If you live in a warm climate or in zones 8 through 11, you can prune your knockout roses in the winter. One season occurs in the spring and another in the fall. Pruning mature knockouts in the winter makes sense, but get expert advice from your local nursery or extension service. To prune knockouts in the winter, follow these suggestions.
Prune only mature plants that are established and are 2 or 3 years old.
Sharpen your clean pruning equipment and always cut at a 45-degree angle.
Remove any thin canes or any diseased or damaged canes. If your temperature only rarely drops below freezing, you may have rose pests and eggs that do not die as they would in an area where it freezes hard. Be on the look out for insect damage and remove any canes that look questionable.
Strip most of the leaves from the roses in the winter, which makes them vulnerable to pests, and spray with a horticultural oil.
Activate the knockout into growth and blooming by pruning it back a little in the late winter before your first growing season in the spring.
Keep the roses under control. If you want it smaller than the standard 3 x 4-foot tall size, prune it back in the late spring or early fall after it has bloomed. Be careful not to clip off flower buds.
Remove dead or old canes during mild climates ideally, but this can be done at any time. Also cut very thin canes that might be coming up from the ground. You don't have to deadhead knockouts, but you can if you like to keep them looking groomed.
Prune mature plants by using the 1/3 rule each year in the winter. Remove 1/3 of the oldest canes and any stems that are damaged, diseased or dead. This will keep the bush vibrant.