The U.S. Department of Agriculture (USDA) developed a map of average annual winter minimum temperatures across North America that determines plant survivability, called hardiness.
Helping gardeners select plants that tolerate winter temperatures in their region became a widespread standard. Each hardiness zone is defined by a 10-degree Fahrenheit (5.6-degree Centigrade) range of the expected minimum temperature in winter. Additional coding of "a" or "b" further divides each zone, allowing differentiation between the chillier "a" sub-zone from the warmer "b" sub-zone.
Undertaken by Alfred Rehder, the first published map for plant hardiness occurred in 1927 in his Manual of Cultivated Trees and Shrubs. In 1940 the second edition of this book displayed the first map based on annual minimum winter temperatures, the precedent for modern zones. Peter del Tredici, editor of the Arnoldia of the Arnold Arboretum of Harvard University, discusses these precursors to and history of the USDA map at arnoldia.arboretum.harvard.edu/pdf/articles/807.pdf.
History and Evolution
In 1960 the U.S. National Arboretum, with assistance from the American Horticultural Society and numerous horticulturists and meteorologists, released the first "Plant Hardiness Zone Map." The first revision occurred in 1965, with another large-scope expansion and revision in 1990. The 1990 version, the most common map encountered in publications, uses the numeric zone with a/b sub-zones. Some maps, as published or modified, no longer display sub-zones.
Online resources usually cite the 1990 or electronic 2003 version (usna.usda.gov/Hardzone/ushzmap.html).
The USDA is no longer the only entity publishing a cold hardiness zone map. The 2006 map created and copyrighted by the National Arbor Day Foundation reflects recent climatic changes evident in the last decade of the 20th century; in comparison it shows a general northward shift of the traditional USDA cold hardiness zones.
In 2008, final drafting of a USDA-updated map was pending, taking into account climatic changes by averaging them with data that was used in the 1990 map. Adding four zones, 12-15, that better delineate winter minimum temperatures in tropical regions of North America, such as Mexico and the Caribbean, was anticipated and already referenced in some horticultural books, such as the AHS's A-Z Encyclopedia of Garden Plants (2004).
Although most heavily utilized in English-speaking countries and occasionally across Europe, horticulturists worldwide apply these zone labels for universal climate and hardiness comparisons as new plants are discovered, bred or shared across national borders.
The USDA map is a standard in the horticulture and plant nursery industry for the appropriate selection and use of ornamental plants. Professionals and home gardeners determine which hardiness zone matches their location and should seek plants rated hardy for their zone. The zone rating is found on plant labels or in published literature and is seen in a format such as "USDA zone (or winter hardiness zones) 4 to 6" (i.e. the plant is adapted to survive the winter minimum temperatures in zones 4, 5 and 6 without human intervention).
Seasonal variations in temperature can temporarily render the USDA zone information slightly skewed, but on the average it is relevant for the educated selection of plants hardy to local winter temperatures in a particular zone.
The majority of horticulturists and plant growers only cite the bona-fide USDA zones, their respective temperature parameters and most recently approved maps when promoting or referencing plant materials. Confusion occurs when new, unapproved or conversational zone maps that follow USDA parameters are used as a means to market new plants in regions previously labeled as a "colder zone."
Although significant merit is given to the USDA Hardiness Map, it is not an absolute guarantee of any individual plant's survivability or performance in the garden. This tool only rates hardiness based on survivability to the average minimum winter temperatures. The map does not address other factors affecting plant hardiness, such as soil type or moisture, humidity, duration of cold, snow cover, expected winter sunshine and wind speeds.
Additional Hardiness Zones
Other tools developed bring into account factors of plant hardiness. In 1997 the Plant Heat-Zone Map divided the continental United States into 12 zones based on the average number of days in summer above 86 degrees Fahrenheit (30 degrees Centigrade). Sunset magazine created a tool for American horticulturists that brings elevation, latitude, microclimates and ocean influences into a broaden scope of plant hardiness in more than 40 categories for all of Canada and the United States.