American Rose Annual 41: 118-122 (1956)
Difference In Resistance To Spring Freezes In Rose Varieties
Dr. H. R. Rosen
1
PROFESSOR EMERITUS OF PLANT PATHOLOGY, UNIVERSITY OF ARKANSAS
1Published with the approval of the Director of the Arkansas Agricultural Experiment Station.

Exceedingly severe injury and much complete killing of rose plants were due to late March freezes in the northern and hilly parts of Arkansas in 1955. Following a period of mild and above normal temperatures experienced throughout the state in the first half of March, nearly all roses, including climbers, shrubs and small bush varieties, had broken their winter dormancy and were growing rapidly when the first freeze came. The new growth was as much as six to twelve inches in length and numerous blossom buds had appeared, some of which were beginning to show color.

At the University farm where the experimental rose plots are located, the temperature dropped to 12° F. on March 22, and to 9° F. on March 26. Three successive days with temperatures below freezing followed, so that the soil remained frozen to a depth of four to six inches for several days.

These freezes, coming at a time when most plants had broken dormancy and were growing rapidly, caused much damage not only to roses but to nearly all fruit crops and to many different types of ornamental trees as well as to shrubs. Many sizable evergreens including varieties of hardy junipers and nearly all Arbor Vitae were either completely killed or severely injured. It is therefore not surprising that there was so much injury to roses.

It is well known that rose varieties differ considerably in their ability to resist winter injury. It is also well known that plants in general lose much of their winter hardiness as they break their winter dormancy and go into sustained growth. Is there any correlation between the degree of winter hardiness possessed by any one variety when it is in a dormant stage and its degree of hardiness once dormancy is broken? The answer appears to be yes, if a relatively large number of varieties are considered. However, certain modifying factors were noted which tended to obscure or vitiate such a relationship.

In general those varieties which have an inherent tendency to break their dormancy ahead of other varieties are apt to suffer more from late spring frosts and freezes. For example, Climbing Mme. Edouard Herriot, one of the most beautiful and hardier sports of a Hybrid Tea when it is dormant, having withstood temperatures below zero while unprotected in my own garden, has the habit of breaking its dormancy considerably ahead of such varieties as Paul's Scarlet Climber, New Dawn, Bonfire, and Chevy Chase. It had much new growth and numerous blossom buds when the March freezes struck and although about twenty years old, it was completely killed. On the other hand, while the other varieties just mentioned suffered a loss of 50 to 90 per cent of their wood, all of them survived.

Another factor which tended to obscure the relationship of winter hardiness to resistance to spring freezes is rapidity of growth. Contrary to what might be expected, those varieties which made the most new growth prior to the freezes were the ones which were hurt the worst. Thus in the fungicidal test plots where two old standard varieties have been used for many years, Etoile de Hollande and Edith Nellie Perkins, the former suffered far worse than the latter although both broke their dormancy about the same time, as they usually do. Etoile de Hollande usually makes about twice the growth in a given length of time, as it did prior to the freezes. Out of 88 plants of this variety, 49 were completely killed and the remainder were set back to such an extent that they probably fell far below average in seasonal amount of growth and of blossom production, while of 96 plants of Edith Nellie Perkins, only 34 were lost and the growth and seasonal blossom production of the remainder appeared to have suffered very little. However, as no blossom counts were made this year because of loss and injury, the data are incomplete.

A third factor that tends to obscure the relationship between winter hardiness and resistance to spring freezes is the age of the plant or the length of time existing between the setting out of a given variety and the appearance of the freezes. Thus, nearly all varieties that were planted in the late fall or early winter prior to the freezes suffered considerably more than those which were planted in February or early March, shortly before the occurrence of the freezes. This was true not only of bush roses but also of hardy climbers. The explanation appears to be connected with the relative amount of new growth. Having had a longer time to become established, the fall planted bushes broke their dormancy sooner and made more new growth than those planted in the spring. Consequently they suffered far more than the latter. The spring injuries of 1955, while much more severe than those previously observed in a 36‑year period, were otherwise similar to those previously experienced and constitute the main reason why February planting has been recommended in Arkansas (see "Keeping Roses in Good Health," Arkansas Extension Circular 412, 1938, 1950, and 1952 editions).

A fourth obscuring factor concerns differences in soil fertility or soil growth‑promoting qualities. In general those gardens or parts thereof that were adequately or heavily fertilizied sometime prior to the freezes sustained a great deal more injury and casualties than those fertilized lightly or not at all. Here again the amount of injury seemed to be directly proportional to the amount of new growth, the greater the amount of this growth, the more killing of new as well as old wood. The stimulatory effect of fertilizer on growth as well as the greater amounts of killing were noted not only on rosés but on other ornamentals and field crops.

Since such severe freezes as those of late March, 1955, are not likely to be experienced again, at least not in Arkansas, for a long time, the writer hesitates to recommend withholding fertilizer applications where needed until all danger of late spring frosts is over. But it seems obvious that for any region where such frosts are fairly common, it would be well to use quick‑acting nitrogenous fertilizers sparingly early in the season until the danger is over. However, if a slow‑acting organic fertilizer is used such as animal manure, cotton seed meal, soybean meal, or fish meal, the stimulatory effect would probably be less and therefore result in less injury.

In Table 1 the number of plants and the percentage of survival of varieties of Hybrid Teas and Floribundas are given. Climbers, pillars, shrub roses, old tea varieties and many breeding numbers are excluded for brevity although their behavior in general was included in the discussion. Wherever any variety is represented by less than three plants the data obviously are not as reliable as where three or more plants are represented.

2 The writer is happy to acknowledge the helpfulness of Dr. Walter D. Brownell, Little Compton, Rhode Island, and of the L. C. House and Sons Nurseries, Tyler, Texas, in furnishing plants for experimental purposes.

Out of the 36 varieties listed, 14 were bred by the Brownells.2 With one or two exceptions, this latter group is very outstanding not only because of the resistance they showed to injury of the March, 1955, freezes but also because of the high degree of winter hardiness they displayed in previous years. Without any protection, they have withstood temperatures of zero and below with less injury than that shown by most Hybrid Teas and Floribundas present in the plots. Among the hardiest of these are Pink Princess, Lafter, and Orange Ruffels. These three are also highly resistant to blackspot and come close to being fool‑proof and less bothersome than most varieties known to the writer. While none of these are of the large, exhibition show‑type, they will often produce blossoms when Peace, The Doctor, or Show Girl will have none. In addition to the three mentioned, the other Brownell varieties in the table are Anne Vanderbilt, Country Doctor, Curly Pink, Dolly Darling, Handsom Red, Lady Lou, Old Fashioned Red, Pink Bouquet, Shades of Autumn, Treasure Gold and V For Victory.

Another variety in the table that deserves special mention is Sierra Glow. It has fully as much winter hardiness as its close kin, Charlotte Armstrong, but usually suffers far less injury from late spring frosts and freezes because of its tendency to break dormancy later and also because it usually does not produce as much early growth in a given length of time. Add to these qualities considerable tolerance to blackspot and extraordinarily beautiful opening buds and one wonders why its introducers dropped it. Is it because it is not as striking or deeply colored in the wide open blossom?

Table 1. NUMBER OF PLANTS AND PERCENTAGE OF SURVIVAL FOLLOWING MARCH, 1955, FREEZES

Variety Number
Plants
Per Cent
Survival
Variety Number
Plants
Per Cent
Survival
Variety Number
Plants
Per Cent
Survival
Anne Vanderbilt
3
100
Edith Nellie Perkins
96
65
Old Fashioned Red
2
100
Betty Prior
3
100
Embers
7
0
Orange Ruffles
5
100
Buccaneer
9
0
Etoile de Hollande
88
44
Pink Bouquet
2
50
Capistrano
10
0
Fashion
1
100
Pink Princess
3
100
Charlotte Armstrong
6
17
Frolic
6
0
Rose of Freedom
1
100
Chrysler Imperial
10
0
Golden Scepter
1
0
Rouge Mallerin
4
0
Country Doctor
2
50
Handsom Red
2
50
Serenade
3
100
Crimson Glory
4
75
Irene of Denmark
11
0
Shades of Autumn
3
100
Curly Pink
3
100
Lady Lou
1
0
Sierra Glow
5
80
Dolly Darling
1
100
Lafter
4
100
Treasure Gold
1
100
Easter Parade
9
11
Ma Perkins
2
0
V for Victory
1
100
Eclipse
4
75
Mission Bells
10
0
World's Fair
1
0

CybeRose note: Hardiness in roses is not a simple trait. To be fully hardy in a given climate a rose must go dormant before the first killing frosts of Autmn, remain dormant during warm warm spells that may occur in Winter, and continue to sleep (or grow slowly) late enough in Spring to avoid late frosts.

See Bugnet on early dormancy.