The Right Time to Trim Knock Out Roses

Annual Pruning Time

• Prune back your Knock Out rose bushes in very late winter to early spring, just before the dormant buds open to reveal reddish young foliage. In the southernmost United States, pruning should be completed by late February, and by April 1 farther north. Often the old gardener's adage applies: prune your roses when the yellow forsythia shrubs are starting to bloom. Cut back the Knock Out rose to main branches that are 12 to 18 inches tall.

Relevance of Annual Pruning

• When Knock Out roses are pruned back each year in late winter, you help control the size of the shrub in the landscape. Although pruning isn't necessary for these roses to bloom and dazzle a crowd, plant branches do eventually approach 5 to 6 feet in length. Larger, lanky Knock Out rose bushes beckon for trimming, as the gardener soon realizes the plants are too big or out of control. Pruning every late winter improved plant vigor, but also ensures the rose bushes mature only 3 to 5 feet tall and wide by the time they go dormant by the end of autumn.

Deadheading

• Deadheading -- the process of trimming off old, spent flower heads -- Knock Out roses is not necessary. These hybrid roses continue to produce more flowers even though old flowers may be developing into rose hips. You may still opt to deadhead the roses if you wish to tidy the look of your Knock Out shrubs over the summer. Often, deadheading the last bunch of flowers off the shrubs in late midsummer when blooming tends to lull, results in a more prolific flowering in September.

Trimming for Shape

• Gardeners should feel empowered to trim Knock Out roses during the growing season if issues arise about plant size or shape. While these roses are disease-resistant, they are not incapable of occasionally having a disease or insect dilemma. Dead, broken or diseased twigs or branches may be trimmed out. Trim back or fully remove errant branches that pose a safety hazard, such as blocking access to a sidewalk, or threaten property, such as rubbing against a building or scratching car finishes.

Caveat

• Overall, avoid pruning Knock Out roses in fall and winter, when temperatures drop. Don't prune back plants too early in winter, either. You don't want to prune when it's mild enough to coax leaf buds to emerge, but subfreezing temperatures may still occur. The only exception to winter-pruning is in USDA zone 9, where early February pruning is timely. Winters are so mild in this zone that pruning is done in midwinter, since roses end their dormancy so early that leaves start emerging around March 1.