When a plant mentions a zone they will grow in, the number is the farthest zone north they can be planted safely. Since you are in zone 5 and astilbes will grow as far north as zone 4 you are safe. I am in zone 6 (Blue Ridge Mtns of southcentral PA) and they grow beautifully for me. I also grew astibles in zone 7 (South Carolina)

Enjoy
Karen

Karen is absolutely right, you'll do just fine. But it seems I never see this mentioned about them - they love water. I have lived in 2 houses where they were planted under or near large trees and the only way to keep them looking nice was to water them practically every other day. Good luck! cin