How large is "quite large"? My Double Knock Outs are about 3-4 ft tall. I have no problem with that height, so I only prune a bit if one side grows a bit faster and the bush tends to look a bit lop-sided--just to even it out a bit. Otherwise, I let them do their own thing.

There is nothing wrong with pruning KO, but they don't really need it. Personally, I would never give it a drastic prune--think it might take quite a while for the bush to recover from that.

Kate

You can prune KOs for shape and size anytime you want to and it shouldn't be any problem. You can prune them down to 8 to 12 inches and let them grow back if you like. You really can't hurt them by pruning them.

However!

In your zone 5 now is probably not the best time to start hacking them back because winter is coming on sooner than we'd like to think. If they're way overgrown and you want to shape them a little that's OK but don't cut them way down now. If you want to do that wait until next spring to do it. As soon as you start to see the leaf buds fattening in the spring (think when the forsythia are blooming) is the time to do any kind of hard pruning in your zone. After that during the season you can prune them some for shape and size when ever you dead head them. They don't have to be dead headed but doing so may give you more frequent bloom and is the perfect opportunity for shaping.

In the 3rd year and every year afterwards I like to prune out 1/3 of the oldest canes. Like if you have 9 canes cut 3 of the oldest canes to ground level. New canes will replace those. This keeps the rose fresh with new flowering canes...

My neighbor cuts hers back severely whenever she gets irritated with cercospora fungus on the foliage--she did that around July 1 and they are blooming again now. Don't prune hard during the fall months. I am not sure whether it is already too late in the season in zone 5. I would follow seil's advice.

I notice around here that the regular Knock out grows faster than the double Knockout...More vigor...

What you guys/gals observe in your areas?

Its been cooler here since end of July I have a feeling
that may mean early winter weather this year... :(
I hope not though.....