University of Maryland Extension

A Rose is a Rose...Not always! Look at ‘KnockOut’ Landscape Roses

shrub rose

Everyone enjoys a beautiful and fragrant rose, but not everyone is successful in keeping them healthy, nor can dedicate the time and effort needed to manage the many problems that attack most roses. The most serious problem is a fungal disease called “black spot”. This disease causes dark spots on leaves about a ¼ of an inch in diameter. Infected leaves quickly yellow and drop off. Another fungal disease that affects most roses is powdery mildew. It does not penetrate the leaf tissue but grows on the surface making a bluish-gray layer on the leaf. This too will cause leaves to yellow because the leaf receives less sunlight. Growing high quality roses involves frequent spraying with insecticides and fungicides and regular applications of fertilizer. Even with routine spraying, prolonged wet weather can set the fungicide treatment off schedule allowing diseases to get started.

An Alternative Rose


You can enjoy having roses with minimal effort by growing the Knock Out® landscape roses. These are a group of roses with dark green lustrous foliage that lasts into late fall, blossoms that are plentiful, colorful, fragrant, and long-lasting. KnockOut® roses are practically immune to the fungal diseases of other roses; therefore no preventative fungicide spraying is required. An occasional insecticide application for aphids, Japanese beetles, and rose slugs may still be needed.

KnockOut® roses grow into a shrub four feet tall and four feet wide. They can be used in the landscape just like any other shrub. They look great when planted in groups, as a hedge, a shrub border, or planted in beds along with annuals and perennials. KnockOut® roses are winter hardy to USDA Zone 5. Their blossoms are “self-cleaning” - the old blossoms drop off after they have faded.

History of KnockOut® Roses
The KnockOut® rose was developed by William Radler in the 1970’s. The first true KnockOut® rose started as only one seed in 1989. After many years of dedicated breeding and careful selection, it was first available on the market as a red colored rose in 2000. His new rose received the “All America Rose Selection” award by the American Rose Society. After his first introduction, William Radler created several more colors and forms of KnockOut® roses:

  • Carefree Sunshine (a yellow rose) introduced in 2001,
  • Ramblin’ Red climber rose in 2002,
  • Blushing KnockOut (pale pink) in 2004,
  • Double-flowered KnockOut (red, with a lot more petals) in 2005,
  • Climbing Carefree Sunshine in 2006,
  • Rainbow KnockOut (pink with a yellow center) in 2007.

Radler pursued his life-long interest in horticulture, specializing in roses and earning a degree in landscape architecture and became the Director of the Boerner Botanic Gardens. He currently maintains hundreds of roses in his private collection and continues to breed them - always looking for improvements in uniqueness, hardiness, and beauty.

Pruning


Since KnockOut® roses only reach a maximum height of 4 feet and a width of about the same they do not require much pruning. Simply cut the canes back every year in late winter or early spring to approximately 24 inches. Cutting them back very severely, like a hybrid tea rose is not recommended. They won’t die but will require a long time to recover. Periodically, older, declining, and broken canes will need to be removed. This can be done at any time of the year.

Soils and Location


KnockOut® landscape roses need full sun to thrive. This is a minimum of 6 hours of direct sunlight every day. They will grow in a shadier place but will be sparse with reduced flowering. These roses are quite tolerant of most soils but not poor drainage. Always thoroughly prepare the planting site (not the planting hole) with liberal amounts of compost prior to planting. Have the soil tested to determine its nutritional needs. Mulch the roses with pine bark, hardwood bark, chopped leaves or pine needles to conserve soil moisture, moderate soil temperatures, and prevent weeds. These roses do not require heavy fertilization. However, periodic fertilizing, following soil test recommendations, will give the best flowering and growth.

Pests and Diseases

  • Rose Rosette 
  • Aphids: Aphids are a common early season pest of roses. They feed on the sap of mostly new growth, resulting in a yellowing and distortion/curling of the new growth. Aphids may also transmit viruses to roses from other infected plants. One serious disease that can be transmitted is rose mosaic virus to which KnockOut roses are susceptible. It causes the leaves and stems of new growth to become twisted and spindly. Infected parts should be pruned out promptly. Badly infected roses must be promptly dug out.

    Usually, no control is required for aphids because native insect predators and parasites reduce the aphid population as the season progresses. However, if quicker control is needed, use an insecticide labeled for aphids such as insecticidal soap or horticultural oil.

  • Japanese Beetles: The Japanese beetle is a very common pest of roses. Their peak feeding is early-mid July in central Maryland, afterwards, adult beetle populations usually decline. Handpick this pest and drop them into a container of soapy water. If additional control is necessary, use a labeled insecticide. If you use traps, place them at least 30-50 feet away from your roses, these traps have a very powerful attractant that will lure more beetles into your garden than if you did not use traps.
  • Rose Slug: The rose slug is a very common pest of landscape roses. It is neither a slug or snail but the larvae of a species of sawfly. Sawfly adults are very small flying insects related to wasps. The larva is small and “slug-like” in appearance. It feeds mostly from the underside of the leaf-eating many holes and causing considerable damage. Severe rose slug feeding will stunt the rose’s growth and flowering. Apply insecticidal soap or horticultural oil, or another insecticide labeled for rose slugs.
  • Deer: The whitetail deer lives in almost every suburban neighborhood and enjoys the taste of roses. Deer will eat the blossoms and the canes, especially the tender new growth, where the thorns are not yet sharp, but soft and tasty. Deer usually browse on plants at night, occasionally you will even see them feeding during the day.

    Regular applications of deer repellents are very effective. Deer repellents need to be re-applied especially after hard rain. Commercial deer repellents can get expensive but are worth the cost to protect your plants. Follow the instructions on the label for proper timing and application.

    Another less expensive approach that has been used with good success until recently is hanging bar soap around the rose shrubs. Soap is so widely used that it appears that some deer are no longer repelled by it. If you never used bar soap for deer it might still work for you. Alternate different types of deer repellents if deer are growing accustomed to the repellent you are using.

If you want roses in your landscape but are not interested in an involved maintenance program, the “KnockOut®” landscape roses might be just the answer. They will surely provide beauty, hardiness, and practicality.

This article originally appeared in the Spring 2011 issue of Home and Garden News.

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