A rose bush that would be killed by cold weather during the summer can usually survive below-freezing temperatures after it has hardened or become acclimated for winter. The same is true of other woody plants.
They all need some help and understanding from the gardener. Don't fertilize or prune heavily from early August until they have become dormant.
As days become shorter in autumn, chemical changes occur within the cells, protecting the plants from injury when temperatures go below freezing. The process is called hardening. A plant is hardy only when it has hardened. If cold weather occurs before hardening has taken place, the plant may be seriously hurt or even killed.
Fertilizing or heavy pruning probably would interfere with hardening: New growth could not harden in time for cold weather. If there is a late flower show you want to enter your roses in, fertilizing probably would give you better flowers, but it also would jeopardize the plant.
After hardening, trees and shrubs can be given fertilizer and the roots can use it. It actually may help the plant endure unusually cold weather.
The degree of cold the plant can tolerate after it has hardened depends on its heritage. Some lilacs are good to 40 below zero, while others may not survive much below zero.
Most woody plants in the temperate zone have chill requirements. Once they become dormant they cannot break dormancy until they have experienced a given number of hours at temperatures of 45 degrees or lower.
The chill requirements is built-in protection. If temporary mild weather occurs in midwinter before the chill requirement has been met, they won't break dormancy and be damaged by subsequent cold weather.
Winter cold damage was heavy in many areas in early winter 1971-'72. Azaleas, dogwoods, roses, camellias and hydrangeas suffered particularly. Roses, incidentally, have no chill requirement, which is why they can be grown in tropical and subtropical areas.
December, 1971, was like spring: Roses were in bloom in my garden for Christmas. It stayed warm through the first half of January with day temperatures as high as 78 degrees. On January 16 the temperature fell suddenly to 2 degrees and to 8 on the 17th. Plants were not winter-hardened and the damage was intensified by high winds.