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Knock Out roses require care

knock out rose
Knock Out roses

News Release Distributed 08/20/10

By LSU AgCenter Horticulturists Dan Gill, Kyle Huffstickler and Allen Owings

Many home gardeners and landscape professionals have been talking about the landscape performance of Knock Out roses this spring and summer. We went through a very significant non-blooming time this spring, which was unusual for Knock Out roses. They are the best-blooming landscape shrub rose on the market. So, what happened?

Initial bloom on the Knock Out rose varieties was significant and impressive this spring. Normally Knock Out roses almost immediately transition from the end of one blooming cycle to the beginning of the next blooming cycle. This year, however, saw a four-week period after the initial bloom before the second bloom cycle. This was because cold temperatures this winter prevented Knock Out roses from blooming during winter.

In most winters, especially in south Louisiana, these popular roses continue to bloom to a small degree. We had very little bloom on Knock Out roses this winter, and we had the best-ever initial spring bloom. This “better than ever” first spring bloom resulted in a significant non-blooming period in May and June. Plants around the state have been blooming again this summer. As with all roses, however, summer brings much smaller, less-colorful blooms.

Here are some “best management practices” to keep in mind when maintaining these popular plants.

The soil pH requirement for all roses is 6.5. This indicates a slightly acid soil. Apply lime to raise pH or sulfur to lower pH as needed. Always follow recommendations of a soil test when modifying soil pH.

Knock Out roses need full sun. They require eight hours of sun daily to perform and bloom their best.

Keep bed preparation in mind when you plant Knock Out roses. They’re good, easy-to-grow plants, but they still need properly prepared beds. We recommend improving drainage by making raised beds.

Consider mature size when spacing plants. Knock Out roses are advertised to be 4-5 feet tall with an equal spread. Plants are very hard to maintain at this height and spread. Space them 6-8 feet apart.

Prune in mid-February by conducting a two-thirds height-reduction pruning. Also, prune again late August (north Louisiana) to early September (south Louisiana) by conducting a one-third height-reduction pruning. It is important to eliminate some older interior wood when pruning 2- to 3-year-old and older plants.

Mulch Knock Out roses with pine straw or a similar material in spring and add new mulch in late summer or early fall. Two to three inches of mulch are great for roses.

Fertilize with a three-month, controlled-release fertilizer in late February at the rate of one to two pounds of nitrogen per 1,000 square feet of bed area. Higher rates may be needed for new beds and those showing low levels of fertility. Use a lower application rate for older beds or beds showing high levels of fertility. Fertilize in early September at one-half the early-season application rate.

Rainbow Knock Out is the only variety slightly to moderately susceptible to blackspot. Consider a fungicide application in March, April and May. In addition, a September fungicide application may be beneficial. Other Knock Out rose varieties should not need fungicide.

Sunny Knock Out is the variety most susceptible to insects. Watch for thrips and aphids. They will be the main insect pests. Many general-use insecticides will control these insects.

Normally, Knock Out roses, as well as many other shrub-type roses, have five to seven bloom cycles between April and November. So take care of your plants properly, and you can enjoy blooms for the vast majority of the year.

Visit LaHouse in Baton Rouge to see sustainable landscape practices in action. The home and landscape resource center is near the intersection of Burbank Drive and Nicholson Drive (Louisiana Highway 30) in Baton Rouge, across the street from the LSU baseball stadium. For more information, go to and

Rick Bogren

Last Updated: 1/3/2011 1:32:01 PM

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