The original cherry red-flowered Knock Out rose (Rosa 'Radrazz') and its varieties produce showy flowers and are relatively disease-resistant. The shrubs grow up to 5 feet high and wide, thriving in full sun and well-drained soil with a pH level between 6.0 and 6.5.
Knock Out rose varieties produce 3-inch-wide, lightly fragrant flowers throughout spring and summer, and nearly all year in locations such as Florida. The blooms of Double Knock Out rose (Rosa 'Radtko') and Pink Double Knock Out rose (Rosa 'Radtkopink') have twice as many petals as the flowers of other Knock Out varieties, referred to as single Knock Outs. Single and double Knock Outs are hardy in U.S. Department of Agriculture plant hardiness zones 4 through 10 or zones 5 through 11, depending on the source. Pruning both kinds of Knock Out roses on a regular basis helps keep them healthy and shaped.
Spring and Fall Pruning
Single and double Knock Out roses benefit from an annual late-winter or early-spring pruning, whether they grow in cool or warm climates. Wait until the shrubs have had at least their second year of growth to prune them, however; they are mature then.
The best time for spring pruning is when their leaf buds have swelled but before the new growth appears. In cool regions, the shrubs usually can be pruned from mid-February to early March or April. In warm regions, late January to mid-February is generally a suitable time to prune. You can prune again in late summer or early fall.
Conditions That Warrant Pruning
At any time of the year, you can remove dead, diseased or damaged branches from Knock Out roses. Although the shrubs tend to resist diseases, you may see occasional disease symptoms, including yellow leaves, peeling bark and a white, powdery mildew on the leaves and stems. Prune and destroy branches that show such symptoms; the disease-resistant plants probably will recover on their own.
If your Knock Out rose bushes serve as a privacy screen, then an option is simply to remove their dead wood and allow the shrubs to grow taller.
Promote air circulation and prevent diseases by removing branches that rub against other branches.
Pruning Gear and Methods
Wear a long-sleeved shirt and long, protective gloves to avoid injury from Knock Out roses' thorns. Prune the shrubs with bypass pruners, which have a scissorlike cutting action. Avoid using anvil pruners, which damage stem tissue, according to North Carolina Cooperative Extension.
Before you prune, sterilize the pruners by soaking them for five minutes in a solution that is 1 part 70-percent isopropyl rubbing alcohol and 1 part water to prevent spreading plant diseases; rinse the tool with clean water. Also sterilize the pruning tool occasionally between cuts on the same bush and before you prune a different bush. Keep pruning tools and disinfectants away from children and pets.
During spring and fall pruning, remove one-half to one-third of each Knock Out rose's growth,* but do not cut a plant shorter than 2 feet tall. Make each cut slanted and about 1/4 inch above an outward-facing leaf bud, with the cut slanting away from the bud. Staggering the cuts will give a shrub a dome shape.
Removal of Old Flowers
In summer, you have the option to deadhead, or remove, spent flower clusters from Knock Out rose bushes. Although deadheading is not a necessity, the practice helps control plant size and promotes late-season blooming. Deadheading is not a severe form of pruning and minimizes stress to the plants during hot summer months.
Deadhead a faded flower cluster by cutting its stem just above the first five-leaflet leaf below the flower cluster. If you want to trim leggy or uneven growth, then make the cut above a leaf that is 6 to 8 inches below the old flower cluster.