This post is part of the Science Tuesday feature series on the USDA blog. Check back each week as we showcase stories and news from the USDA's rich science and research portfolio.
With spring not that far away, the garden catalogues are starting to fly through the mail, but the “must read” of the moment for America’s estimated 80 million gardeners is the new Plant Hardiness Zone Map rolled out by USDA last week at www.planthardiness.ars.usda.gov.
Plant hardiness zones are 10-degree-Fahrenheit geographic bands based on the annual lowest winter temperature in a particular location, averaged over a period of years. The map doesn’t aim to depict the coldest it’s ever been at that spot, or the coldest it might ever be, but just an average low over a specific period of time.
The zones—and now there are 13, with the addition of two new ones on this map—provide a common vocabulary with which gardeners and the people who breed and grow plants for them can talk about which plants will grow well in which locales.
The new map is the first update since 1990, and it’s the most sophisticated and accurate one ever created. It offers an interactive Geographic Information System (GIS)-based format that can be honed to a resolution of about half a mile, based on the user’s ZIP code.
No broadband Internet connection? That’s not an obstacle, because users still can peruse static images of a specific state, region or the nation to pinpoint their zone.
This map is as far from guesswork as anything could be: Its developers used a complex algorithm that incorporated data from 8,000 weather reporting stations, an equation that took into account geographic changes such as slope and elevation of the land, prevailing winds and nearness of large bodies of water. What’s more, it takes into account 30 years of that data, from 1974 to 2005.
Still, no map can replace a savvy gardener’s insider knowledge of his or her own garden, so hardiness maps are just a guide, not a guarantee of green-thumbed success. But this new map is one terrific guide.
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I looked at the map for my part of California and was sorely disappointed in its gross inaccuracy. The figures used appear to match up only with the warmest urban data collection points and the topography of our very micro-climate filled region seems to be completely not in sync with the new map. Both areas that I farm are noted as the same zone and sub-zone letter, yet the one has 5-10 degree colder maximum lows almost every year. The zip code focus also in our case can have a 3000 foot elevation difference in the mapped area. The map does not even take that into account at all.