Knockout roses have quickly become the most popular roses in America. Their ease of care, free-flowering character, long season of bloom and disease resistance has made them landscape stars from coast to coast. However, as easy to care for as Knockouts are, they are not carefree. Like most roses, Knockouts need water, fertilizer and pruning to perform their best. And as quite a few gardeners are finding out, while they may be resistant to common rose diseases like powdery mildew, rust and black spot, they are not immune to them. Knockouts can become infected with common rose diseases.
It’s also true that resistance to common rose diseases does not translate to resistance to common rose insect pests like aphids, mites and Japanese beetles. And simply because of the sheer number of Knockouts that are being grown, minor pest problems can turn into major headaches.
Here’s Help With Common Insects and Diseases of Roses
At BioAdvanced Science Based Solutions, we’re committed to helping you get the most out of every rose you grow. Go to Flower Care 101 for information on how to recognize and control common insects and diseases of roses. There are also videos and stories on growing healthy roses and how to use all kinds of beautiful flowers in your garden.
Rose Diseases. When weather conditions are ideal, Knockout roses can become infected with powdery mildew, rust and even some black spot. More trouble is rose rosette, a new virus disease spread by a small eriophyid mite. It can turn healthy roses into monsters of contorted, crinkled, red stems and leaves. Plants can die within just a couple of years. Even though many types of roses (multiflora roses are the main host) can be infected with rose rosette, Knockouts seem especially susceptible. Unfortunately, there is no surefire control for either the disease or the mite. The most common recommendation is to dig up the rose and destroy or dispose of it to prevent further spread of the disease.
Insects. Almost any common insect pest of roses, including aphids, Japanese beetles, mites, rose slugs and thrips, can infest Knockout roses. Particularly devastating in southern states has been the chilli thrip, which attacks many fruits, vegetables and ornamentals, including roses but especially Knockouts. Chilli thrips are attracted to the young growth of roses, rasping the tender leaves causing them to be crinkled, distorted and dried. Plants can quickly lose their leaves and become stunted. Chilli thrips can also distort flowers.
Knockout roses are tough, vigorous plants, but they still need regular care to bloom prolifically 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 summer areas. Avoid overhead watering to help reduce diseases like black spot. Feed every 4 to 6 weeks or use BioAdvanced Rose & Flower Fertilizer, which provides slow release nutrients for up to 4 months. Prune as needed to keep plants within bounds. Keeping the center of the plants open will increase air circulation help prevent disease.
In winter, prune Knockout rose back by about ½ to 1/3. In USDA Zone 5 and colder, apply winter protection after the first hard frost of fall but before the ground is covered with snow. Winter protection can be as easy as mounding 12 inches of soil over the crown of the plant.
To protect roses from insects and disease for up to 6 weeks, use BioAdvanced All-In-One Rose & Flower Care beginning with new growth in spring and continuing throughout the growing season. Always read and follow label instructions carefully.