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• USDA Hardiness Zones
• How USDA Maps Created
• How Map was Started
• Basic Map  Requirements
• Why New USDA Hardiness Map

Every plant can adapt to a range of environments.  Gardeners have learned through experience where the great variety of landscape plants can be grown.  Over the years many schemes have been proposed to help gardeners locate those environments when they introduce new species, forms, and cultivars.  The pooling of many of the schemes culminated in the development of the widely used "Plant Hardiness Zone Map," under the supervision of Henry T. Skinner, the second director of the U.S. National Arboretum.  In cooperation with the American Horticultural Society, he worked with horticultural scientists throughout the United States to incorporate pertinent horticultural and meteorological information into the map.

The elements of that map were:

Zones.  The contiguous United States and southern Canada were divided into 10 zones based on a 10ºF (5.6ºC) difference in average annual minimum temperature.

Winter Hardiness.  Survival of landscape plants over winter was selected as the most critical criterion of their adaptation to the environment.

Classification.  The zone ratings were intended to indicate excellent adaptability of the plants.  Many plants may survive in warmer or colder zones.  Usually, mere survival does not represent satisfactory performance.

Interactions With Other Environmental Factors.  Many other factors may come into plant in determining satisfactory growth.  Wind, soil type, soil moisture, humidity, snow, and winter sunshine may greatly affect the adaptability of plants.

Interactions With Cultural Factors.  The way plants are placed in the landscape, how they are planted, and their size and health can greatly influence satisfactory adaptability.

© Notice: All information on this and related pages is directly from the USDA Agricultural Research Service, Miscellaneous Publication Number 1475 "USDA Plant Hardiness Zone Map".  This publication is not copyrighted, and permission to reproduce all or any part of it is not required.