Dormancy is not exclusive to vegetation. Many animals are dormant while they hibernate through cold winter weather. Some animals are dormant while they aestivate through hot and dry weather. Fungi and bacteria can maintain dormancy for many years or centuries. Dormancy is a technique for avoiding unpleasant situations, such as cold or hot weather.

Like some animals, fungi and bacteria, some vegetation is dormant through dry weather. This is evident within local chaparral climates and nearby desert climates. Even within a few coastal forests, buckeye defoliates during summer. Such vegetation resumes activity in response to late autumn rain. However, most vegetation here prefers winter dormancy.

Such winter dormancy is especially evident among deciduous species. Many evergreen species merely halt or decelerate their growth, but without defoliation. Several evergreen species defoliate only partially. A few only shed their old foliage as new foliage replaces it during the following spring. Few species prefer to grow most vigorously through winter.

There are a few advantages to defoliation for winter. Wintry wind blows much more freely through defoliated canopies. Evergreen canopies are therefore more vulnerable to wind. In other harsher climates, defoliated canopies do not accumulate much snow. Evergreen canopies might accumulate more snow than they can support. Some get heavy with rain.

Foliage is not as useful through winter as it is through summer anyway. The sun is lower to the south. Consequently, less sunlight gets through more of the atmosphere. Days are shorter while nights are longer. Furthermore, cooling autumn and winter weather inhibits photosynthesis. For deciduous species, foliage becomes less of an asset than a liability.

Dormancy, whether it involves defoliation or not, facilitates a few horticultural techniques. Bare root stock becomes available at the beginning of winter. Transition from its farms to home gardens is much less stressful during dormancy. So is the aggressive pruning that roses and deciduous fruit trees require. Most pollarding is safest during winter dormancy.