From Seed to Seed:
Plant Science for K-8 Educators


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Do plants hibernate?

How do trees know that the cold weather is coming? What makes them begin slowing down and preparing for dormancy? The obvious answer would be that plants begin the process of entering dormancy in response to the cooler temperatures of autumn. In truth, many trees and shrubs begin this process during the "dog days" of August. They can't possibly be responding to cooler temperatures during this hot weather!

Plants have evolved a range of strategies for thriving in challenging environments. Not only are these adaptations interesting, but many are also of particular interest to teachers using gardening as a teaching tool. Let's take a look at how plants change in response to temperature on a seasonal basis.

    How Plants Prepare for Winter

In all but the warmest regions, most plants go through a period of dormancy during the winter. Actively growing plants cannot withstand freezing temperatures for an extended time. As a result, plants have adapted by going dormant during the coldest months. During dormancy, growth stops and the plant remains in a state of rest until good growing conditions return.

    Hardening Off

Although August and September can bring some of the summer's warmest weather, by this time most perennial plants have begun preparations for their annual rest. The hardening-off process is, at least in part, a photoperiodic response; that is, it is a response to changes in day length. Many plants, notably the spring-flowering trees and shrubs, set their flower buds at this time. As the days grow shorter, plants slow and finally stop any new growth. Then the plant withdraws nourishment from the leaves and enters a fully dormant phase.

Different types of plant tissues respond differently to temperature extremes. A peach tree's leaves and branches, for instance, may be able to withstand temperatures of 10F. But the flower buds may be damaged at temperatures below 0F. If the buds are damaged, then the next season's flowers and fruit will be affected. Peach trees thrive in places like Georgia where minimum winter temperatures generally do not go below 0F.

Plants that have not finished the hardening-off process can be damaged by the onset of cold temperatures. Some factors that influence whether or not damage occurs include: the type of plant and its overall health, what stage it had reached in the hardening-off process, and whether the onset of low temperatures was gradual or sudden.

    Chilling Requirements

Let's take a break from all of 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 warm spell-or "January thaw"-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 is really spring-and it is safe to begin growing?

Plants have evolved strategies to keep from being "fooled" into thinking that it is 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 that 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 do not 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.

Relevant Books
Simon, Seymour. 1997. Ride the Wind: Airborne Journeys of Animals and Plants. Browndeer Press. ISBN: 0-1529-2887-1.






















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