During the mid-1900s, the U.S. Department of Agriculture (USDA) mapped out the entire United States, Mexico, and Canada by lowest annual minimum temperature groupings. Each zone represented a 10 degree Fahrenheit difference. This was invaluable advice for the agriculture industry. Now plants could be rated by hardiness zones, taking the guesswork out of choosing plant varieties. You had a gauge, other than experience, for picking plants.
The maps have been revised over the years to reflect changes in climate. When cities and towns were moved from one zone to another, gardeners were left to wonder what would happen to their existing garden plants. While our climate may be shifting, these changes did not occur overnight. Plants are adaptable, surviving in many different climates. (They also cannot read maps!)
In 1990, the zones were further divided, with each numbered zone being broken down into an A and B sectors with A representing the lower temperature end of the zone, and B the higher. Unfortunately, plant breeders have yet to start using these distinctions, so they are really only useful if a gardener wants to push the envelope a bit; for example, a gardener in zone 6B might be tempted to dabble in 7A plants.
The Newest USDA Plant Hardiness Zone Map for the United States
Gardeners have come to rely on USDA Hardiness Zone Maps to tell them what plants will survive in their climates. In 2012, USDA released updated Plant Hardiness Zone Maps for the United States, each individual state, each geographic region, and Puerto Rico.
Plant hardiness zones are based on the average annual minimum temperature over a 30-year period, not the lowest temperature the thermometer has ever hit. They are guides to help you determine which plants will survive in your area; they are not steadfast law. Many circumstances besides hardiness zones, like soil quality, sun exposure, moisture, and sudden or prolonged temperature swings, can affect how well plants will do in your yard. There are also always microclimates that will vary within zones. Trust your own knowledge of your property and your garden.
About the 2012 Plant Hardiness Zone Maps (PHZM)
The 2012 PHZM boasts some significant improvements over earlier versions, though they may still not reflect very recent climate changes. Among the improvements:
- The resolution is noticeably better. Mapping was improved by using a Geographic Information System (GIS). The higher resolution makes seeing the distinction between zones clearer and easier. It also makes it possible to separate small anomalies, like cities, which tend to be warmer than surrounding areas, cooler high elevation area, and areas near mitigating large bodies of water, into their proper zones.
- It's the first map designed to be viewed via the internet. Zones can be zoomed in on a finer scale.
- A more sophisticated algorithm was used to compile low-temperature values from actual weather reporting stations.
- The zones are based on 1976–2005 weather data. The 30-year period was chosen as a means to factor in year-to-year fluctuations and variations. They did a trial check on more recent data and it did not make a measurable difference in zones. (The previous map was based on 1974–1986 data.)
Climate Change Conclusions: Climate changes are usually based on weather averages in a 50- to 100-year period. This map uses only a 30-year average and is not meant as evidence of temperature changes. While most zones are ½ zone warmer than the previous map, the change can in part be attributed to better mapping and weather tracking. The USDA points out that some mountainous regions that had been hard to observe are now in cooler zones.
Interactive Map: There's an interactive map that allows you to enter your zip code or click on the map and get your zone, actual average temperature, the temperature range for your zone, and your longitude and latitude.
USDA Plant Hardiness Zone Maps for Alaska, Hawaii, and Puerto Rico
Alaska and Hawaii are not included on the same map with the lower 48 states. They have their own separate map.
Two new zones were added to the warm end of the scale. They apply only to the climates of Hawaii and Puerto Rico, but they will also be useful to anyone who grows tropical and semitropical plants indoors, by better defining the growing conditions these plants favor and in particular, their sensitivity to cold.
- Zone 12: average annual extreme minimum temperatures above 50 F
- Zone 13: average annual extreme minimum temperatures above 60 F
USDA Regional Plant Hardiness Zone Maps
In addition to the main U.S. map and the individual state maps, there are also six regional maps:
You won't get as much detail from the regional maps, but you do get a sense of microclimates and how the geography affects the zones. Within miles of each other, higher altitudes in the mountains and coastal areas of Arizona can be two to three zones cooler than urban and desert areas.
International Hardiness Zones
Both Canada and Mexico were included in the 1990 Plant Hardiness Zone Mapping. Canada has released its map, but there is currently no new map available for Mexico. Maps are also available for a number of international locations.