficiently maintains warmth and humidity for healthier seedlings. Greenhouse cover securely fits the 11"x 21" tray.