Knockout roses have quickly become the most popular roses in America. Their care-free, non-stop flowering character and disease resistance has made them landscape stars from coast to coast. But as easy to care for as Knockouts are, they are not completely carefree. Like most roses, Knockouts need regular water, fertilizer and pruning to produce the great results you expect. And as many rose growers are finding out, while they may be resistant to common diseases like powdery mildew, rust and black spot, they are not immune to them. Knockout roses can be infected with these diseases.
You should also know that disease resistance does not translate to resistance to common rose insect pests, such as aphids, mites and Japanese beetles. And simply because of the large number of Knockouts that have been planted, minor pest problems can turn into major outbreaks.
Help With Common Rose Insects and Diseases
Troublesome Diseases. When weather conditions are just right, Knockout roses can be infected with powdery mildew, rust and even black spot. Worse is rose rosette, a new virus disease transmitted by a small eriophyid mite. It can turn normal roses (even Knockouts) into hideous monsters of twisted, crinkled, red stems and leaves. Plants can die within a couple of years. Even though many kinds of roses (multiflora roses are a prime host) can be infected with rose rosette, Knockouts seem especially susceptible. And unfortunately, there is no surefire control for either the mite or the disease. The usual recommendation is to dig up the plant and destroy or dispose of it to prevent further spread of the disease.
Damaging Insects. Any of the common insect pests of roses, including aphids, Japanese beetles, mites, rose slugs and thrips, can infest Knockout roses. Especially troublesome in southern climates has been Chilli thrips, which attacks many fruits, vegetables and ornamentals, including roses, but particularly Knockouts. Chilli thrips are attracted to the new growth of roses, rasping the tender leaves, causing them to be crinkled, distorted and desiccated. Plants can quickly drop their leaves and become stunted. Feeding damage from Chilli thrips can also mangle blooms.
Thrips can be controlled on roses with Natria Rose & Flower Insect, Disease and Mite Control. Always read and follow label instructions.
Caring for Knockout Roses
Knockout roses are tough, vigorous plants but they still need regular care for the most colorful bloom and to minimize insect and disease problems. In the heat of summer, they should receive at least one good deep watering per week during dry spells; more in the hottest areas. Avoid overhead sprinkling to help prevent spreading diseases like black spot. Fertilize every 4-6 weeks to keep the flowers coming. Prune regularly to keep plants from getting out-of-bounds and remove spent flowers (dead heading) frequently. Keeping the center of the plants open will increase air circulation and help prevent disease.
In the winter dormant season, prune Knockout roses back to about ½ to 1/3 of their size. In USDA Zone 5 and lower, apply winter protection after the first hard frost but before the ground is covered with snow. Winter protection can be as easy as shoveling 12 inches of soil over the crown of the plant.
To prevent insects and diseases listed on their labels, use Natria Rose & Flower Insect, Disease & Mite Control or Natria Neem Oil beginning as the new leaves unfurl in spring and throughout the growing season. Always read and follow label instructions and check the label for a list of insects and diseases controlled.