Cement In Plastic Planter . Image and tutorial by 46spruce. Apply a sealer to the concrete to reduce alkalinity. Round Cylinder Concrete Planter Concrete Planters Site Furnishings from www.godawn.com Tutorial steps for transforming a plastic plant pot into a durable cement planter step 1. This old green pot was a perfect size but i really didn’t like the color. Plastic planters can come in an endless number of sizes and shapes.