Q: How low can I prune a Knockout rose in winter?
A: Much depends on how high you want them to be in summer. I like mine 36 inches tall, so I prune to 18 inches in January, February or early March. They will grow to five feet tall if left unpruned but they look a bit open and ragged for my taste at that size.
Cut out any dead branches first, then any that cross through the middle. Finally, reduce the height of the rose to 16″ lower than where you want the first flowers to appear.
The best place to make your final cut is a quarter-inch above an outward-facing bud or sprout. The small red buds are easy to see after a few days of warm weather.
Also see Knock Out Rose – Pruning in Summer