Prune Knock Out roses for size control, not out of necessity. The Knock Out variety of roses does not require typical rose pruning. But pruning does encourage the plant to grow and produce more flowers. Wait until early spring, after any chance of frost has passed, before tackling the pruning chore.
Select rose bushes larger in size than you would like or those that look unkempt. Leave Knock Outs unpruned for the first two years to allow for ample growth.
Clip off the top of each cane with a pair of clean, sharp hedge trimmers to about half of the plant's current height. Focus on pruning the older canes primarily, and prune starting from the base of the plant and moving outward. Make each cut clean and at a 45-degree angle.
Remove all broken, weak or dead branches by hand, or use a pair of garden shears, if not easily removed by hand.
Throw the removed branches in an outside trash container.
Water the bush well after pruning, but spray the water gently at soil level to avoid getting soil or water into the bush's open wounds.