The original Knock Out rose (Rosa "Radrazz"), hardy in U.S. Department of Agriculture plant hardiness zones 5 through 11, has exceptional disease resistance and requires little to no maintenance, including fertilizers. Joined by an expanded collection known as the Knock Out Family, the original Knock Out rose requires no fertilizer under normal conditions in healthy garden soil. With varieties hardy from USDA zones 4 through 11, Knock Out Family members share the same simple-care, low-nutrient requirements of the original Knock Out rose. If you choose to fertilize your Knock Out roses, timely applications and a light hand are essential.
First-Year Knock Outs
Although Knock Out roses don't require fertilizer at any stage of life, leaving them fertilizer-free is especially important their first year. Fertilizing newly planted roses can damage their tender young roots and delay the plants' establishment. Instead of fertilizers, focus on a proper planting site that suits Knock Out roses' other needs. Give your roses a location with well-drained soil and at least six to eight hours of full sunlight daily. More hours of direct sunlight translate to better health and more prolific blooms. Knock Outs do best in soil with a near-neutral pH level -- the point at which soil nutrients stay most available to plants.
If you chose to fertilize Knock Outs after they become established, wait until they have at least one full bloom cycle. Water before and after you fertilize them. Plants need water to absorb nutrients, and fertilizing dry soil can burn roots. Wait until the roses have 4 to 6 inches of new growth before you feed them in spring. Then wait until after the first flush of Knock Out blooms to feed them again. Use slow-release, water-soluble, granular fertilizer, with all three of its numbers -- for nitrogen, phosphorus and potassium -- in the single digits, and follow its label instructions. One rose food manufacturer recommends feeding Knock Outs 3/4 cup of 3-4-3 fertilizer per bush every six weeks during the active growth period. Work the granules into the soil around the roses, several inches from their stems or canes.
In most areas, Knock Out roses grow 3 to 4 feet tall, but long, southern growing seasons produce Knock Outs two or more times that height. Their blooms continue almost year-round in some regions. These hard-working Knock Outs may benefit from extra nutrients. Louisiana State University Agricultural Center recommends twice yearly Knock Out pruning, with each session followed by fertilizer. Frequent watering of container-grown Knock Outs leaches nutrients from their soil, leaving the roses short on natural nutrition. In both cases, follow through with 3-4-3 fertilizer or a similar product at the same rate as for established roses. Sterilize your pruning blades with household disinfectant before and after pruning each rose bush.
In northern climates, unfertilized Knock Out roses naturally wind down and harden off as winter approaches. They drop their leaves and redirect their energy to below ground. If you fertilize Knock Out roses in northern climates, stop providing all fertilizers at least six to eight weeks before your location's average first fall frost date. Late-season fertilizer stimulates vulnerable new growth and inhibits the rose bushes from entering dormancy, lessening their cold hardiness and putting the entire plants at risk of cold damage or death. High-nitrogen fertilizers are particularly harmful. In early spring, cut your Knock Outs back to 12 inches tall because they'll triple in height, but forgo the follow-up fertilizer.