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

The "Plant Hardiness Zone Map" was published in 1960 and revised in 1965.  Since then, many changes, new interests, and new responsibilities have emerged in North American Landscaping.

Changes in Weather.  We have been losing from our landscapes plants that apparently survived in the 1940's to the 1960's.  Many of the hardiness zone classifications of plants are no longer considered valid.  In North America, the ranges of temperature and moisture for the past decade were wider than those recorded for the 1940's through the 1960's.

Introduction of new Elite Forms.  Our landscape industry has worked with plant explorers and breeders to introduce many new forms of traditional plants that are adapted to a wider range of environments than the older forms.

Scope.  The continental United States is contiguous with Canada to the north and Mexico to the south.  We share more than a common border -- we share indigenous plants and many introduced plants that can be grown successfully.  We needed to expand the scope to include the whole of North America.

© 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.