Knock Out roses are a patented variety of shrub rose with abundant crimson, pink or yellow blooms. Although highly regarded for their hardiness and disease resistance, Knock Out roses are not indestructible. Often recommended for beginners because they require less maintenance than most roses, Knock Outs experience problems from time to time. Since Knock Outs are such a hardy species, they recover quickly from problems if corrected in time. Take a moment to familiarize yourself with several Knock Out rose problems so you are not caught off guard if an issue arises.
Gardeners sometimes experience flowering problems with Knock Out roses, which are bred for their showy blooms. Bloom-less Knock Out roses are disappointing unless you can determine the cause of the problem and correct it. Several factors result in poor flowering. If Knock Out roses do not receive enough sunlight, they will not bloom. Situate roses where they receive between four and six hours of sun per day, and remember that long stretches of overcast weather will reduce flowering. Lack of blooms may also result from improper fertilization. Knock Out roses require high amounts of phosphorus for abundant flowering and moderate amounts of nitrogen, which can cause excess green, leafy growth but decreased flowering in high amounts. Bone meal or another high-phosphorus fertilizer applied two to three times per year encourages plentiful blooms.
Though highly resistant to common rose fungal problems, Knock Out roses can succumb to conditions such as powdery mildew and black spot. Black spot appears as circular black dots surrounded by a lighter yellow color on the rose’s leaves and stems. To reduce the risk of fungal problems, use soaker hoses instead of overhead watering and avoiding overcrowding a space. High humidity or poorly drained soil also cause black spot on Knock Out roses. Powdery mildew appears as a white dust covering the leaves and results from indirect sunlight and high humidity. When faced with a persistent fungal problem, occasional fungicide applications are effective, but Knock Out roses require proper growing conditions for long-term health.
Since Knock Out roses are hardier and less susceptible to pest problems than most roses, many gardeners grow them with minimal pesticide use. However, if weakened by drought or disease, Knock Out roses become vulnerable to pest invasions. Common insect threats include aphids, spider mites, leafhoppers, beetles, thrips, rose slugs, borers, scale insects and rose midges. Besides regularly watering and providing a disease-free environment for Knock Out roses, gardeners also work to accommodate natural predators of common pests. Because they feed on pests, spiders in the garden should not be disturbed. Other natural Knock Out rose pest predators include lady beetles and lacewings, which typically increase in population naturally when aphid populations increase.