When (how much) should I prune my knockout roses?
Thank you for your question about KNOCKOUT roses.
Many rose authorities recommend for floribundas and shrub roses, including KNOCKOUTs, is to wait until the second season of growth to prune; some even say as late as the third season. This ensures that you are working with a mature bush. (KNOCKOUTs are considered fully-grown at about 4 feet high and 4 feet wide.) KNOCKOUTs, specifically, respond very well to an annual Late-Winter/Early Spring Cleaning, although some sources say you can go a couple of years without this type of “hard pruning.” These mature KNOCKOUT's healthy shoots can be cut back by one-third to one-half, especially if your KNOCKOUTs have grown vigorously in the previous season. Remember that KNOCKOUTs are designed to grow vigorously, so prune them down to about two feet below the height you want them to reach during the growing season.
Clean up each KNOCKOUT rose before the first frost. Rake debris out from the base of each plant, and snip back stray branches. You don't need to prune the rose shrubs until spring, but trimming is optional. When the weather starts to get cold, add a 2- to 3-inch thick layer of mulch around the base of each KNOCKOUT rose plant. Keep the mulch away from the canes. The mulch will keep the ground temperature even.
Check out our Pruning Roses guide for more information. Note: always use clean pruners, disinfecting when moving from one rose to the next, especially different kinds.
Courtesy of the NYBG Plant Information Service