|Should I prune back my Knock Out Rose bushes after being damaged by 20 degree weather? If so, how much?|
|I would delay pruning to wait and see what happens. It may take a month after the freeze for them to begin to grow again. At that time you will be better able to determine how much to prune, if any.
Once you are certain the cold weather is truly over, you could check for dead vs live wood. To do that begin at the tip of each cane. Live wood will be somewhat supple, plump and firm and consistently colored, with a layer of green just inside the bark; the buds will be plump and firm. Dead wood will be off color brown or gray, and possibly shriveled or brittle. There will be no green inside the bark. Dead tips can be pruned off as they will not regrow. The reason to wait until you are sure the cold is done is that they still provide a little protection to the lower part of the plant.
Browned foliage can be gently removed or you can wait for it to drop off naturally. If the foliage has some green portion left, leave it on the plant to help provide energy to the plant.
The new growth will probably come from dormant buds or buds lower on the canes. Once you can see that new growth starting vigorously, trim the plant back. Remove any badly damaged portions and prune a little more if needed to restore a somewhat symmetrical look.
Try to be patient and not worry too much. The plants should recover just fine and bloom for you this summer -- these roses are able to withstand winter damage killing them back to the ground and will still bloom.