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Apple Trees Get Ready for Winter Cold

Apple harvest is finally drawing to a close, and orchard and equipment clean-up is in progress. Apple trees are losing their leaves, but a fascinating process is actively underway inside trees to prepare their tissues for the coming winter cold. This process is called plant cold acclimation, and the goal is to achieve adequate levels of cold hardiness to avoid winter injury.

Much research has been done on cold hardiness in the past century, and recent advances in understanding this mechanism continue to increase. We now know that there are 3 stages in the acclimation process. The first actually started when shoot growth stopped in early summer, and the tree began to store carbohydrates in branches, roots and fruit buds. So even, at this point, trees can tolerate about -21°C (although fruit buds are much less hardy).

Although leafless trees appear dormant, the second stage of cold acclimation is now underway. Cells are actively exporting water through their cell walls into the intracellular space. Each cold event encourages more water export, which will reach its maximum level by mid-January.

If water remains in the cells, the crystals that form on freezing destroy important structures and cell walls, so contents can leak out and the cells will die. But for cells that have dehydrated themselves by exporting their water content, all the crystals form outside the cells and cause less or no damage.

After mid-January, cells will begin to rehydrate as temperatures warm, but if cold temperatures return, the process will reverse. So the third stage can actually be like a yo-yo effect within trees.

Although warm temperatures have been pleasant this November, if they continue, the trees will not increase in hardiness, and sudden drops in temperature that are common in December can cause severe damage. Also, cold during November and December will better prepare trees for the inevitable severe cold that often occurs in January.

So what's my weather wish? Gradually cooling temperatures throughout the fall, moderate uniform cold by Christmas, continuing until March, with a gentle snow fall to cover the ground evenly to protect roots with no thawing until spring. It's not likely to happen just like this, but at least as you bundle up for the coming cold, remember that it's helping fruit trees survive the winter ready to produce another good crop next year.