American Rose Annual 45:91-94 (1960)
Hardness and Hardiness
Roy E. Shepherd Medina, Ohio
THESE TWO WORDS which are distinguished from each other by but one letter are practically synonymous insofar as winter survival of roses is concerned. Wood that is hard or mature contains a minimum amount of sap, or moisture in its tissues and is far better prepared to withstand the alternate freezing and thawing temperatures of the average winter than soft growth. It is true that hardiness is an inherent quality in some roses and not in others but it is also controlled to a great extent by the late summer performance of the plant.
Observe the wild roses as they grow in the fields or along the roadside and you will note they make but little growth late in the season and they are invariably green to the tips when spring arrives. They enter winter with all wood mature as Nature seemingly provides an extended period of preparation. To prove this theory you might select a wild rose known to be dependably hardy when grown as Nature intended and give it the same culture many growers accord their cultivated roses; in all probability much of its wood will be dead the following spring.
Late summer pruning, watering and feeding are definitely destructive agents of winter hardiness. They may result in larger plants at the end of the season but, profitwise, there is no gain as the buds resulting from this treatment rarely develop before the arrival of killing frosts and the wood contains a disastrous amount of moisture. Behavior similar to the wild roses is consistent throughout the entire rose family.
The old Tea roses lost their popularity in the North because winter casualties were normally too great. This is unfortunate as the roses comprising this group are frequently referred to as "the aristocrats of the rose kingdom" and they definitely are worthy of that title. Their loss of popularity is due to the fact that they grow continuously and the wood does not harden as winter nears. This is understandable as their ancestors were native to a climate where winter preparation was not necessary.
The Hybrid Perpetual class represented by such old favorites as Général Jacqueminot, Paul Neyron, etc., and containing the blood of roses native to colder areas, suffers less winter injury than the Teas and Hybrid Teas as they make comparatively less growth after midsummer (unless forced to do so) and the wood produced previously to then has an opportunity to mature.
Before the Hybrid Perpetuals were replaced in favor by the more consistant blooming Hybrid Teas there were many arguments as to their hardiness. Some growers contended they survived the winters unprotected; others that they were not winter hardy. The latter probably pampered their plants in late summer, and the first permitted them to mature as Nature intended.
Climbing roses tell a similar story. Those considered winter hardy produce but little new wood in late summer, while those that freeze back considerably seemingly do not realize only mature wood can survive the approaching severe weather and grow vigorously until late in the fall.
The moral is apparent; the only care your roses require after mid‑August (where winters are severe) is an occasional dusting or spraying to prevent disease.
BLACKSPOT VERSUS HARDINESS
A friend recently made a statement, however, that is contradictory to our previously accepted ideas on the relationship between disease prevention in the fall and winter hardiness. This was to the effect that roses in her garden which were most severely defoliated by blackspot late last fall survived the disastrous winter of 1958‑59 better than those which were in full foliage when winter suddenly arrived. The author's first reaction to this was one of doubt but a comparison between the disease susceptible and disease resistant varieties in his own garden adds credence to the thought. At least it deserves some consideration as we know that the severity of winter injury is usually in direct proportions to the moisture content of the plant cells when the first extremely cold weather arrives. In other words, the almost complete destruction of unprotected canes last winter occurred because they were subjected to near, or below zero temperature while in nearly full growth and before natural dormancy had reduced the moisture content of the cells. The sudden drop in temperature caused the moisture laden cells to rupture which resulted in the death of the canes.
Isn't it quite possible those plants which were badly defoliated by blackspot had reached a comparatively premature stage of dormancy? As they were less healthy and vigorous than their fully foliated companions, it seems reasonable to believe their cells contained less moisture and were, therefore, injured less by the sudden drop in temperature. This is completely contradictory to our previous accepted beliefs that a rose must enter the winter in a healthy condition if it is to survive. Many long accepted rules may be broken, however, during a winter as out‑of‑the ordinary as 1958‑59. As we may never experience another one as severe, it is still probably good cultural procedure to combat blackspot as late in the fall as it develops. At least your plants will be more attractive and productive.
NATIVES TO MILD CLIMATES
Our modern roses of all types are derived in great part from roses native to climates having comparatively mild winters. They are, therefore, not genetically prepared to withstand the rapidity of temperature changes experienced throughout a great portion of our country. We must compensate for this inherent shortcoming if our roses are to survive. Of course, we have no control over one of the major growth inducing factors, rainfall, but we can do our part by withholding nitrogenous fertilizers after mid‑August, by doing no pruning and by giving the plant a protective mound of soil which prevents too rapid fluctuations in temperature.
Hardiness is not actually a matter of severe cold alone but is controlled primarily by two other factors; each of which is related to the canes' moisture content when the first severe winter temperatures occur. Plant tissues should contain a certain amount of moisture but if this is excessive when the ground is frozen and strong winds and sunlight prevail, the too rapid dehydration caused by these factors, plus the inability of the roots to replenish the loss of moisture will cause irreparable damage to the plant cells.
TEMPERATURE FLUCTUATIONS DANGEROUS
Rapid fluctuations in temperature resulting from extremely cold nights followed by sunny days cause a somewhat dissimilar but related condition. In this instance the moisture in the cells, if abnormally great, expands and contracts too rapidly and causes fracture, or rupture, of the cell walls. A healthy cane is composed of normally functioning cells and when they are injured beyond repair the cane cannot survive. The effects of too rapid expansion and contraction of plant tissues is evidence to anyone who has observed the splits, or cracks, that frequently occur on the side of a cane that is exposed to the winter sun.
Although a low night temperature of 20 to 25 degrees followed by a sunny day may, under some conditions, cause considerable damage in late fall, most Hybrid Teas and Grandifloras show no injury at a consistent temperature of from 5 to 10 degrees later in the season. The Floribunda type roses are slightly more cold resistant. Most of us anticipate a certain amount of freeze‑back each winter but it is somewhat difficult to realize that the period of severe injury due to cold alone rarely exceeds 3 or 4 days each year in the lower Great Lakes and similar areas. Very little injury of mature canes occurs during the balance of an average winter as the canes when hard are able to survive quite low temperatures without great injury. Regardless of the mildness of the winter it seems that we rarely escape a few toll exacting days.