The blooms of double Knock Out roses look like traditional roses, but the resemblance ends there. The plants are tough, grow fast, and require little maintenance. Plant breeder William Radler worked for years to develop a shrub rose that is not affected by many of the common rose diseases and pests. In addition, it is tolerant of drought, heat and cold. Double Knock Out roses grow in U.S. Department of Agriculture plant hardiness zones 4 through 9.
Double Knock Outs typically grow in compact mounds up to 4 feet tall and wide, but they can grow up to 7 feet tall. The plants bloom heavily with flushes of roses every five to six weeks from late spring or early summer until the first frost in fall or winter. One double Knock Out rose makes a beautiful specimen or accent plant, while a group of roses provides a mass of color in the garden.
Growth Rate and Pruning
Pruning encourages the growth of healthy new foliage and improves the quality of the blooms on roses. Allow two or three years after planting a double Knock Out rose for it to grow to its mature size. Afterwards prune as needed to maintain the desired shape and size. They are vigorous plants and can grow up to 2 feet a year.
Bypass pruners are better for pruning double Knock Out roses than anvil pruners, which crush the stems. Cut 1/4 inch above an outward-facing bud at the same angle as the bud. New growth will sprout from the bud. Occasionally, dip the pruners in disinfectant or a 70-percent alcohol solution to help prevent the spread of disease.
Pruning to shape double Knock Out roses should be done in the spring just as the buds swell, but before any new growth appears. Cut the stems back to 24 inches or shorter, depending on the desired size of the shrub during the summer. In the fall, remove crossed branches that are rubbing together, and long, leggy branches. Remove diseased, damaged or dead branches any time of the year. Cut back overgrown bushes by one-third to one-half in late winter or early spring.