Hardiness zone


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 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 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 is instead defined by annual high temperatures; the American Horticultural Society 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, microclimates 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.

United States hardiness zones (USDA scale)

The USDA system was originally developed to aid gardeners and landscapers in the United States.
In the United States, most of the warmer zones 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 and Puerto Rico. The southern middle portion of the mainland and central coastal areas are in the middle zones. The far northern portion on the central interior of the mainland have some of the coldest zones 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. 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 and the coldest is in north-central Minnesota. 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.

Definitions

History

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, and the second by Donald Wyman in 1938. 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 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 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 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 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 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, 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.

Selected U.S. cities

The USDA plant hardiness zones for selected U.S. cities as based on the 2023 map are the following:
CityZone
Albuquerque, New Mexico7b/8a
Anchorage, Alaska5a/5b
Atlanta, Georgia8a/8b
Austin, Texas9a
Baltimore, Maryland7b/8a
Boise, Idaho7a/7b
Boston, Massachusetts6b/7a
Buffalo, New York6b
Charlotte, North Carolina8a
Chicago, Illinois6a/6b
Cincinnati, Ohio6b
Cleveland, Ohio6b/7a
Colorado Springs, Colorado5b/6a
Columbus, Ohio6b
Dallas, Texas8b
Denver, Colorado6a
Detroit, Michigan6b
El Paso, Texas8b/9a
Fairbanks, Alaska2a
Fort Worth, Texas8b
Fresno, California9b
Greensboro, North Carolina8a
Honolulu, Hawaii12b
Houston, Texas9b
Indianapolis, Indiana6a/6b
Jacksonville, Florida9a/9b
Kansas City, Missouri6b/7a
Las Vegas, Nevada9a/9b
Los Angeles, California10a/10b
Louisville, Kentucky7a
Memphis, Tennessee8a
Miami, Florida11a
Milwaukee, Wisconsin5b/6a
Minneapolis, Minnesota5a
Nashville, Tennessee7b
New Orleans, Louisiana9b
New York, New York7b
Oklahoma City, Oklahoma7b
Omaha, Nebraska5b/6a
Orlando, Florida10a
Philadelphia, Pennsylvania7b
Pittsburgh, Pennsylvania6b/7a
Phoenix, Arizona9b/10a
Portland, Oregon8b/9a
Raleigh, North Carolina8a
Reno, Nevada7a/7b
Sacramento, California9b
Salt Lake City, Utah7a/7b
San Antonio, Texas9a
San Diego, California10b/11a
San Francisco, California10a/10b
San Jose, California9b/10a
San Juan, Puerto Rico13a/13b
Seattle, Washington9a
St. Louis, Missouri7a
Tampa, Florida10a
Tucson, Arizona9a/9b
Tulsa, Oklahoma7b
Virginia Beach, Virginia8a/8b
Washington, D.C.7b/8a
Wichita, Kansas7a