USDA Miscellaneous Publication No. 1475, Issued January 1990
Authored by Henry M. Cathey while Director, U.S. National Arboretum
Origins of the Zone Map
Originally developed in 1960 by Henry Skinner while he was the Director of the U.S. National Arboretum, the Plant Hardiness Zone map has become widely used by horticulturists, landscape professionals and recreational gardeners. Director Skinner worked with the American Horticultural Society in an effort to provide guidance on horticultural and meteorological information into one map. The map shows plant hardiness by zone. However, it only shows hardiness to cold temperatures, not warm or hot temperatures. This map was first revised in 1965 and again in 1990.
Development and Revisions
In 1990, 2 more zones were added to include Canada and Mexico. Additionally, all other map information was adjusted as necessary to accommodate any major climate trends over the past 30 years. In order to gain accurate data for this revision of 14,500 stations that measured temperature during the period of interest, almost 8,000 could be identified by latitude and longitude and by a valid average annual minimum temperature (i.e., an average based on at least 10 years of data). Data from only the latter stations were used in the map. (1)
The current version of the map can be accessed using the U.S. National Aboretum's web site, http://www.usna.usda.gov/Hardzone/ushzmap.html
Description of the Map
The USDA Hardiness Zone map divides North America into eleven zones based on temperature and each is referred to by the zone number, i.e. Zone 5. Additionally, each zone is subdivided into two sections, A and B to further delineate temperature ranges, i.e. Zone 5a. The zones can be thought of as a general guidance as there is no clear-cut delineation between zones. Any location may be warmer or colder based on topography, drainage or wind exposure.
Plant Hardiness
Plant hardiness refers to a plant’s ability to withstand and survive temperature extremes. " Hardiness is affected by duration and intensity of sunlight, length of growing season, amount and timing of rainfall, length and severity of summer drought, soil characteristics, proximity to a large body of water, slope, frost occurrence, humidity, and cultural practices."(1) Remarkably, many plants that are grown as annuals in colder climates, are actually perennials in warmer climates depending upon its hardiness. When purchasing plants or seeds, check the label or packaging. Most will state the specific zone hardiness for that particular cultivar.
1) http://www.usna.usda.gov/Hardzone/index.html
2) Erv Evans, Consumer Horticulturalist, http://www.ces.ncsu.edu/depts/hort/consumer/weather/hardiness_zones.html