Start with wood pot supports (E) for the potted plants. Draw a 5-1/2-in. circle centered on the pot support (E) boards. Standard six-inch pots will stand a bit proud with the 5-1/2-in. hole.

Plastic pots are cheap and retain moisture well, but will need wrapping with bubble wrap over winter to prevent plant roots from freezing. Topiary specimens look good in wooden half barrels or ...