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Welcome to My Jungle

Welcome to My Jungle - January, 2019

Does unseasonably warm weather in the middle of winter cause woody perennials (trees and shrubs) to "wake up" too early? As with all things in nature, it all depends. Most trees from temperate climates require the accumulation of winter chill (500 and 1,500 chill hours) and subsequent heat during their dormant phase to resume growth and initiate flowering in the following spring. Chilling hours are the number of hours of exposure to about 45°F, and are measured from leaf drop in autumn until mid-February to early March. Chilling hours for fruit plants usually accumulate between 35°F and 55°F, with 45° considered optimal. The Midwest Regional Climate Center is close to this ideal; they assume if the temperature at each station at the top of the hour is greater than or equal to 35°F and less than or equal to 45°F (35°F <= T <= 45°F), then the station's season's chilling hour accumulation is increased by 1 unit. As of January 6, 2019, the collar counties of St Louis, with a few exceptions, have accumulated at least 501-600 chill hours so far. This means any temperate woody perennial requiring less than 600 chill hours has most likely met its cool-temperature requirement and is ready for stage-two of breaking dormancy. In stage-2, the temperate woody perennial must then receive a certain number of growing degree-hours (warm-temperature requirement) in order to resume growth. Normally, most temperate woody plants adapted to our climate accumulate sufficient chilling hours by roughly mid-January, then begin accumulating growing degree-hours before resuming growth in the latter half of March. Where we normally run into problems is when temperate woody perennials accumulate their chill requirement too early in the season, resulting in accelerated budbreak and an increased risk of exposure to freeze damage. For this reason, low-chill cultivars should be avoided in our region. They tend to break bud at the first winter warm up, then are severely damaged by returning freezing temperatures. For example, 'Cresthaven' peach is well adapted to our region and is reported to require 950 chill hours. Compare that to 'UFBest,' a peach cultivar developed for central and south Florida, requiring only 100 chilling hours. If 'Cresthaven' is following the books, it should still be snug in its dormancy, while 'UFBest' would be primed for a winter kill.