Knock Out roses are a shrub rose growing to 3 feet wide and high. These popular roses will bloom all summer long. According to the University of Wisconsin Division of Agriculture, this species was developed by Wisconsin rose breeder William Radler. In the 1980s, he began trying to develop a repeat-blooming, cold hardy rose that was resistant to disease, especially black spot. The Knock Out was introduced in 2000 and became the fastest selling new rose in history, with 250,000 sold the first year alone, according to UWDA. While Knock Out roses are indeed quite hardy and resistant to disease, they are not totally immune to the problems that all roses face.
Blackspot is a fungus that appears on the foliage of roses. The leaves develop black spots with yellow rings on both sides of the leaves. Eventually the entire leaf turns yellow and drops. An extensive infection can kill the rose bush. According to a Louisiana State University study, "blackspot evaluation of Knock Out rose cultivars found Knock Out Rainbow to be the most susceptible. In the national Earth Kind rose study, Knock Out, Blushing Knock Out, Pink Knock Out, The Fairy, Belinda's Dream and Folksinger were classified in the low end of slight susceptibility to blackspot."
Roses, even Knock Outs, that don't get enough sun are susceptible to powdery mildew. According to the University of Minnesota Extension, powdery mildew appears as a grayish dusting on rose leaves and branches. It can cause tender leaves to curl, young growth to be dwarfed and badly infected buds to fail to open.
Botrytis blight is a fungus that causes flower buds to droop and remain closed. The buds will then turn brown and decompose. Sometimes an entire bloom will be covered by the gray fungus. The fungus usually lives on dying tissue, making it imperative to keep roses pruned correctly.
Rust fungus causes small, rust-colored lesions on rose leaves or stems. This fungus appears most often when the weather alternately wets and then dries rose foliage.
Tips for Growing Knock Out Roses
Once your Knock Out rose bushes are well established, they are very drought tolerant. Despite their reputation for being resistant to blackspot, it is still a good idea to water the plants from below.
Knock Outs do not require mulching during the winter.