Wednesday, January 25, 2012

Video Wednesday: New Plant Hardiness Zone Map Unveiled

This video is from today's press conference held at the U.S. National Arboretum. The U.S. Department of Agriculture (USDA) today released the new version of its Plant Hardiness Zone Map (PHZM), updating a useful tool for gardeners and researchers for the first time since 1990 with greater accuracy and detail.

The new, interactive map is available online at:

According to the USDA:

"Compared to the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period; the new map uses data measured at weather stations during the 30-year period 1976-2005. In contrast, the 1990 map was based on temperature data from only a 13-year period of 1974-1986.
However, some of the changes in the zones are a result of new, more sophisticated methods for mapping zones between weather stations. These include algorithms that considered for the first time such factors as changes in elevation, nearness to large bodies of water, and position on the terrain, such as valley bottoms and ridge tops. Also, the new map used temperature data from many more stations than did the 1990 map. These advances greatly improved the accuracy and detail of the map, especially in mountainous regions of the western United States. In some cases, they resulted in changes to cooler, rather than warmer, zones."

The USDA representatives did not think this was conclusive evidence of climate change, in that the data is from a 30 year period and that a much longer period of data (50-100 years) would need to be examined to warrant that assertion.

Kim Kaplan of the USDA stressed that the plant zone map is merely a guide and that home gardeners should use their best judgement when selecting plants for their own nano-climates.

What I find most exciting about this new zone map, aside from the interactive search and increased detail, is the listing of cold hardiness ratings of woody plants that will thrive in each zone. That will be a truly valuable addition for home gardeners to explore.

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