Autumn or spring. It can also depend on what you're planting - bare-rooted or container-grown. The former are more delicate in terms of root development, the latter more sturdy. I wouldn't plant bare-rooted in autumn. Wait for spring. Container-grown can go in in either season.
Duuno how it is where you are, but it's pretty much already winter here. The autumn definition usually means when the leaves start to fall. Ours vamoosed weeks ago. You could try a container-grown specimen and give it some cover.