Cold Enough For You?
Posted: January 7, 2014
A. Die-back on peaches pruned in December. B. Southwest injury on apple trunk. C. Peach twig damage from January 1994 cold snap.
Rich Marini's article on pruning and cold hardiness is in the January 2013 Fruit Times. One good thing at least here in Central Pennsylvania is that we have some snow cover. Snow acts as an insulator. In 1994 when we had record January lows we also had snow cover which probably prevented a lot of trunk splitting. I found one limb on one tree in a peach orchard in Berks county that summer that had 6 peaches on a branch that had been buried beneath the snow.
These past two days really saw temperatures plunge in Pennsylvania. Below is a table of high temperatures and minimum temperatures beginning at midnight January 6 and running through 11:00 AM January 7. While some of the temperatures are below zero their values in themselves would not necessarily indicate damage might occur to fruit trees. However, what is more troublesome is the difference between the maximum and minimum temperatures. In nearly all the locations that difference is 40+ degrees. As Rich mentioned in his article trees can go through a de-acclimation process and that a rapid drop of 40 to 50° can result in winter injury occurring. You can see how rapidly this change occurred in the graph of hourly temperatures at Rock Springs that occurred from midnight January 6 until 10:00 AM January 7.
Over the past few days there have been many discussions on the internet sources about cold hardiness and what minimum temperatures trees can stand. Most people are concerned about damage to flower buds, particularly peaches and nectarines. Peter Hirst at Purdue University related what he learned from Dick Hayden, former Extension Specialist at Purdue. Dick said he expected to see some damage at -10°F and for every degree below -10°F to lose 10% of the remaining flower buds with 100% mortality at -20°F. Bill Shane at Michigan State talked about temperatures that would damage trees being -30°F for apples and pears, -13°F for peaches and nectarines, and -25°, -20°, and -15° for apricots, tart cherries and sweet cherries, respectively. This was assuming it was on varieties and rootstocks adapted to the area and no significant warm spell shortly before the cold event. Mark Longstroth, also from Michigan, mentioned that temperatures January 3rd went down to -17 to -19°F. He expects a light peach crop especially in areas away from Lake Michigan.
Another point of discussion has been what effect wind chill has on trees. Wind chill is actually a human perceived phenomenon. Trees only perceive temperatures. Wind, however, can cause desiccation to trees and plants.
When I first came to Penn State there was a lot of effort put into the problem of peach tree decline. We had a large extension/research effort funded by PDA to look at trying to keep peach orchards alive longer. Some of the problem was related to Cytospora canker weakening trees, damage from lesser peach tree borers, winter injury and tomato ringspot virus. The practice of painting trunks with white latex paint, especially on the southwest side of the tree was promoted as helping to reduce trunk splitting. Some growers still routinely utilize this practice, but in the last few years we have not seen the cold temperatures like the past two days and it is not as universally done as before.
Maximum and minimum temperatures and differences at various orchards
in Pennsylvania from midnight January 6 through 11:00 AM January 7.
Hourly temperatures recorded at Rock Springs.
*NEWA from Cornell University, SkyBit from ZedX, Inc.