Milk jugs, coffee creamer containers (dh must have his hazelnut creamer), plastic bags from grocery store, empty 4" pots and six packs from annuals fill the bottom of my containers. One of the worst filling I ever used was packing popcorn. When I decided to empty it out to replenish with fresh potting medium and forgot the Styrofoam popcorn was at the bottom I think it took me a year to finally pick it all out of the garden.
