Pruning Knock Out™ roses
by Patricia Lunn Adsit, Extension Master Gardener in Guilford County
“Rose is a rose is a rose is a rose.” So said Gertrude Stein. Really?
Most modern roses fall into one of six categories: Hybrid teas, Floribunda, Grandiflora, Climber/Rambler, Miniatures, and the bush or Shrub roses. Popular selections in this category include: David Austin “English” roses and the Knock Out ™ (Rosa ‘RADrazz’) series, created by rose breeder William Radler. Home gardeners choose roses from this group because they are prolific bloomers throughout the entire growing season and tend to have better disease resistance to black spot, one of the main problems of roses.
All roses, regardless of category, benefit from the basics:
selection of problem-resistant cultivars appropriate to your garden, zone, and lifestyle;
6-8 hours of sun per day;
good growing medium (soil);
proper spacing; and
good quality care throughout the growth cycle.
This last “basic” includes pruning to bring out their best in both appearance and performance. While Shrub roses such as the Knock Out are sometimes touted as being carefree, remember: all flowering shrubs are revitalized by proper pruning practices, and the Knock Out rose is no different. Pruning improves the health of the bush and the quality of the blooms.
When should you prune? Rose pruning should be done just as buds break dormancy, usually around mid-to-late February or early March in Zone 7. A good rule of thumb to help you remember is “prune your roses when the forsythia is in bloom.” Have the buds swelled but no new growth has yet appeared? Then, “it is the ideal time to prune bush or shrub type roses,” according to Clemson University’s Laurens County Extension Master Gardeners.
One caveat that many rose authorities recommend for floribundas and shrub roses, including the Knock Out, is to wait until the second season of growth to prune; some even say as late as the third season. This ensures that you are working with a mature bush. (Knock Outs are considered fully-grown at about 4 feet high and 4 feet wide.)
You will also want to plan a little corrective pruning in the hot summer months, to allow the bush to direct more energy to fullness of the foliage. You will then get a flush of flower production as the days get shorter.
What should you prune? Most shrub roses in general, and Knock Outs specifically, respond very well to an Annual Late-Winter/Early Spring Cleaning, although some sources say you can go a couple of years without this type of “hard pruning.” Healthy shoots can be cut back by one-third to one-half, especially if your Knock Outs have grown vigorously in the previous season. Remember that Knock Outs are designed to grow vigorously, so prune them down to about two feet below the height you want them to reach during the growing season.
You can also do some shaping pruning during the growing season. If several shoots become much longer than others, trim them up to maintain a more uniform height. Of course, you should practice removal of the Three D’s at any time during the growing season: dead, diseased, and damaged wood. And keep pruning away leggy growth throughout the season to maintain the appearance of your bush.
How should you prune? For this Annual Cleaning, gather the tools needed, including a good pair of rose gloves (the kind with long cuffs are great for protection against thorns) and sharpened bypass pruners. Bypass pruners, which cut like a pair of scissors, are recommended for pruning roses over anvil-type pruners, which can crush the stems.
Make your pruning cuts at an angle, 1/4″ above an outward facing leaf bud, with the slant away from the bud. Prune the bush for height, width, and to make it more open in the center in order to increase air circulation and help prevent diseases. Whenever two canes cross each other, removed one to prevent rubbing and cut down on places for diseases to enter. Periodically dip pruners in a disinfecting solution to decrease spread of disease. Dr. Linda Chalker-Scott of Washington State University suggests that research shows a common household cleaner such as Lysol to be the best choice over either bleach or alcohol-based solutions.
Believe it or not, shrub roses such as the Knock Outs can even be pruned with hedge clippers or trimmers. Aim to remove the third to half of old growth, and strive for a dome shape for each individual bush. Before you get too energetic with that hedge trimmer on your Knock Out, remember you want your rose to pour its energy into making flowers instead of regrowing too much lost foliage!
Read more about this topic:
Pruning Basics: http://www.hort.purdue.edu/ext/HO-4.pdf
Pruning of Roses (in general): http://urbanext.illinois.edu/roses/prune.cfm
Research on disinfecting pruning tools: http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/FactSheets/Pruning.pdf
Plant fact sheet on Knock Out Rose: https://www.ces.ncsu.edu/depts/hort/consumer/factsheets/roses/knockout.htm
Patricia Lunn Adsit and her husband garden in High Point, NC, where they have 66 rose bushes, including one healthy, happy Knock Out.