outdoor plants (trees?) in moderate climates

I'm finally getting close to setting up an outdoor enclosure for my chameleons and I'm wondering what plants to use.
The standard chameleons plants aren't preferable since they don't survive the Maryland winter. I'm really looking for plants or trees that can stay in the enclosure year round even though the chameleons will of course spend the winters inside.

Any thoughts / experiences?

thanks!
Suzanne