American Rose Annual 28: 95-99 (1943)
How Frost Damage Occurs

CONTINUED ROSE RESEARCH AT CORNELL
By DR. R. C. ALLEN and GEORGE N. ASAI
Department of Floriculture and Ornamental Horticulture, New York State
College of Agriculture, Cornell University, Ithaca, N.Y.

EDITOR'S NOTE.—Again careful scientific study replaces guesswork, and the study of definite rose knowledge as to frost injuring is advanced. Our Cornell friends do the rose world a service in Dr. Allen's reports. See plate facing page 47.

THE American Rose Society for the past four years has fostered a rose-experimental program at the Cornell Test Gardens at Ithaca, N. Y. Each year, in the Annual, a report has been published dealing with the results of experiments in soil-preparation, moisture and fertilizer requirements, time of planting, mulching and propagation. During the current year most of the emphasis has been placed on a scientific study of the factors involved in winter injury. Some of the findings in this study are here reported.

WINTER INJURY STUDIES

Just how to protect roses is a problem which confronts every rose-grower in the colder parts of the country. In many cases winter injury is thought to be the direct or indirect cause of lack of success, and there seems to be no doubt that the severity of the winter is an important factor in the growth and vigor of roses during the summer.

Injury Due to Drying Out of the Canes. To protect roses intelligently it is necessary to know specifically what they are being protected against. Drying out of the canes during the winter has been suggested as a possible cause of winter killing. In the careful observations which have been made so far, it appears that the moisture content does not vary greatly as long as the canes remain alive. If the canes are killed by some other factor, water is rapidly lost so that by spring the canes may be completely dry.

The following Table I gives the average moisture content of some of the test varieties. Unfortunately, the observations could not be continued throughout the winter because most of the varieties were killed by low temperature soon after January 6.

TABLE I. MOISTURE CONTENT OF ROSE CANES
Per cent of Moisture
Variety Oct. 13 Nov. 10 Dec. 15 Jan. 6
Radiance 54 53 54 51
Radio 56 53 54 50
Frau Karl Druschki 54 53 54 54
Dorothy Perkins 53 52 53 52
Ames No. 6   46 48 48

The data show that the moisture content did not vary to any appreciable extent during the period of the observation. They indicate that at least during the early part of the winter loss of water from the canes is insignificant.

To determine just how much water a rose cane could lose before being injured, some canes of Dorothy Perkins were cut from the plants and allowed to lie on the ground out-of-doors. One month later they were examined, and it was found that they contained only 35 per cent moisture. When placed under favorable conditions they proved to be perfectly viable. From this it seems clear that canes of Dorothy Perkins may lose water until the moisture content is as low as 35 per cent without injury. Just how much more they may dry is not known, but experiments are under way to determine this fact.

In some experiments the canes were waxed to prevent loss of water. Waxing did not reduce winter injury, even though no water was lost.

While drying out did not seem to be a cause of winter injury under the conditions of the experiment, it is still probable that in some regions it may be very important. As a result of some discussion in the American Rose Magazine and personal correspondence these findings are being tested under a wide range of conditions, artificially controlled to simulate hot drying winds and other climatic phenomena. In addition, closer observations are being made during the spring period which is thought to be critical. The data from these experiments and observations should conclusively demonstrate the influence of the drying or desiccation factor.

Injury Due to Low Temperature. Minimum temperature records were made close to the roses under observation. Even though a standard weather bureau minimum thermometer was used, it was noted that there was no agreement between the official Weather Bureau temperature report and the readings near the plants. In some instances the temperature around the plants was as much as 11 degrees lower than the reported temperature. This inconsistency of observation is a familiar source of misunderstanding.

Observations on the plants were made after each cold period. On December 8 the minimum temperature reached 4°F. No injury was noted in any of the test varieties.

By December 12 the minimum temperature had dropped to -1°F. Some injury was noted in the pith and inner portions of the xylem in Radio and Radiance, and in the upper portions of the canes of Frau Karl Druschki. However, when these plants were transplanted to the greenhouse they grew normally, indicating that the injury observed was not great enough to impair the subsequent growth.

On January 6 the minimum temperature reached -20°F., and it was noted that all portions of the canes above ground of the varieties Radio, Radiance and Frau Karl Druschki were killed. The upper half of the canes of Dorothy Perkins were killed, but the lower half recovered and produced somewhat abnormal growth. Examination showed that pith, xylem rays, inner cortex and pericycle were seriously injured. Some injury to the pith was noted in Ames No. 6 and Rosa multiflora.

It appears that the injury from severe freezing depends largely upon the minimum temperature reached. Other factors which were observed to exert some effect and are being studied further were the rate of freezing, the length of the period of minimum temperature and the degree of hardening which the plants have undergone. Thus it is not possible to predict accurately the degree of injury to a given variety from the minimum temperature alone.

With the aid of our artificial freezing equipment it was possible to subject the test varieties to various temperatures. Table II shows the minimum temperatures at which the test varieties were killed on different dates.

TABLE II. APPROXIMATE KILLING TEMPERATURES IN FREEZING CHAMBERS
Variety Oct. 13 Oct. 27 Nov. 10 Nov. 24 Dec. 15 Jan. 14
Radiance 21°F. 3°F. 3°F. 3°F. -4°F.
Radio 21°
Frau Karl Druschki 18° -4° -4° -4°
Dorothy Perkins 18° -4° -4° -4° -12° -12°
Thornless Rambler
-12° -12° -18° -18°
Ames No. 6
-18° -18° -18° -18°

It should be noted from Table II that although the varieties Dorothy Perkins, Thornless Rambler and Ames No. 6 were killed at -18°F. under artificial conditions, they were not completely killed when subjected to an outdoor minimum temperature of -20°F. This lack of entire agreement between natural and artificial conditions is probably due to a more rapid rate of cooling and a longer exposure in the cold chamber.

The data presented are also interesting since they show the development of cold resistance through the fall period. On October 13 all varieties were killed at a relatively high temperature. Their resistance increased until about December 15. Certain varieties develop cold resistance earlier than others.

The question of the relative hardiness of the different tissues of a rose stem was investigated. In the early fall before the plants had hardened to any extent the pericyclic portion of the phloem rays was the first to show injury. The inner cortex, phloem, cambium, outer cortex, xylem rays and pith were next in order of killing. The pith appeared to be the most resistant.

Later, after considerable hardening had occurred, the pith and xylem rays were the first to be killed but the other tissues remained in the same order of resistance. The outer cortex which is the tissue seen as the green bark on the outside of a rose stem is the last to be injured by low temperature. This fact complicates the detection of cold injury because, since the outer cortex is the hardiest tissue, the stem appears perfectly normal and healthy even though the inner tissues may be dead. This explains why after spring pruning many apparently healthy canes fail to develop shoots. In Climbers the canes may appear normal in the spring, the buds may develop until almost flowering-time and then begin to wither and the whole cane die. The canes appeared normal from the outside, but the inner tissues may have been winter-injured so that they were unable to recover sufficiently to sustain growth. When competition for water, food and nutrients by the developing shoots became great, they failed.

The Mechanics of Freezing Injury. Observations were made to determine just what happens inside a rose stem, when it freezes. Microscopic sections of frozen rose stems studied in the garden or cold chamber showed masses of ice crystals. As shown in the photomicrograph (Fig. I) these ice masses were in general lenticular in cross sections, occupying 75 per cent or more of space outside the cambium.

When the temperature of the rose stem drops below freezing, ice crystals begin to form in the intercellular spaces. As the temperature drops these crystals grow in size—by drawing the water from the cells of the surrounding tissue. As the water is removed, the cells shrink and the enlarging ice masses exert a pressure from the outside and thus greatly distort the normal arrangement of the cells. In stems frozen at very low temperatures, the tissues are seriously forced out of shape.

The hardiness of a rose stem is related to its ability to withstand the strains and stresses set up by the ice crystals. In very hardy types, when the ice melts, the water returns to the cells and the tissue returns to normal with little or no ill effects. Less hardy types do not have this capacity.

There seems to be some correlation between the size of the cells and resistance to freezing. In general, the hardier varieties normally have small cells in the phloem and outer cortex, while in the tenderer ones the cells are comparatively large.

COMPARATIVE HARDINESS OF ROOTS AND TOPS

To determine the relative hardiness of roots and tops, plants of the comparatively hardy Ames No. 6 variety that had been propagated by cuttings were dug in late February. The eight plants were divided into two groups. One group was cooled to 3°F. and the other to -4°F. and held at these temperatures for two hours when the temperature was slowly raised. Neither the tops nor roots showed any injury at 3°F. At -4°F. no injury occurred in the tops but the roots were completely killed. It is doubtful if the actual soil temperature ever gets as low as 3°F. in any region where roses are grown.

ICE FORMATION IN ROSE STEMS

Photomicrographs showing cross sections of a small portion of the stems of Radiance and Ames No. 6 (R. multiflora x R. blanda.)

A. Radiance before freezing
B. Radiance frozen at -20°F.
C. Ames No. 6 before freezing
D. Ames No. 6 frozen at -20°F.

Note the large masses of ice crystals and the consequent distortion of the cells and tissues in B and D. To illustrate "How Frost Damage Occurs", on page 95.