American Rose Magazine 5(8): 158-159 (Mar-Apr 1944)
The Effect of Repeated Freezing and Thawing
George N. Asai
Cornell University, Ithaca, N.Y.

In the article "How Frost Damage Occurs" by Dr. R. C. Allen and G. N. Asai, which appeared in the 1943 American Rose Annual, evidence was presented to show the relation between the minimum temperature reached and the degree of injury sustained by the rose canes. However, from past experiences there seemed to be other factors which also affect the degree of injury. Among the most important of these is the affect of repeated freezing and thawing.

During the course of a single winter. several cold periods of varying intensity may occur, resulting in repeated freezing and thawing of rose canes. Freezing and thawing may also result from changes in temperature between night and day, etc. and by temporary interception of the sun's rays by clouds on cold, bright days.

Some evidence has pointed to the fact that such repeated freezing and thawing causes more injury to plants than a single freeze. Other evidence, has seemed to be contradictory.

To test the effect of repeated freezing and thawing, rose canes of the variety America were frozen once, twice, and four times and allowed to thaw each time at 40° F. One set was frozen at 10° F. and the other set at 3° F. The canes were then placed in a warm, moist atmosphere to allow for the development of the brown color which normally develops in injured tissues. After one week the canes were examined microscopically to determine the degree of injury. If bud and callus growth failed to take place, the cane was considered dead.

TABLE 1.
The effect of repeated freezing and thawing on the degree of injury.

Number of freezings Freezing Temperature
  10° F. 3° F.
1 4 to 6% of the cells in the cortex dead. Bud and callus growth excellent. Canes green. Bud and callus growth fair. Injury severe in the cortex and pith. Canes green on the outside with a few small brown spots.
2 6 to 12% of the cells in the cortex dead. 1 to 2% of the cambium cells injured. Bud and callus growth excellent. Canes green. No bud or callus growth. Canes brown.
3 25 to 40% of the cortical cells dead. Numerous cells in the phloem and cambium dead. Growth of buds and callus good. Canes green. No bud or callus growth. Canes brown.

It is evident from the data in Table 1 that the degree of injury is directly correlated with the number of freezings and thawings. When frozen at 3° F. one freezing did not result in the death of the canes, whereas freezing and thawing two and four times did. When frozen at 10° F., repeated freezing and thawing did not kill the canes, but the degree of injury, as measured by the number of cells killed, was greater than when frozen only once.

When canes are frozen only once, the number of cells injured increases as the temperature is lowered. Death of the canes occurs when a certain percentage of cells are killed. Thus, for a single freeze, there is a critical temperature at which the canes are killed. However, for freezing at a given temperature, the number of cells injured increases with the number of freezings and thawings. If the number of cells killed at each freezing is small, then repeated freezing and thawing may not cause the death of the cane. If, however, the number of cells injured at each freezing is large, the cane may be killed even though the temperature is considerably above the point necessary to seriously injure the cane in a single exposure.

In the garden, injury to rose canes may be progressive throughout the course of the winter if repeated freezing and thawing occurs. This is especially so if the freezing temperature approaches the killing temperature for a single freezing. In other words, winter injury may be severe even when the temperature does not fall to the critical killing point, because of the accumulative effect of alternate freezing and thawing.