I live in zone 4. I have been given two topiary forms about 5 feet tall. I need to know, for outside on a deck with lots of sun, what ivy's or plants would be best. The finished topiaries would be quite tall, the planter pots quite large.

I call yew & box the Big 2. Both very popular plants as topiary. 3rd biggie is holly. I've also seen juniper and other common conifers. If you want flowers, marguerite daisy, lantana seem to work well as standards. Good luck!

I have seen the following plants such as;- Euonymus, Camella, Myrtle, Rosemary, Boxwood, Rose, Hemlock, and Yew being used. Hope a few of the suggested plants work out for your area.
Happy Growing!

Supannee