The term “winterize” is associated more with automobiles than garden plants, but just as motorists want their cars to withstand the rigors of winter, gardeners can take steps to help their fragile plants do the same.
Most of our modern roses, such as hybrid tea, floribunda and grandiflora, need protection during the winter, said University of Missouri Extension horticulturist David Trinklein. Early December is an ideal time to get rose plants ready for winter.
“Since the 2017 growing season was fairly kind to garden plants, most roses are going into the winter in relatively good condition,” said Trinklein. “The goal of winterization is to keep them that way.”
Protection from cold is important for the graft union—the point where the cultivar was budded onto more vigorous rootstock. If the rose was properly planted, the graft union should be at or slightly above soil level.
During severe winters, the root system of unprotected plants might be the only part that survives. This results in the production of root suckers the next season. If the suckers flower at all, the flowers will be different from the cultivar that was budded onto the rootstock.
Winterizing roses starts with thoughtful pruning. Remove excessive top growth, especially where rose plants have become overgrown. Tall canes—4 to 5 feet—should be cut back about two-thirds of their length. Prune short canes with abundant, bushy top growth to allow about three-fourths of their height to remain. At the same time, remove weak, thin canes lower on the plant.
After pruning the top of the rose plant, apply mulch around the base to protect the delicate graft union. Before applying mulch, carefully remove all plant debris around the base of the plant. Diseases such as black spot may be present on debris and infect plants the next growing season. Also remove diseased leaves that remain on the rose plant.
A number of mulches are suitable to protect roses over the winter. Soil is excellent, readily available and inexpensive, Trinklein said. If using soil, do not dig it from between plants unless spacing is wide.
Bark, wood chips or aged sawdust are also good choices. For best results, form a mound about 10-12 inches high and 18 inches wide at the base of the plant. Cover the stem and graft union. Add more mulch over the winter if the first application settles.
Timing of rose winterization is important, Trinklein said. Applying mulch too early will keep the rose stem warm and moist. This encourages stem cankers to form. Delay winterization until after there have been several hard or killing frosts but before the soil freezes.
Treat stems with a fungicide to control black spot before mounding as a good precautionary measure.
Finally, it is not necessary to winterize “old garden” and shrub roses such as the popular cultivar “Knock Out” at our latitude, said Trinklein. Both types are extremely tolerant of cold weather.
Source: University of Missouri
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