In your climate, I'd wait until late winter to prune. When you see new growth emerging (buds plumping) or about the same time the forsythia is blooming.

And you might want to check the other recent thread on making a big mistake :-) SAC does not play well with others - it is a big, rampant and aggressive vine and will rather easily overwhelm the others, given enough time.

Trimming back a SAC won't phase it a bit. Mine is trimmed back in late December every year and it hasn't yet phased it in the subsequent growing season one iota.

I prune my SAC back in April to somewhere between 18-30 inches high. I don't cut her back earlier because it can still be pretty cold here and the ground is too mucky. I don't cut her back any lower simply because it leaves some of her still on her support and I don't have to do anything to get her going. When I cut her back there is sometimes some substantial growth on her already. Believe me it is not a problem cutting here back. If she got any bigger she would topple the fence line. I also chop the heck out of her whenever I want space (or more sunlight) for something else. SAC does not care! Since your zone 4 has a shorter season than even me, the only thing you need to worry about is frost in the fall. Picture was take 9/19/06. When are you likely to get your first hard frost this fall?

{{gwi:264885}}