A hardiness zone is a geographic area defined to encompass a certain range of climatic Edating zones for plants relevant to plant growth and survival. 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 annual extreme minimum temperature.
It has been adapted by and to other countries such as Canada in various forms. 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": In the United States, most of the warmer zones zones 9, 10, and 11 are located in the deep southern half of the country "Edating zones for plants" on the southern coastal margins.
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 Edating zones for plants mainland are the coldest zones zones Edating zones for plants, 4, and small area of zone 3and 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.
The low latitude and often stable weather in Floridathe Gulf Coastand southern Arizona and California "Edating zones for plants," are responsible for the few episodes of severe Edating zones for plants relative to normal in those areas.
The warmest zone on the USA mainland is the Florida Keys zone 11bwhile the coldest zone is in north-central Minnesota zone 3a. The first attempts to create a geographical hardiness zone system were undertaken by two researchers at the Arnold Arboretum in Boston: The first map was issued inand revised in It used uniform 10 degree Fahrenheit ranges, and gradually became widespread among American gardeners.
The USDA map was revised and reissued in with freshly available climate data, this time with 5-degree distinctions dividing each zone into new "a" and "b" subdivisions. The map placed many areas approximately a half-zone higher warmer than the USDA's map.
Reviewers noted the map zones appeared to be closer to the original USDA 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. Inthe Arbor Day Foundation released an update of U. It revised hardiness zones, reflecting generally warmer recent temperatures in many parts of the country, and appeared similar to the AHS draft.
In the USDA updated their plant "Edating zones for plants" map based on — weather data, using a longer period of data to smooth out year-to-year weather fluctuations. 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 map.
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: 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 zoneswhile in the US, zones 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 ; "Edating zones for plants" 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, Edating zones for plants summer locations like Seattle and Londonas 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. In the colder zones, another issue is that the hardiness scales do not take into account the reliability of 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 Edating zones for plants 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 Montreallocated to the southwest in zone 5, it is sometimes difficult to cultivate plants adapted to the zone because of the unreliable snow cover. Another problem is that many plants may survive in a locality but will not flower if the day length is insufficient Edating zones for plants 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 so on. The annual extreme minimum temperature is a useful indicator, but ultimately only one factor among many for plant growth and survival. An alternative means of describing plant hardiness is to use "indicator plants".
In this method, common plants with known limits to their range are used.