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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 hardiness or heat zone are both highly specific with respect to altitude, amount of sunlight, rainfall, humidity levels, wind chill, wind speed, amongst other conditions, regardless of latitude and longitude. Trees and greenery foliage may provide dappled or full shade like they can break wind to provide, as do greenhouses without artificial climate control. As a result, Microclimate are created where some plants may unexpectedly thrive. Always include caution, knowledge from personal experiences and trial and error when determining whether a plant will grow or not in a particular area.https://evergreenbotany.com/blogs/news/plant-growing-zones
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.
| < −65 °F (−53.9 °C) | |||
| −60 °F (−51.1 °C) | |||
| −55 °F (−48.3 °C) | |||
| −50 °F (−45.6 °C) | |||
| −45 °F (−42.8 °C) | |||
| −40 °F (−40 °C) | |||
| −35 °F (−37.2 °C) | |||
| −30 °F (−34.4 °C) | |||
| −25 °F (−31.7 °C) | |||
| −20 °F (−28.9 °C) | |||
| −15 °F (−26.1 °C) | |||
| −10 °F (−23.3 °C) | |||
| −5 °F (−20.6 °C) | |||
| 0 °F (−17.8 °C) | |||
| 5 °F (−15 °C) | |||
| 10 °F (−12.2 °C) | |||
| 15 °F (−9.4 °C) | |||
| 20 °F (−6.7 °C) | |||
| 25 °F (−3.9 °C) | |||
| 30 °F (−1.1 °C) | |||
| 35 °F (1.7 °C) | |||
| 40 °F (4.4 °C) | |||
| 45 °F (7.2 °C) | |||
| 50 °F (10 °C) | |||
| 55 °F (12.8 °C) | |||
| 55 °F (12.8 °C) | 60 °F (15.6 °C) | ||
| 13 | a | 60 °F (15.6 °C) | 65 °F (18.3 °C) |
| b | > 65 °F (18.3 °C) | ||
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.
| 6a |
| 7b/8a |
| 7a |
| 7a |
| 5a/5b |
| 8a/8bMost of the city proper is 8a while Hartsfield–Jackson Atlanta International Airport is south of the city and 8b. |
| 8b |
| 9a |
| 9b |
| 7b/8a |
| 9a |
| 8a/8b |
| 4b |
| 7a/7b |
| 6b/7a |
| 6b |
| 10b |
| 9a |
| 8a |
| 6a/6b |
| 6b |
| 6b/7a |
| 5b/6a |
| 6b |
| 8b |
| 6a |
| 5b |
| 6b |
| 8b/9a |
| 2a |
| 6a |
| 9b |
| 6a |
| 8a |
| 8a/8b |
| 13b |
| 6b/7a |
| 4b/5a |
| 12b |
| 9b |
| 6a/6b |
| 3a |
| 9a/9bThe urbanized portion of Jacksonville is 9b while much of the west end of Duval County is 9a. |
| 6b/7a |
| 6b/7a |
| 11b/12a |
| 9a/9b |
| 8a |
| 10a/10bSmall portions of the city of Los Angeles extend into zones 9b and 11a. |
| 7a |
| 8a |
| 11a |
| 5b/6a |
| 5a |
| 7b |
| 9b |
| 7b |
| 7b |
| 5b/6a |
| 10a |
| 7b |
| 9b/10a |
| 6b/7a |
| 10a |
| 6a |
| 8b/9a |
| 7a |
| 8a |
| 7a/7b |
| 7b |
| 10a |
| 9b |
| 7a/7b |
| 9a |
| 10b/11aSome northern areas of the city proper away from the coast are 9b or 10a. |
| 10a/10b |
| 9b/10a |
| 13a/13b |
| 6b/7aMost of the city proper is 6b, with 6a on Montage Mountain and 7a from downtown downriver. |
| 9a |
| 6b/7a |
| 7a |
| 10a |
| 6bThe city proper is 6b while Toledo Express Airport is over ten miles west of Lake Erie and is 6a, |
| 9a/9b |
| 7b |
| 8aThe climate is subpolar oceanic ( Cfc). |
| 8a/8b |
| 7b/8a |
| 7a |
| 6a/6b |
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 Desert 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.
The hardiness scales do not take into account the reliability of snow cover in the colder zones. 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.
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).
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 humanmade 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.
Sunset Magazine publishes a series that breaks up climate zones more finely than the USDA zones, identifying 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. Sunset National Garden Book. Sunset Books Inc. Menlo Park, California (1997)
In addition, the Köppen 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. The Trewartha climate classification is often a good "real world" concept of climates and their relation to plants and their average growing conditions.
The spread of weather stations may be insufficient and too many places with different climates are lumped together. Only 738 Australian stations have records of more than ten years (one station per ), though more populated areas have relatively fewer hectares per station. 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. Sydney residents are split between zones 3a and 4b. Different locations in the same city are suitable for different plants.
The formula used is:
where:
| 4a |
| 3b |
| 6a |
| 4b |
| 4b |
| 3a |
| 7a |
| 5b |
| 8b |
| 9a |
| 3b |
| 2a |
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. The Canadian government publishes both Canadian and USDA-style zone maps.
| 8b | |
| 10a | |
| 7b/8a | |
| 7a/7b | |
| 7a/7b | |
| 9b/10a | |
| 8b/9a | |
| 8a/8b | |
| 8b | |
| 6b | |
| 6b/7a | |
| 7a | |
| Lisbon, Portugal | 10b |
| London, England | 8b/9a |
| 7b | |
| 9a | |
| 8b | |
| 9a | |
| 5a | |
| 8b/9a | |
| 9a | |
| 7b | |
| 6b | |
| Rome, Italy | 9b |
| 4a | |
| 7a/7b | |
| 6b | |
| 9a | |
| 7a/7b | |
| 6a/6b | |
| 8b | |
| 7b | |
| 10a | |
| 7b/8a | |
| 2a/2b | |
| Warsaw, Poland | 6b |
| Zurich, Switzerland | 7b/8a |
| Antwerp, Belgium | 8a |
| Belfast, Northern Ireland | 8b/9a |
| Berlin, Germany | 7a |
| Birmingham, England | 8b |
| Cardiff, Wales | 8b/9a |
| Copenhagen, Denmark | 8a/8b |
| Düsseldorf, Germany | 8a |
| Funchal | 11b |
| Gdańsk, Poland | 7a |
| Hamburg, Germany | 8a |
| Istanbul, Turkey | 8b/9aSome inland regions of Istanbul are in zone 8a. Meanwhile, a small strip of land in the far-south of the city, as well as the neighboring Princes' Islands, are in zone 9b. |
| La Coruña, Spain | 10b |
| 12b | |
| Marseille, France | 9a/9b |
| Minsk, Belarus | 5a |
| Munich, Germany | 6b |
| 4a | |
| 8a/8b | |
| Nicosia, Cyprus | 9b |
| Oslo, Norway | 7a |
| 10b/11a | |
| Plymouth | 9a/9b |
| Porto, Portugal | 10a |
| Poznań, Poland | 6b |
| Reykjavík, Iceland | 7b/8a |
| Saint Petersburg, Russia | 5a |
| Simrishamn, Sweden | 8a |
| Sofia, Bulgaria | 6b/7a |
| Strasbourg, France | 8a |
| Thessaloniki, Greece | 8b/9a |
| Tromsø, Norway | 7a/7b |
| Umeå, Sweden | 5a/5b |
| Valletta, Malta | 10b |
| 2022-05-09 | 5b/6a |
| 6b | |
| Zagreb, Croatia | 7b/8a |
| 9a |
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 USDA hardiness zones relevant to Britain and Ireland are quite high, from 7 to 10, as shown below.
In 2012 the United Kingdom's Royal Horticultural Society introduced new hardiness ratings for plants, not places. These run from H7, the hardiest (tolerant of temperatures below ) to H1a (needing temperatures above ). 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.
In Sweden and Finland generally, at sea level to , zone 3 is north of the Arctic Circle, including cities like Karesuando and Pajala. Kiruna is the major exception here, which being located on a hill above , is in zone 5. Zone 4 lies between the Arctic Circle and about 64–66°N, with cities such as Oulu, Rovaniemi 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. Åland, 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.
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 Helgoland, 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. Another notable example is Waksmund, a small village in the Poland Carpathians, which regularly reaches during winter on calm nights when cold and heavy airmasses from the surrounding Gorce Mountains and Tatra Mountains descend down the slopes to this low-lying valley, creating extremes which can be up to colder than nearby Nowy Targ or Białka Tatrzańska, which are both higher up in elevation. Waksmund is in zone 3b while nearby Kraków, only to the north and 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.
The Croatian () 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 Spain and Portugal Atlantic coast, much of Andalusia and Murcia, coastal and slightly inland southern Valencian Community, a part of coastal Catalonia, the Balearic Islands, southwestern Sardinia, most of Sicily, coastal southern Italy, some areas around Albania, coastal Cyprus and southwestern Greece are in zone 10.
In Europe, the zone 11a is limited only to a few spots. In the Iberian Peninsula, it can be found on the southern coast, in small Spain areas inside the provinces of Almería, Cádiz, Granada, Málaga and Murcia. In Portugal, zone 11a can be found in the Southwest on a few unpopulated sites around the municipalities of Lagos and Vila do Bispo. In mainland Greece, zone 11a can be found in Monemvasia and also in areas of Crete, the Dodecanese, Cyclades and some Argo-Saronic Gulf islands. The Mediterranean islands of Malta, Lampedusa and Linosa belong to zone 11a as well as a few areas on the southernmost coast of Cyprus.
The Cape Verde islands, located much further south inside the tropics, have hardiness zones that range from 12 to 13 in the coastal areas, while the lowest hardiness zone is found in the island of Fogo, in the country's highest peak Pico do Fogo.
The criterion is the average number of days per year when the temperature exceeds . The AHS Heat Zone Map for the US is available on the American Horticultural Society website.
| < 1 | |
| 7 | |
| 14 | |
| 30 | |
| 45 | |
| 60 | |
| 90 | |
| 120 | |
| 150 | |
| 180 | |
| 210 | |
| >210 | |
|
|
