You may be surprised to learn that knockout roses are an actual variety of roses and require careful attention though little to no pruning. In fact, the Knockout rose bush is one of the hottest selling roses in North America. They were invented by breeder Bill Radler and have been taking the gardening world by storm, thanks to their easy to fill requirements. Knockout roses are a plant it and forget it varietal with a five to six week bloom cycle. They require no dead heading and are resistant to disease and pests.
There is no need for you to prune your Knockout roses. If you do not prune your Knockouts, they will grow to three or four feet tall and three or four feet wide. Your roses bushes will stay perfectly healthy and continue to bloom readily with no pruning. If you do want to prune your Knockouts, or if you do not want them to get so big, that is okay too. In areas with mild winters, you can prune your Knockouts in early spring. Prune them to 12 to 18 inches above the ground. You can prune them down to about two feet shorter than you want them to grow by midsummer, but don't prune them to less than 12 inches. In areas with harsh winters, you can prune your Knockouts down to about three inches above the ground in order to remove the dieback of the canes. Give the Knockouts a good early spring pruning in order to help get the top blooming performance out of them.
Always prune the Knockouts before the new plant growth begins. You can also prune throughout the growing season to keep the size of your plants manageable. This just means that you should clip stray shoots to maintain your plants' shape though.