When to Fertilize Knock Out Roses

Origins

• Knock Out roses are shrub roses developed by William Radner, who served as director of the Boerner Botanical Garden in Minneapolis and as consulting rosarian for the Chicago Botanical Garden. In the 1980s, Radner set out to breed a rose that would be disease-, drought- and insect-resistant, and cold-hardy. He also wanted the resulting rose to bloom abundantly all summer long.

Varieties

• Since its first member came on the market in 2000, the Knock Out rose family has grown to include not only the original red-flowering variety, but also double red, pink, double pink, "blushing," "rainbow" and "sunny" versions. They are now among the bestselling roses in the United States.

Cultivation

• Knock Out roses are easy to take care of. They need to be planted in light, well-drained soil that gets at least 5 hours of full sun every day. If you don't prune them, they will grow into a mound about 3 to 4 feet wide and 3 to 4 feet tall. Once they start blooming in the spring, they will blossom in continuous, 5- to 6-week cycles until the first hard frost of the season.

To Fertilize or Not?

• Because Knock Out roses bloom so prodigiously, you may want to fertilize them regularly to keep them looking their best. According to the Conard-Pyle Company, which raises and sells all the Knock Out roses, these roses don't have to be fertilized to prosper. Nevertheless, Gerald Klingaman, a retired horticulturalist for the University of Arkansas Cooperative Extension, recommends you water and fertilize the plants throughout the season, "to ensure plants continue to grow and set new flower buds."

When to Fertilize

• If you want to fertilize your Knock Out roses, you should wait till they have completed one cycle of blooming. Then you can feed them with commercial rose food according to the product instructions. Water the soil thoroughly before you feed the roses, to keep from burning the roots. Stop fertilizing late in the season, so the plants can prepare to go dormant during the winter.