Richard C. Bogren, Gill, Daniel J.
By Dan Gill
LSU AgCenter Horticulturist
(08/25/17) The Knock Out rose has become amazingly popular over the past 10 years. This rose has singlehandedly changed the market for roses since its introduction and ushered in a whole new way to look at roses and use them in our landscapes.
Its outstanding characteristics are well documented — excellent disease resistance, more frequent reblooming, showy clusters of single cherry red flowers, dark green attractive foliage and a shrubby growth habit that works well in the landscape. Knock Out roses, along with many other excellent rose cultivars in the landscape rose and old garden rose categories, tend to be used in landscape plantings like any other shrub. And they play that role very well.
These plants are remarkably resilient in landscape planting. You will see black spot and yellowing leaves on occasion, particularly during rainy periods. Powdery mildew may show up during ideal weather conditions. As a result, I’ve had gardeners complain that they thought Knock Out roses did not get foliar diseases.
These roses, however, are resistant, not immune, to diseases. That means when more susceptible cultivars are having major disease problems, you will tend to see fewer disease problems on more-resistant types, like Knock Outs. But under hot, humid, rainy weather conditions, even resistant rose cultivars will show spotting and yellowing leaves. But the plants will recover and be fine without fungicide treatments.
Once established, they are remarkably resilient and drought-tolerant. Notice the Knock Out roses thriving around gas stations. They do not need or want to be pampered.
One issue, however, is size. Many people purchase this plant with a tag that indicates the mature size at about 4 feet tall and wide. In fact, they can easily grow to 6 by 6 feet or more. The good news is that pruning them is not difficult and, done about twice a year, will help keep your bushes more compact.
Like all everblooming roses we grow in Louisiana (hybrid tea, grandiflora, floribunda, China, noisette, tea, Bourbon, landscape roses, miniature roses and others), Knock Out roses should receive two major prunings a year.
Use sharp bypass-type hand pruners when pruning roses. They make clean cuts and minimize damage to the stems. Wear a sturdy pair of leather gloves and long sleeves because no matter how careful you are, thorny roses can painfully puncture or scratch your hands and arms. Should you need to cut canes larger than one-half inch in diameter, you should use loppers.
The first pruning is done anytime from late January to mid-February. Pruning any later will delay the outstanding spring and summer flowering. There is no set way to prune a Knock Out rose (or other landscape roses). It depends entirely on the desires of the gardener and the situation.
If you want your roses to grow large to form a screen, you would do only light pruning and cut out any dead wood. In a situation where the bushes have grown too large, decide what size they need to be for the location. Cut the bushes back about 1 foot shorter than you want them to be (within reason, you should never cut them back shorter than 2 feet tall). A general recommendation for the late-winter or early spring pruning is to reduce the height of the bushes by about one-half to one-third.
You may do this even if the shrubs are sending out new growth. If the winter is really mild, they may even be producing some flowers, but you still need to prune. Put any flowers you cut off in a vase to enjoy indoors. Try to shorten canes back to a leaf or dormant bud, but you don’t have to be too picky. I’ve heard of extensive plantings being pruned successfully with electric hedge trimmers. Cutting back these roses stimulates vigorous new growth and improves flowering.
We do not prune roses back hard during the summer. They are stressed by the intense heat of June, July and August. (You will typically notice flowers are smaller and not as pretty.) However, where size control is needed, it is possible to manage the size of rose bushes to some degree during the summer when removing faded flowers — called deadheading.
Typically when we deadhead, we make the cut just above the five-leaflet leaf closest to the flower cluster (or the first five-leaflet leaf you come to as you move down the stem from the cluster of flowers). To control size during summer, however, you may cut back to just above a leaf farther down the stem — down about 6 to 8 inches below the faded flowers — when you deadhead.
Another opportunity to cut the bushes back arrives now in late August to early September. Again, you don’t have to be too fussy about this. This pruning is not as severe as the late-winter pruning. Plants are generally cut back by about one-third their height, more or less, depending on how much control is needed. Don’t forget to remove any dead canes when you cut the bushes back. Fertilize the bushes immediately after this pruning.
The Knock Out family of roses, and other excellent everblooming landscape roses and old garden roses, are great additions to our landscapes. Don’t forget to prune them in late August (north Louisiana) or by early September (south Louisiana). All repeat-flowering roses benefit from late-summer pruning. It stimulates vigorous new growth and better flowering, helps control size, keeps the bushes full and shapely, prevents them from looking so leggy and removes dead growth.
Knock Out roses are popular in Louisiana landscapes. Photo by Dan Gill/LSU AgCenter
We do not prune roses back hard during the summer. Where size control is needed, however, you can manage the size of rose bushes to some degree when removing faded flowers. Photo by Allen Owings
Use sharp bypass-type hand pruners when pruning roses. They make clean cuts and minimize damage to the stems. Wear a sturdy pair of leather gloves and long sleeves because no matter how careful you are, thorny roses. Photo by Rick Bogren/LSU AgCenter