By C. J. Walke
This past winter was one of the coldest and longest winters we’ve experienced in Maine in recent years, and that brought up questions about the cold hardiness of our fruit trees and the potential for winter injury to them. Trees can be damaged by prolonged exposure to extreme cold, as well as fluctuations from deep cold to warmer temperatures and back to cold, such as during a typical “January thaw.” We saw this full range of weather last winter, and the effects may not be obvious until spring fully arrives (I am writing this in late April). Meanwhile I thought it would be good to discuss cold hardiness and the potential for winter injury.
Cold hardiness is simply the measure of a plant’s ability to withstand low temperatures; however the factors involved can be complex. The level of cold hardiness of a fruit tree can vary according to when the low temperatures occur, how fast the temperature changes and how long the low temperatures last. Considering these factors, a fruit tree may be affected differently by different cold events during winter, and the same variety may have different levels of hardiness depending on the region where it is grown.
In the Northeast, fruit trees start to harden off and prepare for winter in early August, when vegetative growth ceases and terminal buds are set. Disrupting this “hardening off” period can reduce the cold hardiness of the tree for the winter, so it is best to limit disruptions to and manipulations of the trees at this time. This is why we tend to avoid in August any nitrogen applications that can induce growth, to cease watering spring-planted trees and to avoid pruning cuts that expose the vascular system of the tree.
Fruit trees harden off gradually as they are exposed to lower and lower temperatures as late summer transitions into fall and early winter. Shorter day length also contributes, as leaves sense the change, which in turn reduces daily photosynthesis and chlorophyll production. (This initiates fall foliage color as the amount of green chlorophyll decreases, and yellow and orange pigments are allowed to reflect more light.) As time moves from September to December, the trees are exposed to increasing cold and gain increasing hardiness.
A tree can maintain its level of hardiness through winter as long as temperatures remain cold, but a warm spell (think January thaw) can “break” its level of hardiness, and when temperatures drop again, winter injury can occur. This drastic change more often causes winter injury in mid to late winter when the tree has satisfied its chilling requirement (the amount of cold required to induce spring bloom) and is less able to withstand the deep cold.
Considering all the factors involved, the only way to know for sure whether a tree has suffered from winter injury is to wait for flower buds to “pop” and shoot growth to begin. Or not.
A recent report from Highmoor Farm (a UMaine research farm) on fruit tree flower bud survival did not provide the most encouraging data from fruit trees observed at Highmoor’s Monmouth location in spring 2014. This report focused on tender varieties (peaches, plums and cherries), since they experience winter injury in “normal” winter conditions. Flower bud survival in peaches ranged from 4 to 12 percent, sweet cherries showed 0 percent, and pears showed 0 percent survival, which is surprising because pears are not considered to be tender.
This report also provided data from a location in Turner, Maine, which was far more encouraging: peach at 52 percent, sweet cherry at 83 percent and Japanese plum at 94 percent survival. As I wrote this article, data on apple flower bud survival had not been released.
Flower bud survival can be checked by dissecting a bud and observing the color inside – green is healthy, while brown or black means damage. A damaged bud could potentially bloom but would not be reproductively viable and will most likely brown quickly and fall prematurely from the tree. Damage to last year’s shoot growth may not be evident until healthy leaf buds on unaffected shoots start to swell and open, but damaged leaf buds on affected shoots don’t develop.
Winter injury is most severe when it occurs in the lower trunk, the crown (where the trunk transitions to roots) and the upper region of the root system (closest to the soil line). Damage in these areas usually results in splitting of the cambium layer just under the bark, which disrupts the flow of nutrients and can initiate wood rots or insect damage. Trees with this level of damage will often leaf out in spring, since energy is stored in the buds themselves, but once that energy is spent and the tree can’t pull more resources from the roots, affected areas will start to die back.
We can’t control the weather, but we can limit interference with hardening off of our fruit trees and we can encourage their dormancy. Time will tell whether trees experienced injury last winter, and hopefully damage is not widespread. Let’s plan for many blooms and many fruits to follow.
C.J. Walke is MOFGA’s organic orchardist and librarian. You can address your orcharding questions to him at 568-4142 or [email protected].