Cold Hardiness and Fruit Trees

Recent extremes in cold temperatures have caused concerns for fruit growers, and much speculation about how much cold before fruit crops are damaged. Cold hardiness is an amazing process and understanding what it is and how it works may help explain why it varies so much.

What is cold hardiness: It is the ability of plant tissues to withstand extremes in cold temperatures. It is a complex physiological process that begins in early fall, and progresses until bud break in the spring.

How does it happen: Progressively cooler temperatures in the fall signal the plant tissue to move water from inside their cells into the spaces between cells. Some of this water is lost through transpiration, but what remains in the intercellular space eventually freezes into ice crystals. However, crystals formed in this space do not damage the cell, but crystals forming inside the cell kills it by destroying the cell membranes.

How fast does it happen: Accumulation of cold hardiness is a slow process, and as temperatures get colder, tissues become more cold hardy. The maximum hardiness is reachedin mid-January. The temperatures in the previous weeks dictate the maximum level of hardiness reached. For 2009, the cold weather that preceded this cold snap has provided good incentive for trees to reach their maximum levels.

How does it reverse: Trees move into their dormancy through the fall, but still respire at a much slower rate. The first phase is called quiescence, when outside factors can “wake them up” which reduces their level of hardiness. This is why fall pruning, fertilization or cultivation should be avoided. By January, trees are in the rest phase, and require many days of cold before they re-enter spring quiescence. Warm temperatures signal cells to re-hydrate, making them vulnerable to late cold snaps.

What temperature causes damage: This depends on many factors, and will vary between species and cultivars. Mid-January is the best time for a deep freeze, and constant cold temperatures beforehand is the best scenario. Luckily, trees do not experience wind chill like we do, and healthy trees achieved through best management practices, show maximum hardiness. Also, snow cover helps protect lower trunks and roots, which have less hardiness. It’s early to assess if damage occurred, and the winter is not yet over, but for now, it is important to understand the process of cold hardiness.