Packing Peanuts! They make a great filler for containers, raising the soil level, improving drainage and lightening the vessel. And they can be re-used year after year.
Mint Self Storage has green (corn-starch) peanuts which will not work as they will dissolve in water, but we have added a Polystyrene loose fill peanut in a brand new Medium box for gardening season. On sale now $7.99