An effective item to put at the bottom of a planter for drainage is broken pieces of pot. Utilize unwanted plant pots or chipped crockery - simply smash them up into small to medium-sized pieces (in a bag with goggles to avoid chips flying off.) Adding a layer of broken pieces of pot can prevent compost loss out of the drainage slits. It will also stop the drainage slits from becoming blocked. We supply slate stones ( MFA1016/5) if you don't have any old pots and you will need 1 x 5kg for a 40 x 40 planter.