A new version of the ‘Plant Hardiness’ map has been released by the U.S. Department of Agriculture. It enables users to view the average minimum temperatures for separate latitudinal zones — a useful and anticipated online tool for keen gardeners.
The 2012 USDA Plant Hardiness Zone Map (PHZM) allows gardeners and growers to assess which plants are most likely to thrive at any given location, as well as view warming trends across various U.S. states. Taking the guesswork out of plant growth may encourage different nursery stocks, and could help gardeners experiment with species they may not have attempted before.
Each zone temperature the map represents is divided in to 10 degrees Fahrenheit. There is a total of 13 ‘zones’ — Zone one is the coldest (-60 to -50) whereas Zone 13 is the hottest on record (60 to 70).
Two new zones have been added to this edition — Zones 12 and 13. They only appear on the maps concerning Hawaii and Puerto Rico.
The map uses 30 years of weather data, gathered from 1976 to 2005, and is a long-awaited update to the less accurate 1990s version. This version of the Plant Hardiness map includes smaller areas and encompasses additional factors — such as elevation and proximity to bodies of water — in an attempt to make the data more accurate.
The temperature data came from various sources — including weather stations of the National Weather Service, USDA Natural Resources Conservation Service, USDA Forest Service, and the DOI Bureau of Land Management. Environment Canada and the Global Historical Climate Network contributed to define hardiness zones in Canada and Mexico.
The map is located on the USDA website, where users are able to find their ‘plant hardiness zone’ by entering a ZIP code, down to a half-mile radius. It is also possible to view static maps according to U.S. states. There are no current print versions of the map data, however, users are still able to download state, regional and national images in high enough resolutions to print.
In terms of climate change, the website notes that as climate trend differences are recorded over the course of 50 - 100 years, this kind of 30-year average data is not suitable for predictive use. Changes in temperature trends are not necessarily due to larger-scale climate change, but rather slight shifts in zones and data which is now more accurate than what used to be available.
Image credit: USDA
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