This edition of the USDA Plant Hardiness Zone Map (PHZM) is GIS (Geographic Information System)-based for the first time. This is also the first USDA PHZM that is specifically designed for the Internet. It enables viewers to examine plant hardiness zones at a much finer scale than ever before. For the first time, a very sophisticated algorithm was used to interpolate low-temperature values between actual weather reporting stations. A personal ZIP Code zone finder is also included with this version of the map. Simply type your ZIP Code in the proper box and your zone will be reported.
Zones in this edition of the USDA Plant Hardiness Zone Map (PHZM) are based on 1976–2005 weather data. A trial check did not find that the addition of more recent years of data made a significant difference in the definition of these zones. Each zone represents the mean extreme minimum temperature for an area, calculated from the lowest daily minimum temperature recorded for each of the years 1976–2005. This does not represent the coldest it has ever been or ever will be in an area, but it simply is the average of lowest winter temperatures for a given location for this time period.
The previous edition of the USDA PHZM, revised and published in 1990, was drawn from weather data for 1974–1986. The longer period (30 years) of data was selected by the group of horticultural, botanical, and climatological experts who led the review of the latest revision as the best balance between smoothing out the fluctuations of year-to-year weather variation and the concept that during their lifetimes, perennial plants mostly experience what is termed "weather" rather than "climate."
Two new zones have been added to this edition of the USDA PHZM. Zones 12 and 13 have been introduced for regions with average annual extreme minimum temperatures above 50 degrees and 60 degrees F, respectively. They only appear on the maps for Hawaii and Puerto Rico. But the additional frost-free zones will enable better definition of conditions for tropical and semitropical plants, which often serve as house or patio plants in many parts of the country. The two new zones will provide a way to share information about differences in cold sensitivity of tropical ornamental plants and may help gardeners decide when to bring tropical plants indoors from a deck or patio as the temperature cools.
Because this map was created digitally with GIS technology, it has a higher level of resolution and can show smaller areas of zone delineations than ever before. For example, cities tend to hold more heat because they have large amounts of concrete and blacktop, so a city or town may be assigned to a zone warmer than the surrounding countryside. Higher elevations tend to be colder than surrounding lower areas, so the top of a mountain may be an area of cooler zones. A location near a large body of unfrozen water may provide milder winter weather and be in a warmer zone.
Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years. Because the USDA PHZM represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming.
Compared with the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new PHZM is generally one half-zone warmer than the previous PHZM throughout much of the United States, as a result of a more recent averaging period (1974–1986 vs. 1976–2005). However, some of the changes in the zones are the results of the new, more sophisticated mapping methods and greater numbers of station observations used in this map, which has greatly improved accuracy, especially in mountainous regions. These changes are sometimes to a cooler, rather than warmer, zone.
Canada and Mexico
The 1990 PHZM included Canada and Mexico for the first time. In this version, the latest mapping technology was focused on creating the highest-quality PHZM for the United States and Puerto Rico. Canada has developed its own plant hardiness zone map, which can be accessed at http://atlas.agr.gc.ca/agmaf/. Mexico does not yet have a similar map.
U.S. National Arboretum
The U.S. National Arboretum (USNA) website includes a list of cold hardiness ratings for selected woody plants which may be found at the following link: http://www.usna.usda.gov/Hardzone/hrdzon4.html#5.