In fall, deciduous trees show signs of going into a dormant period. The leaves of these trees change colors from green to brown and fall. But trees are not the only plants to go dormant in winter. Nearly every perennial plant that grows outdoors in temperate climates experiences a dormancy period.
Not all plants in temperate climates go dormant during winter. Annual plants do not go dormant in winter and instead die with the first cold frost. Plants that enter a dormant period survive through the winter cold.
During dormancy, plants slip into a period of rest that could be likened to an animal's hibernation. Because most plants cannot withstand the harsh temperatures for an extended period of time, the dormancy period protects them from damage. Plants that are in their dormant phase do not grow during this time.
Although it looks like a plant such as a tree goes dormant in fall when temperatures begin to cool off, many plants actually begin the process of going dormant in late summer when temperatures are at their hottest. This process is called hardening off. A plant begins to harden off for the winter in response to shorter daylight hours. Even though evergreen trees do not shed their needles the way that deciduous trees shed their leaves, the plants still stop growing and go dormant.
If plants did not harden off before temperatures turned cool, they would continue to produce new growth. This new growth would become damaged from sudden frosts and cool weather as temperatures begin to fall. During this time, trees and shrubs that produce flowers in spring also begin to set flower buds.
In early spring, many regions experience a temporary warm period. During this time, a plant will not emerge from dormancy and become injured when cold weather returns. This is because the plant has not yet been exposed to weather temperatures between 32 and 45 degrees F for a long enough time period. This length of time is known as a plant's chilling requirements. Plants that go dormant have a specific number of cold weather hours that they must experience before they will break dormancy.