A hardiness zone is a geographic area defined as having a certain average annual minimum temperature, a factor relevant to the survival of many plants. In some systems other statistics are included in the calculations. The original and most widely used system, developed by the United States Department of Agriculture (USDA) as a rough guide for landscaping and gardening, defines 13 zones by long-term average annual extreme minimum temperatures. It has been adapted by and to other countries (such as Canada) in various forms. A plant may be described as "hardy to zone 10": this means that the plant can withstand a minimum temperature of .
Unless otherwise specified, in American contexts "hardiness zone" or simply "zone" usually refers to the USDA scale. However, some confusion can exist in discussing buildings and HVAC, where "climate zone" can refer to the International Energy Conservation Code zones, where Zone 1 is warm and Zone 8 is cold.
Other hardiness rating schemes have been developed as well, such as the UK Royal Horticultural Society and US Sunset Western Garden Book systems. A heat zone (see below) is instead defined by annual high temperatures; the American Horticultural Society (AHS) heat zones use the average number of days per year when the temperature exceeds .
A plant's survival depends on factors other than its hardiness, including the amount of sunlight, rainfall, humidity, and frequency of snow cover, among other conditions. Trees and greenery foliage may provide dappled or full shade and they can act as wind breaks, as do greenhouses without artificial climate control. As a result, microclimates are created where some plants may unexpectedly thrive.
United States hardiness zones (USDA scale)
The USDA system was originally developed to aid gardeners and landscapers in the United States. In the United States, most of the warmer zones (zones 9, 10, and 11) are located in the deep southern half of the country, on the southern coastal margins, and on the Pacific coast.
Higher zones can be found in Hawaii (up to 12) and Puerto Rico (up to 13). The southern middle portion of the mainland and central coastal areas are in the middle zones (zones 8, 7, and 6). The far northern portion on the central interior of the mainland have some of the coldest zones (zones 5, 4, and small area of zone 3) and often have much less consistent range of temperatures in winter due to being more continental, especially further west with higher diurnal temperature variations, and thus the zone map has its limitations in these areas. Lower zones can be found in Alaska (down to 1). The low latitude and often stable weather in Florida, the Gulf Coast, and southern Arizona and California, are responsible for the rarity of episodes of severe cold relative to normal in those areas. The warmest zone in the 48 contiguous states is the Florida Keys (11b) and the coldest is in north-central Minnesota (2b).
A couple of locations on the northern coast of Puerto Rico have the warmest hardiness zone in the United States at 13b. Conversely, isolated inland areas of Alaska have the coldest hardiness zone in the United States at 1a.
Definitions
Zone From To0a< −65 °F (−53.9 °C)b−65 °F (−53.9 °C)−60 °F (−51.1 °C)1a−60 °F (−51.1 °C)−55 °F (−48.3 °C)b−55 °F (−48.3 °C)−50 °F (−45.6 °C)2a−50 °F (−45.6 °C)−45 °F (−42.8 °C)b−45 °F (−42.8 °C)−40 °F (−40 °C)3a−40 °F (−40 °C)−35 °F (−37.2 °C)b−35 °F (−37.2 °C)−30 °F (−34.4 °C)4a−30 °F (−34.4 °C)−25 °F (−31.7 °C)b−25 °F (−31.7 °C)−20 °F (−28.9 °C)5a−20 °F (−28.9 °C)−15 °F (−26.1 °C)b−15 °F (−26.1 °C)−10 °F (−23.3 °C)6a−10 °F (−23.3 °C)−5 °F (−20.6 °C)b−5 °F (−20.6 °C)0 °F (−17.8 °C)7a0 °F (−17.8 °C)5 °F (−15 °C)b5 °F (−15 °C)10 °F (−12.2 °C)8a10 °F (−12.2 °C)15 °F (−9.4 °C)b15 °F (−9.4 °C)20 °F (−6.7 °C)9a20 °F (−6.7 °C)25 °F (−3.9 °C)b25 °F (−3.9 °C)30 °F (−1.1 °C)10a30 °F (−1.1 °C)35 °F (1.7 °C)b35 °F (1.7 °C)40 °F (4.4 °C)11a40 °F (4.4 °C)45 °F (7.2 °C)b45 °F (7.2 °C)50 °F (10 °C)12a50 °F (10 °C)55 °F (12.8 °C)b55 °F (12.8 °C)60 °F (15.6 °C)13a60 °F (15.6 °C)65 °F (18.3 °C)b> 65 °F (18.3 °C)
History The first attempts to create a geographical hardiness zone system were undertaken by two researchers at the Arnold Arboretum in Boston; the first was published in 1927 by Alfred Rehder, and the second by Donald Wyman in 1938. The Arnold map was subsequently updated in 1951, 1967, and finally 1971, but eventually fell out of use completely.
The modern USDA system began at the US National Arboretum in Washington. The first map was issued in 1960, and revised in 1965. It used uniform ranges, and gradually became widespread among American gardeners.
The USDA map was revised and reissued in 1990 with freshly available climate data, this time with five-degree distinctions dividing each zone into new "a" and "b" subdivisions.
In 2003, the American Horticultural Society (AHS) produced a draft revised map, using temperature data collected from July 1986 to March 2002. The 2003 map placed many areas approximately a half-zone higher (warmer) than the USDA's 1990 map. Reviewers noted the map zones appeared to be closer to the original USDA 1960 map in its overall zone delineations. Their map purported to show finer detail, for example, reflecting urban heat islands by showing the downtown areas of several cities (e.g., Baltimore, Maryland; Washington, D.C., and Atlantic City, New Jersey) as a full zone warmer than outlying areas. The map excluded the detailed a/b half-zones introduced in the USDA's 1990 map, an omission widely criticized by horticulturists and gardeners due to the coarseness of the resulting map. The USDA rejected the AHS 2003 draft map and created its own map in an interactive computer format, which the American Horticultural Society now uses.
In 2006, the Arbor Day Foundation released an update of U.S. hardiness zones, using mostly the same data as the AHS. It revised hardiness zones, reflecting generally warmer recent temperatures in many parts of the country, and appeared similar to the AHS 2003 draft. The Foundation also did away with the more detailed a/b half-zone delineations.
In 2012 the USDA updated their plant hardiness map based on 1976–2005 weather data, using a longer period of data to smooth out year-to-year weather fluctuations. Two new zones (12 and 13) were added to better define and improve information sharing on tropical and semitropical plants, they also appear on the maps of Hawaii and Puerto Rico. There is a very small spot east of San Juan, Puerto Rico, that includes the airport in coastal Carolina, where the mean minimum is 67 degrees F (19 C), which is classified as hardiness Zone 13b, the highest category, with temperatures rarely below . The map has a higher resolution than previous editions, and is able to show local variations due to factors such as elevation or large bodies of water. Many zone boundaries were changed as a result of the more recent data, as well as new mapping methods and additional information gathered. Many areas were a half-zone warmer than the previous 1990 map. The 2012 map was created digitally for the internet, and includes a ZIP Code zone finder and an interactive map.
In 2015, the Arbor Day Foundation revised another map, also with no a and b subdivisions, showing many areas having zones even warmer, with the most notable changes in the Mid Atlantic and Northeast, showing cities like Philadelphia, New York City and Washington D.C. in zone 8, due to their urban heat islands.
In November 2023, the USDA released another updated version of their plant hardiness map, based on 1991–2020 weather data across the United States. The updated map shows continued northward movement of hardiness zones, reflecting a continued warming trend in the United States' climate.
Selected U.S. cities The USDA plant hardiness zones for selected U.S. cities as based on the 2023 map are the following:
CityZoneAlbuquerque, New Mexico 7b/8aAnchorage, Alaska 5a/5bAtlanta, Georgia 8a/8bAlmost all of the city proper is 8a while the airport is 8b.Austin, Texas 9aBaltimore, Maryland 7b/8aBirmingham, Alabama 8a/8bBoise, Idaho 7a/7bBoston, Massachusetts 6b/7aBuffalo, New York 6bCharlotte, North Carolina 8aChicago, Illinois 6a/6bCincinnati, Ohio 6bCleveland, Ohio 6b/7aColorado Springs, Colorado 5b/6aColumbus, Ohio 6bDallas, Texas 8bDenver, Colorado 6aDetroit, Michigan 6bEl Paso, Texas 8b/9aFairbanks, AlaskaThe coldest city over 10,000 people in the United States. 2aFort Worth, Texas 8bFresno, California 9bGreensboro, North Carolina 8aHartford, Connecticut 6b/7aHonolulu, Hawaii 12bHouston, Texas 9bIndianapolis, Indiana 6a/6bJacksonville, Florida 9a/9bKansas City, Missouri 6b/7aLas Vegas, Nevada 9a/9bLos Angeles, California 10a/10bSmall portions of the city of Los Angeles extend into zones 9b and 11a.Louisville, Kentucky 7aMemphis, Tennessee 8aMiami, Florida 11aMilwaukee, Wisconsin 5b/6aMinneapolis, Minnesota 5aNashville, Tennessee 7bNew Orleans, Louisiana 9bNew York, New York 7bOklahoma City, Oklahoma 7bOmaha, Nebraska 5b/6aOrlando, Florida 10aPhiladelphia, Pennsylvania 7bPittsburgh, Pennsylvania 6b/7aPhoenix, Arizona 9b/10aPortland, Oregon 8b/9aRaleigh, North Carolina 8aReno, Nevada 7a/7bSacramento, California 9bSalt Lake City, Utah 7a/7bSan Antonio, Texas 9aSan Diego, California 10b/11a9b and 10a also exist in areas of the northern city proper away from the coast.San Francisco, California 10a/10bSan Jose, California 9b/10aSan Juan, Puerto Rico 13a/13bSeattle, Washington 9aSt. Louis, Missouri 7aTampa, Florida 10aTucson, Arizona 9a/9bTulsa, Oklahoma 7bVirginia Beach, Virginia 8a/8bWashington, D.C. 7b/8aWichita, Kansas 7a
Limitations As the USDA system is based entirely on average annual extreme minimum temperature in an area, it is limited in its ability to describe the climatic conditions a gardener may have to account for in a particular area: there are many other factors that determine whether or not a given plant can survive in a given zone.
Zone information alone is often not adequate for predicting winter survival, since factors such as frost dates and frequency of snow cover can vary widely between regions. Even the extreme minimum itself may not be useful when comparing regions in widely different climate zones. As an extreme example, due to the Gulf Stream most of the United Kingdom is in zones 8–9, while in the US, zones 8–9 include regions such as the subtropical coastal areas of the southeastern US and Mojave and Chihuahuan inland deserts, thus an American gardener in such an area may only have to plan for several nights of cold temperatures per year, while their British counterpart may have to plan for several months.
In addition, the zones do not incorporate any information about duration of cold temperatures, summer temperatures, or sun intensity insolation; thus sites which may have the same mean winter minima on the few coldest nights and be in the same garden zone, but have markedly different climates. For example, zone 8 covers coastal, high latitude, cool summer locations like Seattle and London, as well as lower latitude, hot-summer climates like Charleston and Madrid. Farmers, gardeners, and landscapers in the former two must plan for entirely different growing conditions from those in the latter, in terms of length of hot weather and sun intensity. Coastal Ireland and central Florida are both Zone 10, but have radically different climates.