During the past century there have been several extreme cold events that have damaged orchards in the eastern U.S. Following the winter of 1935-36 there were a number of observational reports from the Mid-west and the East describing how winter injury seemed to be related to various cultural practices and varieties. A summary of a survey of Pennsylvania apple orchards, published in the Proceedings for the American Society for Horticultural Science by Anthony, Sudds and Clarke (1936), described tree injury following a very rapid decline in temperatures in mid-January 1936. In general trees that were most injured were those that lacked adequate vigor, those that were too vigorous, and those that had been pruned before the cold event. Trunk injury was greater than I would have expected considering that the lowest temperature was only -15˚F, but this was accompanied by a rapid drop of 40 to 50˚. There was another report from Indiana by Burkholder (1936), describing observations in the orchard at Purdue University. Students in the Tree Fruit course had heavily pruned some trees in November as part of their pruning lab exercises. The first half of January was fairly mild followed by a ten-day stretch of temperatures below zero with a minimum temperature of -20˚F. By the following September all 43 heavily pruned 'Jonathan' trees were dead or nearly dead, the 8 trees that were lightly pruned had slight trunk injury, and none of the non-pruned trees were injured. Other reports of orchard observations suggested that trees pruned shortly before a cold snap were injured more than trees that were not pruned. Similar observations have also been reported for woody ornamentals. These lessons from the past may be important because it seems that we are experiencing more unusually warm periods during mid- and late-winter, so trees may be more susceptible than in the past to moderately low winter temperatures. In this review I will summarize the current state of knowledge concerning pruning and cold hardiness of fruit trees.
Cold acclimation is the process leading to the development of freezing tolerance in plants. Fruit trees, like other woody plants, gradually acclimate to low temperatures in the autumn in three stages. Short photoperiods, sensed by the leaves, trigger the onset of the first stage of acclimation and plants will develop 10 to 15 degrees of cold tolerance as the days become shorter. Warm temperatures (60 to 70˚F) combined with short days promote maximum hardening due to increased metabolic activity. Following a period of warm temperature preconditioning, cold acclimation is continued by a period of cool nonfreezing temperatures (60˚ days and 40˚ nights). A second stage of acclimation occurs upon exposure to subfreezing temperatures (23 to 27˚). This increase in hardiness is very rapid and trees may be as much as 10˚ hardier the day after a frost than they were the previous day. The final stage of acclimation, resulting in maximum cold tolerance, is initiated by exposure to temperatures approaching zero degrees F.
During the winter, trees remain hardy as long as the temperatures remain fairly cold. However, trees de-acclimate in response to warm temperatures. Trees can also re-acclimate when warm temperatures are followed by normally cold winter temperatures. A mid-winter severe cold snap may not injure plants unless it is immediately followed by unseasonably mild conditions. Later in the winter, when the chilling requirement has been satisfied, trees begin to lose the ability to re-acclimate to hardiness levels obtained earlier in the winter, and may only partially re-acclimate. The chilling requirement for most apple varieties is about 1,000 hours of temperatures between 50 and 35˚ F. Most peach varieties grown in the Northeast require about 800 hours of chilling, but varieties from southern or California breeding programs may require less chilling. During the winter, the temperature required to kill peach flower buds may vary by as much as 10˚F. During bloom, the frost tolerance of an open blossom may vary by about 5˚F depending on the temperature conditions for several days before a frost. This is why it is so difficult to predict the temperature that may kill trees or flower buds during the winter and even during bloom.
What is Cold Hardiness?
Cold hardiness is the ability of a plant to withstand low temperatures. "Cold hardiness" is a vague and often misleading term because low temperature injury can vary depending on when the low temperatures occur (early vs. mid- or late-winter), how fast the temperature drops, what the temperatures were during the previous few days, and how long the low temperatures are sustained. For this reason every cold event is fairly unique and a plant may be affected differently by different cold events. For example, the peach rootstock 'Siberian C' survives sub-zero temperatures in Ontario where winter temperatures are consistently low, but it is winter killed at above zero temperatures in the South where winter temperatures fluctuate. So 'Siberian C' is considered "cold hardy" in Ontario, but not in South Carolina.
Pruning Influences Cold Hardiness
Although there is quite a bit of anecdotal evidence indicating that pruning early in the winter can reduce the cold hardiness of woody plants, there have been few controlled experiments to determine how hardiness is affected by time of pruning, how long the pruning effect may last, and if pruning severity is involved. Some of the best research on time of pruning of peach trees was done in South Carolina (Nesmith and Dowler, 1976) and Georgia (Prince and Hutton, 1972) in an attempt to identify practices that contributed to peach tree short life (PTSL). PTSL is a complex disorder of peach trees in the southeastern U.S. causing excessive mortality of trees less than 10-years old. Conditions contributing to PTSL seem to include nematodes and other soil pathogens, rootstock selection, and fall pruning. A series of experiments conducted during the 1960's and 1970's showed that fall-pruning of trees on sites with a history of PTSL were susceptible to late winter cold injury that killed the trees. Trees on sites with no history of PTSL were less affected by fall pruning. Pruning studies with other woody species, such as grapes (Wolpert and Howell, 1984), crape myrtle and cypress (Hayns, et al., 1991) showed that pruning in the fall reduced the cold hardiness of the plants during the winter. A couple of interesting facts have emerged from these studies.
- Pruning in November tends to reduce the cold hardiness of woody plants until late February, so the effects of pruning are fairly long-lasting.
- Pruning experiments with peach show that fall-pruned trees had higher levels of the growth regulator indoleacetic acid (IAA). This is the naturally occurring auxin that is mimicked by the synthetic auxins that are used commercially, including NAA, 2,4,5-TP and 2,4-D.
- Cambial activity (the tissue in the bark responsible for cell division contributing to trunk and stem radial growth) seems to be stimulated by the increased IAA levels following pruning. Peach trees pruned in November exhibited enhanced cambial activity in February.
- More recently, summer pruning of peach trees in August was shown to delay leaf abscission and cold acclimation, so flower buds on summer pruned trees were less cold hardy than non-pruned trees in the early winter, but not in the mid-winter. I also made the observation a few years ago that peach trees that were pruned in the pink stage had more injury than non-pruned trees when a frost occurred two days after trees had been pruned. Based on observing peach trees that were approaching bloom, I think that in some years pruned trees begin to bloom and leaf out a little earlier than trees that were not yet pruned.
Based on everything that has been published we can conclude that woody plants do not attain maximum cold hardiness when they are pruned in the fall. Trees are affected more by heavy pruning than light pruning. There is still much that we don't know about the practical implications of how pruning affects cold hardiness. We especially don't know how rapidly pruning causes de-acclimation, or if de-acclimation is similar in the early-, mid-, and late-winter, and we don't know how long the trees remain de-acclimated. We also don't know if the de-acclimation following pruning is affected by mid-winter warm spells, which we seem to be experiencing more frequently. This is an area of research that I am interested in pursuing during the final stage of my career, and I hope to have answers to some of these questions before I retire.