|I have a 3 yr. old knockout rose. I'm sure it could use some pruning, but I don't know when to do it. It is now almost 6ft. tall.
|Wow! Six feet tall? You have a happy rose, don't you? The best time to do radical pruning is in the late winter or early spring, just as new growth is beginning. You can trim it periodically through the summer to keep it looking neat and compact. With such an overgrown rose I think I'd do some light pruning all summer long and wait until late winter or early spring to whack it back to a more manageable size. If you cut it back severely now it will recover, but it won't flower well for a few months.
Best wishes with your garden!