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

USDA Hardiness Zones

Benefits and drawbacks

The hardiness zones are informative in that 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 Maritime Subarctic climate of Shetland, and there are very 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 degrees Celsius) according to the AHS (American Horticultural Society), whereas Alabama is in Zones 7 to 9 (61 to 150 days hotter than 30 degrees Celsius). It is, therefore, efficacious to combine the hardiness zone with the heat zone to gain greater understanding of what can be grown in a particular location.

Another problem 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, and generally present through winter, 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 an important snow cover every year, making it possible to cultivate plants normally rated for zones 5 or 6, whereas 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 will 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 their map); in this method, common plants with known limits to their range are used.

Sunset Books (Associated with Sunset magazine) publishes a series of gardening books that break up climate zones more finely than the USDA zones. Under this system, the US is broken into 45 distinct zones. These zones incorporate ranges of temperatures in all seasons, precipitation, wind patterns, elevation, and length and structure of the growing season.^[1]

Hardiness zones remain a useful source of information, if used in conjunction with other data, such as heat zones, air moisture and rainfall distribution.

Britain and Ireland Hardiness Zones

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.
• 8. Most of England, Wales and Scotland, and parts of central Ireland.
• 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 km of the North Sea), London, and most of Ireland.
• 10. Very low lying coastal areas of the southwest of Ireland including the city of Cork and the Isles of Scilly.

Central Europe Hardiness Zones

Central Europe's climate 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 UK or even the Faroe Islands where zone 9 extends to 62°N. In Central Europe, the relevant zones decrease from zone 9 at the Belgian and Dutch coasts, as well as some of the Frisian Islands and the island of Heligoland, to zone 6 on the far eastern border between Poland and Belarus. 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 in zone 2.

Northern Europe Hardiness Zones

Scandinavia lies at the same latitude as Alaska or Greenland, but the effect of the warm Gulf Stream is even more pronounced here than it is in the British Isles. 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 9, the southern coast of Iceland is in zone 8 at 64°N, as are the outer Lofoten Islands at 68°N. Tromso, 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, or 15°C in Longyearbyen. For this reason, hardiness zones have little meaning in these places, for the same reason as they have little meaning in the Aleutians, which have a similar climate as Iceland or the Faroes despite being 10° further south.

A garden in Simrishamn, southern Sweden.

European cities

The table herein provides hardiness zone data for most European cities (based on climatological data)http://www.backyardgardener.com/zone/europezone.html:

North American hardiness zones

USDA Map of North American hardiness zones, issued 1990 (for data 1974-1986). An interactive version of this map is available at http://www.usna.usda.gov/Hardzone/ushzmap.html

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). While the 1990-issue map utilized approximately double the number of stations, it also divided the temperature zones into five-degree a/b zones for greater accuracy. This revised map showed many areas to be suddenly colder than the 1960 map, due largely 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.

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

Updates

Differences between 1990 USDA hardiness zones and 2006 arborday.org hardiness zones reflect warmer climate

In addition, the National Arbor Day Foundation in the United States recently completed an extensive updating of U.S. Hardiness Zones in 2006, utilizing essentially the same data used by the AHS, the then-most-recent 15 years? data available from more than 5,000 National Climatic Data Center cooperative stations across the United States. Once the Foundation analyzed the new data, hardiness zones were revised, generally reflecting warmer recent temperatures in many parts of the country. The Arbor Day Foundation used the updated versions of the same sources of data as had been utilized by the U.S. Department of Agriculture in the creation of its hardiness zone maps. The 2006 map appears to validate the data used in the 2003 draft completed by the AHS. Like the AHS map, it also did away with the more detailed a/b half-zone delineations.^[2]

U.S. Cities

Here are the USDA plant hardiness zones for major U.S. cities (based on the draft 2003 map):

Australian Hardiness Zones

The Australian continent, though surrounded by ocean, is drier than Europe or North America. Australia numbers its climate zones differently but these can be made roughly equivalent to North American hardiness zones by adding an offset of 6. For example, Australian Zone 3 is roughly equivalent to North America Zone 9. The higher Australian zone numbers have no US equivalents.

• Zone 1 covers the alpine areas of southeastern Australia.
• Zone 2 the tablelands of south east Queensland, New South Wales and Victoria, and the uplands of central Tasmania.
• Zone 3 includes much of the southern half of the continent, except for near the coast.
• Zone 4, because of the warmer seaboard, covers a broad area from coastal Queensland across to Shark Bay and Geraldton in the west, and includes the Mornington Peninsula, areas adjacent to Spencer Gulf and Adelaide, the southwestern coastal zone, Sydney and the north coast of NSW, along with a number of localities dotted all around the southern coast of the continent.
• Zone 5 covers some of the Queensland coast, Western Australia north of Shark Bay, and across the Top End.
• Zone 6 includes the Queensland coast north of Cairns, Cape York Peninsula and the coast of the Northern Territory.
• Zone 7 is mainly restricted to islands off the north coast.

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), 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

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) http://www.gardeningplaces.com/heatzonemap/. The AHS Heat Zone Map for the US is available here http://www.ahs.org/pdfs/05_heat_map.pdf.

European cities

See also

• Hardiness (plants)
• Gardening
• Agriculture

References

Notes

Bibliography

External links

• Hardiness Zone maps for several continents
• Interactive USDA Plant Hardiness Zone Map
• Sunset Magazine Garden Climate Zone Map
• American Horticultural Society Heat Zone Map
• Freeze/Frost data from NOAA
• Data on plant hardiness in Ireland
• Plant Hardiness Data (in Canada)
• Selecting the Best Plants for Your Zone

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