Chilling requirements

Chilling requirements



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Chilling Requirements

Let’s take a break from all this talk about the cold. Close your eyes and imagine yourself outdoors, in your shirtsleeves, on a warm, sunny, windless January day. Most cold regions experience at least one, if not more, warm spells—or "January thaws"—during each winter. We humans come out of our "hibernation" to enjoy the sunshine, so why don’t our plants break dormancy and start growing?

Suppose a tree were to break dormancy during a January thaw. The new, delicate growth would quickly succumb to the next cold spell. How do plants know when it’s really spring—and it’s safe to begin growing?

Plants have evolved strategies to keep from being "fooled" into thinking it’s spring, before it really is. Many plants native to temperate regions have specific chilling requirements. A plant’s chilling requirement is the number of hours the plant must be exposed to temperatures between 32F and 45F before the plant breaks dormancy. (Times when the temperature drops below 32F or rises above 45F don’t count toward the chilling requirement.) Chilling requirements are generally measured in hours; incredibly, plants are somehow able to keep track of the number of hours they are exposed to this very specific range of temperature.

While gardeners in the north worry more about cold hardiness when choosing plants, those in warm regions must also understand chilling requirements. Northern Florida usually gets between 400 and 600 hours of chilling, while Nashville gets close to 1200. Companies specializing in fruit trees for warm regions will list each plant’s chilling requirement—it’s wise to research your area’s average number of chill hours, and try to match the two. Areas north of Tennessee usually get more than enough chilling for most plants!

And in warm regions that don’t experience adequate winter chilling, if you want to enjoy spring-flowering bulbs like tulips and daffodils, you’ll need to dig them up at the end of the growing season and artificially provide them with the necessary chilling. Make room in your refrigerator, and keep the bulbs away from ripening fruit (which can stimulate growth and damage the bulbs).

We’ve been focusing on plant adaptations for enduring cold weather. But plants native to regions that experience hot, dry weather have their own adaptations for enduring those challenging conditions. Some desert plants actually enter dormancy during the summer months to avoid the damaging heat, and do their growing and reproducing during the cooler winter months.

And, finally, no discussion of dormancy would be complete without at least touching on that most incredible of adaptations, the seed. With their hard seed coats protecting the delicate plant embryos within, seeds are miracles of evolution. Many seeds are able to endure long periods of drought, heat, or freezing temperatures—yet are ready to spring to life when favorable conditions arise. Depending on the adaptations they’ve made to their native environments, different plants’ seeds may need darkness, light, warmth, chilling, or even exposure to fire before they’ll germinate!

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Chill Out!
Many spring- flowering bulbs, including tulips, daffodils, and hyacinths, require several weeks of chilling before they’ll begin growing. If you are trying to "force" bulbs to flower indoors, be sure to research and meet the bulbs’ chilling requirements. Some nurseries sell pre-chilled bulbs especially for forcing.

Similarly, pussy willow, forsythia, and fruit tree branches brought indoors in the fall won’t sprout and flower. But the same branches brought indoors in early spring will provide a wonderful display—because their chilling requirements have been met.


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