By: M. Bodman, MCMGA
Chill hours, chill time, chilling requirement are terms bandied about this time of year. These “secret” phrases do not refer to favorite recipes, but to the cold weather needed to send some of our favorite fruit trees into dormancy and keep them there until Spring. Stone fruits (e.g., peaches, plums, cherries) and most pome fruits (e.g., apples, pears) require cold temperatures to trigger dormancy and prepare for fruiting the coming Spring. It is theorized that these trees originated in the temperate regions of Asia and the Middle East where winters are consistently cold, but not brutally freezing. Accordingly, these trees do best where there are long winters and the temperatures range mostly between 32°F and 45°F (0°C and 7°C), with only a few days above 55°F or below 20°F. If winter is too warm (insufficient chilling), the trees may not flower; if they flower, they may not fruit; if they bear fruit, the fruit will likely be misshapen.
This results in the simple rule of thumb that chill time is counted when temperatures are between 32°F and 45°F; above 45°F and below 32°F no chilling time is accumulated. More complicated formulae have been developed that more accurately anticipate the performance of these trees. Montgomery County Master Gardeners use one of these more elaborate formulae when evaluating fruit trees for the area.
It can be a little tricky selecting just the right fruit trees for the Montgomery County area. Records for the past 15 years indicate that the Montgomery County area has experienced ~150 to ~1000 hours of chilling time:
Some of the best-known fruit (e.g., Elberta peaches, McIntosh and Honeycrisp apples) require over 850 hours of chilling. If all stone and pome fruit required over 850 hours of chilling then we would have had fruit only twice in the last 15 years. Fortunately, through selective breeding, cultivars of these fruits have been developed with low chilling requirements. While some varieties have very low chill requirements (200 hours or less) we tend to recommend those requiring about 300 – 400 hours. These plants tend to stay dormant long enough to avoid the typical March freezes. Even then the trees do get caught in really late freezes such as we experienced March 27, 2013:
And there goes fruit production for that year!
Montgomery County Master Gardeners have tested several varieties in test plots and their personal gardens. Our recommendations come from this experience and are in evidence at our annual Fruit and Nut Tree Sales. Wherever gardeners purchase their fruit trees they should check the chilling requirements. Regardless of the selections we make, we still have years that “fail”; this makes us that much more appreciative of the years when conditions are “just right” for a bountiful harvest.