“Of all flowers, methinks rose is best,” said Shakespeare. That sentiment still holds true today, since (arguably) rose is America’s most popular flower. Perhaps the fragile nature of the rose (and the plant on which it is borne) is a part of its mystique. In other words, if roses were easy to grow they might not be as popular among gardeners. A major factor contributing to plant fragility is a lack of cold hardiness of many popular types of rose. Early December is an ideal time to get rose plants ready for the upcoming winter or, in short, to “winterize” them.
Most of our modern roses, (e.g. hybrid tea, floribunda and grandiflora) need some type of protection during the winter. Since the past growing season was fairly kind to garden plants, most roses are going into the winter in relatively good condition. The goal of winterization is to keep them that way.
Protection from cold especially is important for the graft union—the point at which the named cultivar was budded onto a more vigorous rootstock. Abrupt changes in temperature (especially from fairly mild to very cold) can be especially damaging. Many of the things done to winterize rose plants are aimed at keeping temperatures more uniform throughout the winter.
Winterizing roses starts with thoughtful pruning. Remove excessive top growth, especially where rose plants have become over-grown. Tall canes (e.g. four to five feet) should be cut back about two-thirds of their length. Canes that are short but have abundant, bushy growth at their top should be pruned to allow about three-fourths of their height to remain. At the same time, weak, thin canes emanating from lower on the plant should be removed. Canes such as these can be damaged by winter winds.
In addition to pruning the top of the plant, application of mulch around the base of the plant is considered the best way to protect the delicate graft union. As previously mentioned, the graft union is the part of the plant most sensitive to cold temperatures. During severe winters, the root system of unprotected plants might be the only part that survives. The result will be the production of root suckers the following season. If the suckers flower at all, the flower will not be similar to the named cultivar that was budded onto the rootstock.
Any one of a number of mulches can be used to protect roses over the winter. Soil is excellent, readily available and inexpensive. If soil is used, it should not be dug from between plants unless spacing is quite wide. Additionally, bark, wood chips, aged sawdust or any other fairly dense material can be used. For best results, form a mound about 10 to 12 inches high and 18 inches wide at the base of the plant, covering the stem and bud union. Additional mulch may need to be added during the course of the winter should the original application settle.
Carefully remove all plant debris around the base of the plant before applying mulch. The inoculums of troublesome diseases such as blackspot may be present on this debris and will serve as a source of infection the next growing season. Any diseased leaves that remain on the rose plant should be removed as well.
The timing of rose winterization is important. Because of the latent heat contained by the ground, mulch applied too early will keep the stem of the rose warm and moist. The latter encourages the establishment of stem cankers. Delay winterization until several “hard” (killing) frosts have occurred but before the soil freezes should prevent the previously-mentioned problem. Treating the stems with a fungicide used to control blackspot before mounding is a good precautionary measure, but not an absolute necessity.
Winterizing shrub roses such as the very popular cultivar ‘Knock Out’ is not necessary at our latitude. The latter was bred, along with other desirable traits, to be very cold hardy.
REVISED: September 29, 2015