USDA Hardiness Zones

The USDA Hardiness Zones map is a climate guide that helps gardeners understand temperatures and how their gardening relates to their geographic location. The map was developed by the United States Department of Agriculture in 1960, and is based on the average minimum temperature for each location as an indication for when to begin planting and seeding in the Spring.

The zones are numbered from 1 until 11. The Zone numbering starts from southern North America, (Texas, Florida) where the Winter months are not as cold (mostly Zones 9-11), gardeners should allow for a longer growing season, which means people can begin seeding and planting earlier in the Spring. As you move north on the map, the minimum temperature requirements to plant seeds are reached later and later in the year. In areas such as Zones 4 and 5, planters should wait until May to be sure that temperatures are stable enough to support outdoor plants and seeds.

Each zone is separated by 5-degree F differences. The colors on the map range from hot to cold colors. Light colors on the map represent colder sections while darker colored sections represent areas of the country that have warmer weather. Zone 11 represents any area where the average annual minimum temperature is above 40 degrees F. http://www.usna.usda.gov/Hardzone/hrdzon3.html

The first map was developed by Henry Skinner during the time he was the director of the U.S. National Arboretum, and has undergone three major revisions in its history to date. It was first revised in 1965 to add information that was missing from the original map.

That map was used up until 1990, when a new and revised map was developed to include the United States as well as Canada based on information developed between 1974 and 1986. Information gathered from 1971 to 1984 was used to add Mexico to the map. H. Marc Cathey was the designer of the revised map. The planting zones were originally broken down in 10-degree increments, but the revised map lowered it to 5-degree increments.http://www.garden.bsewall.com/topics/hardiness/history.html

A new map was created with assistance from the National Arbor Day Foundation in 2006 to reflect the changing landscape of the hardiness zones. This was done based on new information and factors, such as global warming, stress factors of plants, and new agricultural techniques and technologies developed to extend growing seasons.http://www.usna.usda.gov/Hardzone/index.html#howuse

Other factors not incorporated into Hardiness Maps include: Summer heat levels (like heat waves), snow cover being used as an insulation, soil moisture, humidity, the number of days of frost, and the risk of catastrophic cold snaps or blizzards.

The USDA Hardiness Zones map is an indication for the average lowest temperature in each zone across the country, allowing for a better understanding of where different plants will survive the Winter months. It is an indication also as to when gardeners should begin their Spring plantings when the threat of frost has passed in their area. All gardeners have to do is take a look at the map and locate their city on the map and the corresponding zone number. The higher the zone number, the higher the minimum temperature in each zone is annually and the sooner their growing season can begin and the longer it will last (ex: growing seasons in South Carolina are longer than those in New Jersey).