When Knock Out shrub rose (Rosa 'Radrazz') came on the scene in 2000, it took the rose world by storm. Coaxing blooms from finicky, seasonal roses became a distant memory as gardeners made the switch to Knock Out's exuberant, extended cycles of bloom. Hardy in U.S. Department of Agriculture (USDA) plant hardiness zones 5 through 11, Knock Out and its staying power continue to attract devotees.
Come Out Punching
Like all repeat-blooming roses, Knock Out blooms in cycles. Those cycles begin early in spring in cold climates, right on the heels of Knock Out's winter dormancy. They continue until hard frost puts an end to them for the year. Unlike most roses' bloom cycles, Knock Out's bloom cycles occur every five to six weeks, which is considered one of its best features. Constant flowers result from early spring until late fall. Gardens in the southernmost regions of the United States host Knock Out blooms year-round.
Prepare for the Ring
In order to have a stellar bloom performance, Knock Out's annual routine includes thorough pruning. In cool climates, the rose grows 3 to 4 feet tall and wide. When not pruned in a warm climate, the shrub can more than double that size. Promote heavy bloom cycles by cutting a Knock Out back to 12 to 18 inches above ground each year. Don't be timid; this rose was bred for vigor. Because pruning promotes new growth, wait until after the last hard frost in early spring to prune. Buds will follow soon. Bypass pruning shears are the perfect tool to use for small stems. Lopping shears do the job for larger branches. Wear gloves and a long-sleeved shirt for protection from thorns while pruning. Avoid the spread of plant diseases by always sterilizing tool blades before and after each cut.
Go Toe to Toe
Even with energy poured into near-continuous blooms, Knock Out stands up to conditions and diseases that make many roses retreat. It resists blackspot and mildew, and it tolerates heat and drought. Its spent flowers drop on their own, eliminating the need to remove old blooms by hand. Provide a Knock Out at least six to eight hours of full sunlight each day, well-drained soil and good air movement. Mulch helps to minimize temperature fluctuations and moisture loss in soil. In areas with a cold winter, an extra mulch layer helps to protect the shrub during that time of year.
Stand at the Bell
Not only will your Knock Out still be blooming when frost brings the growing season to an end, the plant should perform reliably year after year. Knock Out made its world debut as an All-American Rose Selection when the Conard-Pyle Co. introduced the rose in 2000. The original seedling, however, was grown in 1989 in USDA zone 5. After years of trials, exposure to disease and intentional hardships designed to prove the plant's worth, the original Knock Out rose was going strong in the front garden of its originator's home in September 2013, according to the Star Roses and Plants, Conard-Pyle Blog website.