By Lois Ann Helgeson, ARS Consulting Rosarian (Excerpted from www.ars.org ©1999)

Cold hardiness has three factors--plant acclimation to cold in the fall months, actual mid-winter hardiness and deacclimation in the spring.  Acclimation in plants takes place in response to shortening day lengths and declining temperatures.  Biochemical and physiological changes gradually occur that make plants more cold tolerant.  Plants, including the different classes of roses, and the varieties within each of these classes, differ in their ability to make these changes.  In fact, this ability can change somewhat from year to year for any particular plant due to changes in plant health and by annual variations in temperature patterns.

Mid-winter hardiness refers to the actual lowest temps that a plant will tolerate, without damage, once it has acclimated during the fall months.   

Deacclimation occurs in the late winter and early spring.  This basically is a decrease in hardiness in response to warming temperatures.  It is a process that is opposite to that of fall acclimation.

Good Health is Important.  Plants, including roses, that acclimate too slowly, can be damaged by early cold temps.  In an unusual year an early cold snap that occurs before acclimation or hardening off can injure plants that normally are considered winter hardy.  A plant weakened by poor health may never reach its normal maximum mid-winter hardiness level and thus may suffer tissue damage at considerably warmer temps than expected. 

Additionally, plants that deacclimate too rapidly during late winter thaws may suffer damage due to late spring frosts.  To at least some extent, the rose varieties that people choose to grow and the methods that they use to protect them will be influenced by these hardiness factors. 

The choices will also depend upon how much dieback--and subsequent reduced bloom that follows and in some cases increased risk of plant loss--that a particular grower is willing to accept.  This is weighed against the amount of additional work that they might be willing to do in the spring and fall for seasonal protection. 

There has been a term coined, dieback hardy, for those roses that generally can be grown with minimal winter protection, that are likely to sustain considerable winter damage, yet are known to be able to regrow the following spring and bloom quite well by June.  It should be acknowledged, however, that while this, with certain varieties, is acceptable to most growers, that these same varieties will perform substantially better in the years when there are milder winters and reduced damage. 

Additionally, growers must understand that healthy plants--of any class and variety--will be better able to survive winter weather using any of the available protection methods.  As an example, plants defoliated by black spot or stunted by heat and water stress face winter with a significantly reduced chance of survival.    

Location Factors.  Some marginally hardy varieties can be helped by careful choice of planting location--such as the east side of buildings or in areas where snow accumulates.  Some areas are subjected to drying winter winds, and if without reliable natural snow cover, this can inflict additional damage on rose canes. 

Protection by shrubbery or buildings may create areas where the desiccating effect of the winds is substantially reduced.  In my yard, the city snowplows push snow around the fence bordering a long perennial bed near the street providing the necessary additional protection for a number of Hybrid Perpetuals.  Most years these otherwise zone 4 marginal plants have experienced little dieback and reward me with outstanding bloom.

Watering.  Hardiness can also be improved by fall watering.  Additionally, stopping nitrogen fertilizers and discontinuing deadheading after August will encourage the hardening off, or maturing, of the canes. 

Mounding the base of the plant with extra soil and mulching in the fall can provide extra protection.  If a plant is grafted, the bud union should be placed 2-4" below the soil surface when planting (not necessary when using the Minnesota Tip).  This will provide additional protection for the bud union and may also result in the plant growing roots from the area above the graft--turning the plant own rooted--which is generally desirable for roses in colder climates.