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(See table below)
Temperature scale used to define USDA hardiness zones. These are annual extreme minimums (an area is assigned to a zone by taking the lowest temperature recorded there in a given year). As shown, the USDA uses a GIS dataset averaged over 1976 to 2005 for its United States maps.



Unless otherwise specified, "hardiness zone" or simply "zone" usually refers to the USDA scale. For example, a plant may be described as "hardy to zone 10": this means that the plant can withstand a minimum temperature of −1 °C (30.2 °F) to 3.9 °C (39.0 °F).

Other hardiness rating schemes have been developed as well, such as the UK Royal Horticultural Society and US Sunset Western Garden Book systems.

United States hardiness zones (USDA scale)Edit

The USDA system was originally developed to aid gardeners and landscapers in the United States.

State-by-state maps, along with an electronic system that allows finding the zone for a particular zip code, can be found at the USDA Agricultural Research Service (USDA-ARS) website.

In the United States, most of the warmer zones (zones 9, 10, and 11) are located in the deep southern half of the country and on the southern coastal margins. Higher zones can be found in Hawaii (up to 12) and Puerto Rico (up to 13). The middle portion of the mainland and central and northern 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, 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 few episodes of severe cold relative to normal in those areas. The Pacific Ocean keeps the Pacific Northwest in warmer zones than nearby inland areas. The warmest zone in the 48 contiguous states is the Florida Keys (11b) and the coldest is in north-central Minnesota (3a).


2012 update of the Hardiness Zone Map
Zone From To
b > +12.8 °C (55 °F)


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,[1] and the second by Donald Wyman in 1938.[2] 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 10 degree Fahrenheit ranges, and gradually became widespread among American gardeners.[3][4]

The USDA map was revised and reissued in 1990 with freshly available climate data, this time with 5-degree distinctions dividing each zone into new "a" and "b" subdivisions.



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.[7] Two new zones 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. The map has a higher resolution than previous ones, and is able to show local variations due to things 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.[8] The 2012 map was created digitally for the internet, and includes a ZIP Code zone finder and an interactive map.[9][10]

Selected U.S. citiesEdit

The USDA plant hardiness zones for selected U.S. cities as based on the 2012 map are the following:

City Zone City Zone
Albuquerque, New Mexico 7b Oklahoma City, Oklahoma 7a
Anchorage, Alaska 4b/5a Omaha, Nebraska 5b
Atlanta, Georgia 8a Orlando, Florida 9b
Baltimore, Maryland 7b/8a Panama City Beach, Florida 9a
Boston, Massachusetts 6b/7a Philadelphia, Pennsylvania 7a/7b
Buffalo, New York 6a Phoenix, Arizona 9b/10a
Burlington, Vermont 5a Pierre, South Dakota 4b/5a
Charleston, South Carolina 8b/9a Pittsburgh, Pennsylvania 6b
Charleston, West Virginia 6b Pocatello, Idaho 5b
Chicago, Illinois 6a Portland, Maine 5b
Chattanooga, Tennessee 7a/7b Providence, Rhode Island 6b
Columbus, Ohio 6a Quad Cities, Iowa/Illinois 5b
Dallas, Texas 8a/8b Raleigh, North Carolina 7b
Denver, Colorado 5b/6a Reno, Nevada 6b/7a
Detroit, Michigan 6b Roanoke, Virginia 7a/7b
Fairbanks, Alaska 2a Sacramento, California 9b
Hartford, Connecticut 6b Salt Lake City, Utah 7a/7b
Honolulu, Hawaii 12b San Antonio, Texas 8b/9a
Houston, Texas 9a San Diego, California 10a/10b
Juneau, Alaska 6b/7a San Gabriel, California 10a
Kansas City, Missouri 6a/6b San Jose, California 9b/10a
Las Vegas, Nevada 9a San Juan, Puerto Rico 12b/13a
Los Angeles, California 10a/10b Savannah, Georgia 8b
Memphis, Tennessee 7b/8a Seattle, Washington 8b/9a
Miami, Florida 10b/11a Tampa, Florida 9b/10a
Minneapolis, Minnesota 4b/5a Tucson, Arizona 9b
Nashville, Tennessee 7a Tuscaloosa, Alabama 8a
New Orleans, Louisiana 9b Utqiagvik, Alaska 2b
New York, New York 7a/7b Washington, D.C.[11] 7a/7b
Norfolk, Virginia 8a Wichita, Kansas 6b


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, 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 days 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 summer temperature or insolation; thus sites which may have the same mean winter minima, but markedly different summer temperatures, will be accorded the same hardiness zone. 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 seasons from those in the latter.



There are many other climate parameters that a farmer, gardener, or landscaper may need to take into account as well, such as humidity, precipitation, storms, rainy-dry cycles or monsoons, and site considerations such as soil type, soil drainage and water retention, water table, tilt towards or away from the sun, natural or manmade protection from excessive sun, snow, frost, and wind, etc. The annual extreme minimum temperature is a useful indicator, but ultimately only one factor among many for plant growth and survival.[9][3][12]




In addition, the Koppen climate classification system can be used as a more general guide to growing conditions when considering large areas of the Earth's surface or attempting to make comparisons between different continents.[14] The Trewartha climate classification is often a good "real world" concept of climates and their relation to plants and their average growing conditions.[citation needed]

Climate change projectionsEdit

Recent research suggests that USDA plant hardiness zones will shift northward under climate change.[15]

Australian hardiness zonesEdit

The USDA hardiness zones are not used in Australia. The Australian National Botanic Gardens have devised another system more in keeping with Australian conditions.[16] They are numerically about 6 lower than the USDA system. For example, Australian zone 3 is roughly equivalent to USDA zone 9. The higher Australian zone numbers have no US equivalents.

There are problems with classifications of this type: the spread of weather stations is insufficient to give clear zones and too many places with different climates are lumped together. Only 738 Australian stations have records of more than ten years (one station per 98,491 hectares or 243,380 acres), though more populated areas have relatively fewer hectares per station. Local factors such as aspect, altitude, proximity to the sea also complicate the matter. For example, Mount Isa has three climatic stations with more than a ten-year record. One is in zone 4a, one in zone 4b, and the other is in zone 5a. Likewise, Sydney residents are split between zones 3a and 4b. Most other cities have similar problems. Different locations in the same city are suitable for different plants, making it hard to draw a meaningful map without publishing a list of weather stations and their zone classification to allow best use of local conditions.

Canadian hardiness zonesEdit

Climate variables that reflect the capacity and detriments to plant growth are used to develop an index that is mapped to Canada's Plant Hardiness Zones.[17] This index comes from a formula originally developed by Ouellet and Sherk in the mid-1960s.[18][19][20]

The formula used:


Y = estimated index of suitability
X₅ = winter factor expressed in terms of (0 °C – X₁)Rjan where Rjan represents the rainfall in January expressed in mm
X₇ = maximum wind gust in (km/hr) in 30 years.

City Zone[17]
Calgary 4a
Edmonton 4a
Halifax 6b
Montreal 6a
Ottawa 5b
Saskatoon 3b
St. John's 6a
Toronto 7a
Vancouver 8b
Winnipeg 4a
Yellowknife 0a

For practical purposes, Canada has adopted the American hardiness zone classification system. The 1990 version of the USDA Plant Hardiness Zone Map included Canada and Mexico, but they were removed with the 2012 update to focus on the United States and Puerto Rico.[8]

European hardiness zonesEdit

British IslesEdit

Britain and Ireland's hardiness zones


  • 7. In Scotland the Grampians, Highlands and locally in the Southern Uplands, in England the Pennines, and in Wales the highest part of Snowdonia.
  • 9. Most of western and southern England and Wales, western Scotland, also a very narrow coastal fringe on the east coast of Scotland and northeast England (within 5 kilometres (3.1 mi) of the North Sea), London, the West Midlands Urban Area, most of Ireland, and most of the Isle of Man.

USDA zones do not work particularly well in the UK as they are designed for a continental climate.[21] The cooler UK summers must be considered. New growth may be insufficient or fail to harden off affecting winter survival.[21]

In 2012 the United Kingdom's Royal Horticultural Society introduced new hardiness ratings for plants, from H7, the hardiest (tolerant of temperatures below −20 °C (−4 °F)) to H1a (needing temperatures above 15 °C (59 °F)).[22] The RHS hardiness ratings are based on absolute minimum winter temperatures (in °C) rather than the long-term average annual extreme minimum temperatures that define USDA zones.[22]



A garden in Simrishamn, southern Sweden.

In Sweden and Finland generally, at sea level to 500 metres (1,600 ft), zone 3 is north of the Arctic Circle, including cities like Karesuando, Pajala, and Rovaniemi. Kiruna is the major exception here, which being located on a hill above frost traps, is in zone 5. Zone 4 lies between the Arctic Circle and about 64–65°N, with cities such as Oulu and Jokkmokk, zone 5 (south to 61–62°N) contains cities such as Tampere, Umeå, and Östersund. Zone 6 covers the south of mainland Finland, Sweden north of 60°N, and the high plateau of Småland further south. Here one will find cities such as Gävle, Örebro, Sundsvall, and Helsinki. The Åland Islands, as well as coastal southern Sweden, and the Stockholm area are in zone 7. The west coast of Sweden (Gothenburg and southwards) enjoys particularly mild winters and lies in zone 7, therefore being friendly to some hardy exotic species (found, for example, in the Gothenburg Botanical Garden), the southeast coast of Sweden has a colder winter due to the absence of the Gulf Stream.

Denmark is in zones 9a, 8b, and 8a.[23]

Central EuropeEdit


Southern EuropeEdit

The southern European marker plant for climate as well as cultural indicator is the olive tree, which cannot withstand long periods below freezing so its cultivation area matches the cool winter zone. The Mediterranean Sea acts as a temperature regulator, so this area is generally warmer than other parts of the continent; except in mountainous areas where the sea effect lowers, it belongs in zones 8–10; however, southern Balkans (mountainous Western and Eastern Serbia, continental Croatia, and Bulgaria) are colder in winter and are in zones 6–7. The Croatian (Dalmatian) coast, Albania, and northern Greece are in zones 8–9, as are central-northern Italy (hills and some spots in Po Valley are however colder) and southern France; Central Iberia is 8–9 (some highland areas are slightly colder). The Spanish and Portuguese Atlantic coast, most of Andalusia and Murcia, almost all the Valencian Community, a part of Catalonia, the Balearic Islands, southwestern Sardinia, most of Sicily, coastal southern Italy, and southwestern Greece are in zone 10. The islands of Malta and Lampedusa belong to zone 11a and the southernmoust zone of Spain (Cádiz, Málaga, where the cultivation of mango, avocado or papaya are extended) belong to zone 11a.[24] Also, a few areas on the southernmost coast of Cyprus are inside zone 11a.

Selected European citiesEdit

The table below provides hardiness zone data for selected European cities (based on climatological data):[25][26][27]

City Zone City Zone
Alicante, Spain 10b Almería, Spain 11a
Amsterdam, The Netherlands 8b Antwerp, Belgium 8
Barcelona, Spain 10a Belfast, Northern Ireland 9
Berlin, Germany 7b Bratislava, Slovakia 7b
Birmingham, England 9a Bucharest, Romania 6b
Cádiz, Spain 11a Cardiff, Wales 9
Belgrade, Serbia 8a Copenhagen, Denmark 8a
Dublin, Ireland 9a Düsseldorf, Germany 8
Edinburgh, Scotland 8b Gdańsk, Poland 7
Glasgow, Scotland 8b Hamburg, Germany 8a
Helsinki, Finland 6a Istanbul, Turkey 8b
Kaliningrad, Russia 6 Kyiv, Ukraine 6a
Kraków, Poland 6 Lisbon, Portugal 10b
Las Palmas, Spain 12b A Coruña, Spain 10b
Ljubljana, Slovenia 7b London, England 9a
Madrid, Spain 9a Málaga, Spain 11a
Marseille, France 9a Milan, Italy 8b
Minsk, Belarus 5b Moscow, Russia 5a
Munich, Germany 7b Murmansk, Russia 5
Nicosia, Cyprus 10a/10b Oslo, Norway 6a
Simferopol, Ukraine 7a Palma, Spain 10a
Paris, France 8b Poznań, Poland 7a
Prague, Czech Republic 7a Reykjavík, Iceland 8a
Riga, Latvia 6a Rome, Italy 9b
Rovaniemi, Finland 4 Saint Petersburg, Russia 5b
Sarajevo, Bosnia and Herzegovina 7b Santander, Spain 10a
Sicily (Catania, Italy) 10a Simrishamn, Sweden 8a
Sochi, Russia 9 Sofia, Bulgaria 7a
Stockholm, Sweden 6b Strasbourg, France 7
Tallinn, Estonia 6b Tuapse, Russia 8
Tórshavn, Faroe Islands 7-8 Tromsø, Norway 7
Trondheim, Norway 6 Umeå, Sweden 5
Valencia, Spain 10b Valletta, Malta 11a
Vienna, Austria 8a Vilnius, Lithuania 6
Vorkuta, Russia 2 Warsaw, Poland 6b
Wroclaw, Poland 7a Zagreb, Croatia 8a
Zürich, Switzerland 8a Zaragoza, Spain 9b

American Horticultural Society heat zonesEdit



Zone From To
1 < 1
2 1 7
3 8 14
4 15 30
5 31 45
6 46 60
7 61 90
8 91 120
9 121 150
10 151 180
11 181 210
12 >210

European cities (AHS heat zones)Edit

City Zone City Zone
Amsterdam, The Netherlands 2 Antwerp, Belgium 2
Belfast, Northern Ireland 1 Berlin, Germany 3
Birmingham, England 2 Bratislava, Slovakia 4
Bucharest, Romania 6 Cardiff, Wales 1
Copenhagen, Denmark 2 Cork, Ireland 1
Derry, Northern Ireland 1 Dublin, Ireland 1
Düsseldorf, Germany 3 Edinburgh, Scotland 1
Gdańsk, Poland 2 Galway, Ireland 1
Glasgow, Scotland 1 Hamburg, Germany 2
Helsinki, Finland 2 Istanbul, Turkey 6
Kaliningrad, Russia 2 Kiev, Ukraine 4
Kraków, Poland 4 Lisbon, Portugal 7
Ljubljana, Slovenia 6 London, England 2
Madrid, Spain 7 Málaga, Spain 7
Marseille, France 7 Milan, Italy 6
Minsk, Belarus 3 Moscow, Russia 2
Munich, Germany 3 Murmansk, Russia 1
Nicosia, Cyprus 9 Oslo, Norway 2
Oulu, Finland 1 Palma, Spain 8
Paris, France 3 Perm, Russia 3
Prague, Czech Republic 3 Reykjavík, Iceland 1
Riga, Latvia 2 Rome, Italy 7
Rovaniemi, Finland 1 Saint Petersburg, Russia 2
Sarajevo, Bosnia and Herzegovina 5 Santander, Spain 3
Simrishamn, Sweden 1 Sochi, Russia 6
Sofia, Bulgaria 6 Stockholm, Sweden 2
Strasbourg, France 4 Tallinn, Estonia 2
Tuapse, Russia 7 Tórshavn, Faroe Islands 1
Tromsø, Norway 1 Trondheim, Norway 1
Umeå, Sweden 1 Vienna, Austria 4
Vilnius, Lithuania 2 Vorkuta, Russia 1
Warsaw, Poland 3 Zürich, Switzerland 4

South AfricaEdit

South Africa has five horticultural or climatic zones.[29]

See alsoEdit


  1. ^ Fernald, M. L. (1927). Rehder, Alfred (ed.). "Rehder's Manual of Cultivated Trees and Shrubs". Rhodora. 29 (339): 48–51. JSTOR 23298457.
  2. ^ Wyman, Donald (1938). Hedges, Screens & Windbreaks: Their Uses, Selection and Care. McGraw-Hill.
  3. ^ a b "History of Plant Hardiness Zone Maps – The Rest of the Story". Plant Delights Nursery. Retrieved 2018-06-03.
  4. ^ Del Tredici, Peter (1990). "The New USDA Plant Hardiness Zone Map" (PDF). Arnoldia. 50 (3): 16–20 – via Harvard University.
  5. ^ "USDA Hardiness Zone Map". American Horticultural Society. Archived from the original on 8 December 2017. Retrieved 8 December 2017.
  6. ^ "New Hardiness Zone Map reflects warmer climate : Latest hardiness zones, based on most current temperature data available, suggest up-to-date choices for best trees to plant". Archived from the original on 2007-12-28. Retrieved 2007-12-27.
  7. ^ "USDA Plant Hardiness Zone Map, 2012". Agricultural Research Service. U.S. Department of Agriculture. Archived from the original on 24 October 2017. Retrieved 7 December 2017.
  8. ^ a b "What's New | USDA Plant Hardiness Zone Map". Agricultural Research Service. U.S. Department of Agriculture. Archived from the original on 19 October 2017. Retrieved 7 December 2017.
  9. ^ a b "About | USDA Plant Hardiness Zone Map". Retrieved 2018-06-03.
  10. ^ "History of USDA Hardiness Zones". Retrieved 2018-06-03.
  11. ^ "7a" areas not part of downtown Washington
  12. ^ McKenney, Daniel W.; Pedlar, John H.; Lawrence, Kevin; Campbell, Kathy; Hutchinson, Michael F. (2007-12-01). "Beyond Traditional Hardiness Zones: Using Climate Envelopes to Map Plant Range Limits". BioScience. 57 (11): 929–937. doi:10.1641/B571105. ISSN 1525-3244.
  14. ^ Köppen, Wladimir; Translated by Volken, E., and Brönninmann, S. "Die Wärmezonen der Erde, nach der Dauer der heissen, gemässigten und kalten Zeit und nach der Wirkung der Wärme auf die organische Welt betrachtet [The thermal zones of the Earth according to the duration of hot, moderate and cold periods and of the impact of heat on the organic world]" (PDF). World Maps of Köppen-Geiger Climate Classification. Meteorologische Zeitschrift, Vol. 20, No. 3, 351-360. Retrieved 24 September 2019.
  16. ^ Dawson, I. (1991). "Plant Hardiness Zones for Australia". Archived from the original on 2010-12-10. Retrieved 2010-11-11.
  17. ^ a b "Natural Resources Canada – Plant Hardiness of Canada". Natural Resources Canada – Plant Hardiness of Canada. Government of Canada. Archived from the original on 29 June 2006. Retrieved 7 April 2018.
  18. ^ Ouellet, C. E.; Sherk, L. C. (1967). "WOODY ORNAMENTAL PLANT ZONATION: I. INDICES OF WINTERHARDINESS". Canadian Journal of Plant Science. 47 (3): 231–238. doi:10.4141/cjps67-044. ISSN 0008-4220. pdf
  19. ^ Ouellet, C. E.; Sherk, L. C. (1967). "WOODY ORNAMENTAL PLANT ZONATION: II. SUITABILITY INDICES OF LOCALITIES". Canadian Journal of Plant Science. 47 (4): 339–349. doi:10.4141/cjps67-064. ISSN 0008-4220. pdf
  20. ^ Ouellet, C. E.; Sherk, L. C. (1967). "WOODY ORNAMENTAL PLANT ZONATION: III. SUITABILITY MAP FOR THE PROBABLE WINTER SURVIVAL OF ORNAMENTAL TREES AND SHRUBS". Canadian Journal of Plant Science. 47 (4): 351–358. doi:10.4141/cjps67-065. ISSN 0008-4220. pdf
  21. ^ a b Martin Crawford (2010). Creating a forest garden : working with nature to grow edible crops. Green Books. p. 13. ISBN 9781900322621.
  22. ^ a b "RHS hardiness rating". RHS hardiness rating. Royal Horticultural Society. Archived from the original on 25 September 2016. Retrieved 24 September 2016.
  23. ^ "Denmark Interactive Plant Hardiness Zone Map". Archived from the original on 11 October 2017. Retrieved 1 May 2018.
  24. ^ "Archived copy". Archived from the original on 2016-03-05. Retrieved 2015-06-09.CS1 maint: archived copy as title (link)
  25. ^ "Hardiness zone – – Plant Encyclopedia and Gardening wiki". Archived from the original on 2012-10-03. Retrieved 2012-10-13.
  26. ^ "Europa Hardiness zone map". Archived from the original on 2012-08-15. Retrieved 2012-10-13.
  27. ^ "Hardiness Zones". Archived from the original on 2012-05-04. Retrieved 2012-10-13.
  28. ^ "AHS Plant Heat Zone Map". American Horticultural Society. Archived from the original on 2017-05-16. Retrieved 2017-05-26.
  29. ^ PlantZAfrica 2016, Horticultural Zones


External linksEdit