The Amazing Cold Hardiness Process in Fruit Trees

Baby, it's cold outside! That's been the main topic of conversation recently, and many have asked how the fruit trees are surviving this winter.

Mother Nature has an amazing process to move trees into dormancy and develop cold hardiness. When daylength got shorter last summer, trees responded by ripening fruit, storing carbohydrates in their roots and wood, and wrapping their buds in thick, waxy protective scales. Leaves fell off the trees, but respiration continued at low rates. This stage is called quiescence, when it is best to avoid practices like fall pruning, fertilization or cultivation that "wake them up" and reduce hardiness.

As temperatures got progressively cooler through the fall, plant tissue started exporting water from inside their cells into the spaces between cells. The goal is twofold: firstly, to avoid cell wall damage by encouraging ice crystals to form between cells instead of inside them, and secondly, to concentrate the cell contents and lower the freezing temperature, similar to antifreeze. By January, trees had entered the rest stage, when they require many days of cold before they re-enter spring quiescence. Fortunately, this chilling requirement keeps trees dormant through early winter thaws.

Cold hardiness is a continuous process, and reaches a maximum by mid- to late January. Each day of cold temperatures results in plant tissues that can withstand progressively colder temperatures. And each warm day results in reduced ability to withstand cold.

So how much cold will fruit trees tolerate? This depends on many factors, and will vary between species and cultivars. For example, some apple cultivars might be damaged below -28 C, while others can tolerate -35 C. Peaches, grapes and berries are less hardy than apples and pears. We know that healthy trees are the hardiest, and that snow cover helps protect lower trunks and roots, which are less hardy tissues. Fortunately, trees do not experience wind chill like we do, although excessive winds can cause desiccation.

Some growers are cutting buds to assess if damage occurred, but the winter is not yet over. The next thaw will reduce hardiness, and the daily temperature changes are critical in late winter. For now, it is important to understand the amazing process of cold hardiness, and assess if management changes could enhance hardiness in the future.

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