Gerten's, at least in the past, had a couple of bins for this purpose. They were located along the main fence just past the rose section (going from main bldg towards watergarden area)...may be different this year.

You can drop them off in the driveway up by the garage at 726 grandview avenue in roseville--the person that lives there has a plant sale at the end of may every year and is always willing to take people's used pots and multi-packs.