USDA Plant Hardiness Zone Maps
By Marie Iannotti
The Newest USDA Plant Hardiness Zone Map for the United States
The Newest (2012) USDA Plant Hardiness Zone Map for the United States. USDA Plant Hardiness Zone Map, 2012. Ag.Research Service, USDA. http://planthardiness.ars.usda.gov/.
Gardeners have come to rely on USDA Hardiness Zone Maps to tell them what plants will survive in their climates. In 2012, USDA released updated Plant Hardiness Zone Maps for the United States, each individual state, each geographic region and Puerto Rico.
Plant hardiness zones are based on the average annual minimum temperature over a 30-year period, not the lowest temperature the thermometer has ever hit. They are guides to help you determine which plants will survive in your area. They are not steadfast law. Many circumstances besides hardiness zones, like soil quality, sun exposure, moisture and sudden or prolonged temperature swings, can affect how well plants will do in your yard.
There are also always microclimates that will vary within zones. (Even within yards.) Trust your own knowledge of your property and your garden.
You can download the map at the USDA Plant Hardiness Map Site.
What's New in the 2012 Plant Hardiness Zone Maps (PHZM)?Appearance and Data
- The resolution is noticeably better. Mapping was improved by using a Geographic Information System (GIS). The higher resolution makes seeing the distinction between zones clearer and easier. It also makes it possible to separate small anomalies, like cities, which tend to be warmer than surrounding areas, cooler high elevation area, and areas near mitigating large bodies of water, into their proper zones. It's the first map designed to be viewed via the internet. Zones can be zoomed in on a finer scale.
- A more sophisticated algorithm was used to compile low-temperature values from actual weather reporting stations.
- The zones are based on 1976–2005 weather data. The 30 year period was chosen as a means to factor in year to year fluctuations and variations. They did a trial check on more recent data and it did not make a measurable difference in zones. (The previous map was based on 1974-1986 data.)
There's an interactive map where you can pop in your zip code or click on the map and get your zone, actual average temperature, the temperature range for your zone and your longitude and latitude.
How Were the Hardiness Zone Maps Developed?In the mid-1900s, the U.S. Department of Agriculture (USDA) mapped out the entire United States, Mexico and Canada by lowest annual minimum temperature groupings. Each zone represented a 10 degree F. difference. This was invaluable advice for the agriculture industry. Now plants could be rated by hardiness zones, taking the guesswork out of choosing plant varieties. You had a gauge, other than experience, for picking plants.
The maps have been revised over the years, to reflect changes in climate. When cities and towns were moved from one zone to another, gardeners were left to wonder what would happen to their existing garden plants. While our climate may be shifting, these changes did not occur over night. Plants are adaptable, surviving in many different climates. They also cannot read maps.
In 1990, the zones were further divided, with each numbered zone being broken down into an ‘a’, the lower temperature end of the zone, and a ‘b’, the higher. Unfortunately plant breeders have yet to start using these distinctions, so they are mostly useful if a gardener want’s to push the envelope a bit. A gardener in zone 6b will be tempted to dabble in 7a plants. Given the variability of climate, it’s a hit and miss situation.
The American Horticulture Society (AHS) introduced a Plant Heat-Zone Map in 1997, intended to supplement the hardiness map. Heat related problems are much harder to quantify. High summer temperatures only told half the story. Plants don’t usually react to a day or two of heat they way they might respond to a frost. However, plants subjected to a two week heat wave could well succumb. Other variables that weren’t considered were things like humidity levels, nighttime temperatures and rainfall USDA Plant Hardiness Zone Maps for Alaska and Hawaii
The newest (2012) USDA Plant Hardiness Zone Map for Alaska and Hawaii. USDA Plant Hardiness Zone Map, 2012. Accessed from http://planthardiness.ars.usda.gov/.
Alaska and Hawaii are not included on the same map with the lower 48 states. They have their own separate map, shown here.
Two new zones were added to the warm end of the scale. They apply only to the climates of Hawaii and Puerto Rico, but they will also be useful to anyone who grows tropical and semitropical plants indoors, by better defining the growing conditions these plants favor and in particular, their sensitivity to cold.
- Zone 12 - average annual extreme minimum temperatures above 50 F.
- Zone 13 - average annual extreme minimum temperatures above 60 F.
USDA Regional Plant Hardiness Zone MapsThe newest (2012) USDA Plant Hardiness Zone Regional Map for the US Southwest. USDA Plant Hardiness Zone Map, 2012. Ag.Research Service, USDA. http://planthardiness.ars.usda.gov/.
taken from https://www.thespruce.com/usda-plant-hardiness-zone-maps-
till next time this is Becky Litterer, Becky's Greenhouse, Dougherty Iowa email@example.com