Preparing Trees for Winter
In order to survive freezing temperatures during winter, trees must quickly acquire hardiness in the fall and maintain it until temperatures rise in spring. Trees gradually become winter hardy as they are exposed to cold temperatures, finally becoming fully hardy in early winter. This process begins in the buds and young shoots and progresses into the larger limbs. The trunk is the last part of the tree to acquire hardiness and the most likely to be injured by an early fall freeze. Failure to harden-off before severely cold weather will cause winter injury, but is not a common occurrence in eastern regions of the US. More frequently, warm weather in winter interferes with cold hardiness and predisposes trees to winter injury in March and April. This type of winter injury causes minor injury to some fruit trees, but can be severe about once every ten years.
Good cultural practices hasten hardening-off and reduce the likelihood of damage from fall freezes. Allowing the tree to bear too many fruit will delay the hardening-off process. Apple, pear, plum, and peach fruit should be thinned in early summer so that the tree does not bear an excessive number of fruit. In addition, fruit should be harvested when ripe instead of being left on the tree. Too much nitrogen fertilizer, particularly in summer, stimulates shoot growth and delays hardening-off. Encourage the timely onset of winter hardiness by thinning and harvesting fruit, and by limiting the amount of fertilizer.
Good cultural practices increase the degree of hardiness in midwinter. Good tree care increases the amount of energy stored by the tree, which is used in spring to grow new shoots and to replace tissues that have been killed by winter injury. Over watering, which is very stressful to the roots, limits the level of hardiness the tree can acquire. Fruit trees are more sensitive to excess soil moisture than to drought. Diseases that attack foliage, such as apple scab and peach leaf curl, reduce winter hardiness because of the stress they place on the tree. Disease prevention is an important step in preparing trees for winter. Pruning at the right time of year also promotes winter survival. Pruning causes a small loss of hardiness, so trees should be pruned in late winter or early spring, after the chance for severe temperature has passed. Tender trees such as peaches and trees younger than three years old should be pruned in early spring rather than winter.