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Canadian Garden Insider says.....
These square growing pots have a better fit when using flood and drain tables. Fill them with soilless mix, soil, rockwool, coco fiber, perlite, vermiculite, or another medium that you like to use. These pots are of commercial greenhouse quality, and are made of durable and recyclable black polypropylene. Drainage is at the base of the pot, and the moulded rim is a convenient handle for moving your plants.