Hardiness zone

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Temperature scale of hardiness zones, showing the average annual minimum temperature in degrees Celsius. The main factors determining average minimum temperature are elevation, latitude and proximity to the coast.

A hardiness zone (a subcategory of vertical zonation)[citation needed] is a geographically defined area in which a specific category of plant life is capable of growing, as defined by climatic conditions, including its ability to withstand the minimum temperatures of the zone (see the scale on the right or the table below). For example, a plant that is described as "hardy to zone 10" means that the plant can withstand a minimum temperature of −1 °C (30 °F). A more resilient plant that is "hardy to zone 9" can tolerate a minimum temperature of −7 °C (19 °F). First developed for the United States by the Department of Agriculture (USDA), the use of the zones has been adopted by other nations.

USDA hardiness zones[edit]

0a< −53.9 °C (−65 °F)
b−53.9 °C (−65 °F)−51.1 °C (−60 °F)
1a−51.1 °C (−60 °F)−48.3 °C (−55 °F)
b−48.3 °C (−55 °F)−45.6 °C (−50 °F)
2a−45.6 °C (−50 °F)−42.8 °C (−45 °F)
b−42.8 °C (−45 °F)−40 °C (−40 °F)
3a−40 °C (−40 °F)−37.2 °C (−35 °F)
b−37.2 °C (−35 °F)−34.4 °C (−30 °F)
4a−34.4 °C (−30 °F)−31.7 °C (−25 °F)
b−31.7 °C (−25 °F)−28.9 °C (−20 °F)
5a−28.9 °C (−20 °F)−26.1 °C (−15 °F)
b−26.1 °C (−15 °F)−23.3 °C (−10 °F)
6a−23.3 °C (−10 °F)−20.6 °C (−5 °F)
b−20.6 °C (−5 °F)−17.8 °C (0 °F)
7a−17.8 °C (0 °F)−15 °C (5 °F)
b−15 °C (5 °F)−12.2 °C (10 °F)
8a−12.2 °C (10 °F)−9.4 °C (15 °F)
b−9.4 °C (15 °F)−6.7 °C (20 °F)
9a−6.7 °C (20 °F)−3.9 °C (25 °F)
b−3.9 °C (25 °F)−1.1 °C (30 °F)
10a−1.1 °C (30 °F)+1.7 °C (35 °F)
b+1.7 °C (35 °F)+4.4 °C (40 °F)
11a+4.4 °C (40 °F)+7.2 °C (45 °F)
b+7.2 °C (45 °F)+10 °C (50 °F)
12a+10 °C (50 °F)+12.8 °C (55 °F)
b> +12.8 °C (55 °F)

Benefits and drawbacks[edit]

The hardiness zones are informative: the extremes of winter cold are a major determinant of whether a plant species can be cultivated outdoors at a particular location; however, the USDA hardiness zones have a number of drawbacks if used without supplementary information.

The zones do not incorporate summer heat levels into the zone determination; thus sites which may have the same mean winter minima, but markedly different summer temperatures, will be accorded the same hardiness zone. An extreme example of this phenomenon is seen when comparing the Shetland Islands and southern Alabama, which are both on the boundary of zones 8 and 9 and share the same winter minima, but little else in their climates. In summer, the humid subtropical climate of Alabama is about 20 degrees Celsius hotter than the temperate maritime climate of Shetland, and there are few similar plants that can be grown at both locations. Due to its maritime climate, the UK is in AHS heat zone 2 (having 1 to 8 days hotter than 30 °C or 86 °F) according to the AHS (American Horticultural Society), whereas Alabama is in zones 7 to 9 (61 to 150 days hotter than 30 °C or 86 °F). Users need to combine the hardiness zone with the heat zone to gain greater understanding of what can be grown in a particular location.

Another issue is that the hardiness zones do not take into account the reliability of the snow cover. Snow acts as an insulator against extreme cold, protecting the root system of hibernating plants. If the snow cover is reliable, the actual temperature to which the roots are exposed will not be as low as the hardiness zone number would indicate. As an example, Quebec City in Canada is located in zone 4, but can rely on a significant snow cover every year, making it possible to cultivate plants normally rated for zones 5 or 6. But, in Montreal, located to the southwest in zone 5, it is sometimes difficult to cultivate plants adapted to the zone because of the unreliable snow cover.

Other factors that affect plant survival, though not considered in hardiness zones, are soil moisture, humidity, the number of days of frost, and the risk of a rare catastrophic cold snap. Some risk evaluation – the probability of getting a particularly severe low temperature – often would be more useful than just the average conditions.

Lastly, many plants may survive in a locality but will not flower if the day length is insufficient or if they require vernalization (a particular duration of low temperature). With annuals, the time of planting can often be adjusted to allow growth beyond their normal geographical range.

An alternative means of describing plant hardiness is to use "indicator plants" (the USDA also publishes a list of these to go with the hardiness zone map). In this method, common plants with known limits to their range are used.

Gardening books are available that provide more information on climate zones. For example, Sunset Books (associated with Sunset Magazine) publishes a series that break up climate zones more finely than the USDA zones. They identify 45 distinct zones in the US, incorporating ranges of temperatures in all seasons, precipitation, wind patterns, elevation, and length and structure of the growing season.[1]

Europe hardiness zones[edit]

Britain and Ireland[edit]

Britain and Ireland's hardiness zones

Owing to the moderating effect of the North Atlantic Current on the Irish and British temperate maritime climate, Britain, and Ireland even more so, have milder winters than their northerly position would otherwise afford. This means that the hardiness zones relevant to Britain and Ireland are quite high, from 7 to 10, as shown below.

Northern Europe[edit]

Scandinavia lies at the same latitude as Alaska or Greenland, but the effect of the warm North Atlantic Current is even more pronounced here than it is in Britain and Ireland. Save for a very small spot near Karasjok, Norway which is in zone 2, nowhere in the Arctic part of Scandinavia does it get below zone 3. The Faroe Islands, at 62–63°N are in zone 8, as are the outer Lofoten Islands at 68°N. Tromsø, a coastal city in Norway at 70°N, is in zone 7, and even Longyearbyen, the northernmost true city in the world at 78°N, is still in zone 5. All these coastal locations have one thing in common, though, which are cold, damp summers, with temperatures rarely exceeding 20 °C (68 °F), or 15 °C (59 °F) in Longyearbyen. This shows the importance of taking heat zones into account for better understanding of what may or may not grow.

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 6. 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.

Central Europe[edit]

Central Europe is a good example of a transition from an oceanic climate to a continental climate, which can be noticed immediately when looking at the hardiness zones, which tend to decrease mainly eastwards instead of northwards. Also, the plateaux and low mountain ranges in this region have a significant impact on how cold it might get during winter. Generally speaking, the hardiness zones are high considering the latitude of the region, although not as high as in the Shetland Islands where zone 9 extends to over 60°N. In Central Europe, the relevant zones decrease from zone 8 on the Belgian, Dutch and German North Sea coast, with the exception of some of the Frisian Islands (notably Vlieland and Terschelling), the island of Heligoland and some of the islands in the Rhine-Scheldt estuary, which are in zone 9, to zone 5 around Suwałki, Podlachia on the far eastern border between Poland and Lithuania. Some isolated, high elevation areas of the Alps and Carpathians may even go down to zone 3 or 4. An extreme example of a cold sink is Funtensee, Bavaria which is at least in zone 3 and maybe even in zone 1 or 2. Another notable example is Waksmund, a small village in the Polish Carpathians, which regularly reaches −35 °C (−31 °F) during winter on calm nights when cold and heavy airmasses from the surrounding Gorce and Tatra Mountains descend down the slopes to this low-lying valley, creating extremes which can be up to 10 °C (18 °F) colder than nearby Nowy Targ or Białka Tatrzańska, which are both higher up in elevation. Waksmund is in zone 3b while nearby Kraków, only 80 km (50 mi) to the north and 300 m (980 ft) lower is in zone 6a. These examples prove that local topography can have a pronounced effect on temperature and thus on what is possible to grow in a specific region.

Southern Europe[edit]

Southern Europe is generally warmer than other parts of Europe; except mountains, it belongs to 8–10 zones; however southern Balkans (Serbia, eastern Bulgaria) are rather cold in winter and are in zones 6–7. Dalmatian coast, Albania and northern Greece are in zones 8–9, just like central-northern Italy (hills and some cold spots in Po Valley are however colder) and southern France; Central Iberia is 8–9 (some areas are slightly colder). The Spanish and Portuguese Atlantic coast, most of Andalusia and Murcia, southern Valencian Community, a part of Catalonia, Balearic Islands, southwestern Sardinia, most of Sicily, coastal southern Italy and southwestern Greece are in zone 10 (some islands possibly even in zone 11).[2]

European cities[edit]

The table below provides hardiness zone data for some European cities (based on climatological data):[3][4][5]

Amsterdam, The Netherlands8Antwerp, Belgium8
Belfast, Northern Ireland9Berlin, Germany7a
Bratislava, Slovakia7aBucharest, Romania6a
Cardiff, Wales9Catania, Italy10
Copenhagen, Denmark8Cork, Ireland9b
Dublin, Ireland9Düsseldorf, Germany8
Edinburgh, Scotland8Gdańsk, Poland7
Glasgow, Scotland9aHamburg, Germany7
Helsinki, Finland5bIstanbul, Turkey8b
Kaliningrad, Russia6Kiev, Ukraine5
Kraków, Poland6Lisbon, Portugal10a
Ljubljana, Slovenia7London, England9a
Madrid, Spain8-9Málaga, Spain10b
Marseille, France9Milan, Italy7-8
Minsk, BelarusMoscow, Russia4
Munich, Germany6Murmansk, Russia5
Nicosia, Cyprus10bOslo, Norway6
Simferopol, Ukraine7aPalma, Spain10
Paris, France8aPortsmouth, England9b
Prague, Czech Republic7aReykjavík, Iceland7
Riga, Latvia6Rome, Italy9b
Rovaniemi, Finland4Saint Petersburg, Russia5a
Sarajevo, Bosnia and Herzegovina7Santander, Spain10a
Simrishamn, Sweden8aSochi, Russia9
Sofia, Bulgaria6Stockholm, Sweden7
Strasbourg, France7Tallinn, Estonia5
Tuapse, Russia8Tórshavn, Faroe Islands7-8
Tromsø, Norway7Trondheim, Norway6
Umeå, Sweden5Vienna, Austria6-7
Vilnius, Lithuania5Vorkuta, Russia2
Valletta, Malta11aWarsaw, Poland6
Zürich, Switzerland7

United States hardiness zones[edit]

Based on the average annual minimum temperature for a given location, the USDA map provides an easy guideline for categorizing locations suitable for winter survival of a rated plant in an "average" winter. Since temperatures in the non-coastal-adjacent areas of the continent rarely present a consistent experience from year to year, and occasionally present a major—and often agriculturally devastating—deviation from the average minimum, the map has limitations for much of the country as a basis for using with long-term reliability, at least in areas close to the margin of a plant's rated hardiness-zone.

The USDA first issued its standardized hardiness zone map in 1960, and revised it in 1965. A new map was issued in 1990, based on U.S. and Canadian data from 1974 through 1986 (and 1971–1984 for Mexico). The 1990-issue map was based on nearly double the number of stations, and it divided the temperature zones into five-degree a/b zones for greater accuracy. This revised map identified many areas as colder than did the 1960 map, due chiefly to a number of severely colder winters in the central and eastern U.S. in the 1974–1986 data-gathering period, as opposed to the mid-20th century data-sampling period used in the 1960 map; for instance, many locations in the southeastern U.S. set all-time record lows during January 1985.

The 1990 map shows 10 different zones, each of which represents an area of winter hardiness for the plants of agriculture and the natural landscape. The USDA introduced zone 11, representing areas that have average annual minimum temperatures at or above 4.4 °C (40 °F) and that are therefore essentially frost-free.


Differences between 1990 USDA hardiness zones and 2006 arborday.org hardiness zones reflects global warming.
2012 update of the Hardiness Zone Map

In 2003, the American Horticultural Society (AHS) produced a draft revised map, using temperature data collected from July 1986 to March 2002. This was a period of warmer winters than the 1974–1986 period, especially in the central and eastern U.S. The 2003 map placed many areas approximately a half-zone higher (warmer) than the 1990 map had. Reviewers noted the map zones appeared to be closer to the original 1960 map in its overall zone delineations. The 2003 AHS draft 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 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; the agency stated it would create its own map in an interactive computer format. As of August 2010 the AHS and the National Arboretum websites still presented the 1990 map as current.

In 2006, the US National Arbor Day Foundation completed an extensive update of U.S. hardiness zones. It used essentially the same data as the AHS. Once the Foundation analyzed the new data, it revised hardiness zones, reflecting the generally warmer recent temperatures in many parts of the country. The Foundation's 2006 map appears to validate the data used in the AHS 2003 draft. The Foundation also did away with the more detailed a/b half-zone delineations.[6]

In 2012 the USDA updated their plant hardiness map to reflect the warmer observed temperatures in the past thirty years.[7]

U.S. Cities hardiness zones[edit]

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

Albuquerque, New Mexico7aOklahoma City, Oklahoma7a
Anchorage, Alaska4b/5aOmaha, Nebraska5b
Atlanta, Georgia7bOrlando, Florida9b
Billings, Montana4b/5aOwensboro, Kentucky6b
Boston, Massachusetts6b/7aPhiladelphia, Pennsylvania7a/7b
Buffalo, New York6aPhoenix, Arizona9b/10a
Burlington, Vermont5aPierre, South Dakota4b/5a
Charleston, West Virginia6aPittsburgh, Pennsylvania6b
Chicago, Illinois6aPortland, Maine5b
Charlotte, North Carolina7b/8aPortland, Oregon8b/9a
Columbus, Ohio6aQuad Cities, Iowa/Illinois5b
Dallas, Texas8a/8bRaleigh, North Carolina7b/8a
Denver, Colorado5b/6aReno, Nevada6b/7a
Detroit, Michigan6bRoanoke, Virginia7a
Fairbanks, Alaska2aSt. Louis, Missouri6b
Hartford, Connecticut6bSalt Lake City, Utah5b/6a
Honolulu, Hawaii12bSan Antonio, Texas8b/9a
Houston, Texas9aSan Diego, California10b
Las Vegas, Nevada9aSan Francisco, California10a/10b
Los Angeles, California10bSan Jose, California9b
Memphis, Tennessee7b/8aSavannah, Georgia8b
Miami, Florida11aSeattle, Washington8b
Minneapolis, Minnesota4b/5aTampa, Florida9b/10a
Nashville, Tennessee7aTucson, Arizona9a/9b
New Orleans, Louisiana9bTuscaloosa, Alabama8a
New York, New York7bWashington, D.C.7b
Norfolk, Virginia8aWichita, Kansas6b

Australian hardiness zones[edit]

The USDA hardiness zones are in use in Australia, but in addition the Australian National Botanic Gardens have devised another system more in keeping with Australian conditions.[8] They are numerically about 7 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 can choose 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. There may even be a case for publishing a list of weather stations and their zone classification to allow best use of local conditions.

AHS heat zones[edit]

In addition to the USDA Hardiness zones there are the American Horticultural Society (AHS) heat zones.

The criterion is the average number of days per year when the temperature exceeds 30 °C (86 °F).[9] The AHS Heat Zone Map for the US is available on the American Horticultural Society website.[10]

1< 1

European cities (AHS heat zones)[edit]

Amsterdam, The Netherlands2Antwerp, Belgium2
Belfast, Northern Ireland1Berlin, Germany3
Bratislava, Slovakia4Bucharest, Romania6
Cardiff, Wales1Catania, Italy8
Copenhagen, Denmark2Cork, Ireland1
Derry, Northern Ireland1Dublin, Ireland1
Düsseldorf, Germany3Edinburgh, Scotland1
Gdańsk, Poland2Galway, Ireland1
Glasgow, Scotland1Hamburg, Germany2
Helsinki, Finland2Istanbul, Turkey6
Kaliningrad, Russia2Kiev, Ukraine4
Kraków, Poland4Lisbon, Portugal7
Ljubljana, Slovenia6London, England2
Madrid, Spain7Málaga, Spain7
Marseille, France7Milan, Italy6
Minsk, Belarus3Moscow, Russia2
Munich, Germany3Murmansk, Russia1
Nicosia, Cyprus9Oslo, Norway1
Oulu, Finland1Palma, Spain8
Paris, France3Perm, Russia3
Prague, Czech Republic3Reykjavík, Iceland1
Riga, Latvia2Rome, Italy7
Rovaniemi, Finland1Saint Petersburg, Russia2
Sarajevo, Bosnia and Herzegovina5Santander, Spain3
Simrishamn, Sweden1Sochi, Russia6
Sofia, Bulgaria6Stockholm, Sweden2
Strasbourg, France4Tallinn, Estonia2
Tuapse, Russia7Tórshavn, Faroe Islands1
Tromsø, Norway1Trondheim, Norway1
Umeå, Sweden1Vienna, Austria4
Vilnius, Lithuania2Vorkuta, Russia1
Warsaw, Poland3Zürich, Switzerland4

See also[edit]


  1. ^ Sunset National Garden Book. Sunset Books Inc. Menlo Park, California (1997)
  2. ^ Toogood, Alan (1992) [1990]. Guida agli alberi ornamentali [Garden Trees Handbook] (in Italian). Bologna: Zanichelli. pp. 114–115. 
  3. ^ "Hardiness zone - Gardenology.org - Plant Encyclopedia and Gardening wiki". Gardenology.org. Retrieved 2012-10-13. 
  4. ^ "Europa Hardiness zone map". Backyardgardener.com. Retrieved 2012-10-13. 
  5. ^ "Hardiness Zones". Havlis.cz. Retrieved 2012-10-13. 
  6. ^ "New arborday.org 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". Retrieved 2007-12-27. 
  7. ^ "USDA Plant Hardiness Zone Map". USDA Agricultural Research Service. 
  8. ^ "Plant Hardiness Zones for Australia". Retrieved 2010-11-11. 
  9. ^ "AHS Plant Heat Zone Map". GardeningPlaces. Retrieved 2012-10-13. 
  10. ^ "American Horticultural Society: Plant Heat-Zone Map". ahs.org. Retrieved 2012-10-13. 

External links[edit]