The United States Department of Agriculture (USDA) released a new plant hardiness zone map in January. Hardiness zones are of interest to all gardeners as they give you some idea of what sort of plants may be expected to thrive in your region. The map is based on the average annual minimum winter temperature. The new version of the map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees F) and 13 (60-70 degrees F). Each zone is a 10-degree Fahrenheit band, further divided into A and B 5-degree Fahrenheit zones.
The new map replaces the 1990 map, which was based on temperature data from a 13-year period of 1974-1986. The 2012 edition uses data measured at weather stations during the 30-year period 1976-2005. I didn’t see any clear explaination as to why a longer period was used. In other situations it might be argued that data from a longer period is more accurate, but as average temperatures have been spiraling upward and the last decade has seen some of the warmest years on record, extending the data period to 30 years disguises the impact of recent changes. Perhaps the USDA was looking to deflect critisism from the Big Oil-Wall Street crowd.
Here’s a chart released by the National Oceanic and Atmospheric Administration (NOAA). The year 2010 tied with 2005 as the warmest year on record since 1880. Last year, 2011, was ranked 11th warmest, tied with 1997. However, 2011 was a La Niña year. La Niña is defined by cooler-than-normal waters in the eastern and central equatorial Pacific Ocean that affect weather patterns around the globe. When compared to previous La Niña years, the 2011 global surface temperature was the warmest on record.
The new USDA map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. What does this mean for gardeners? If there has been a borderline-tender plant you have been itching to try in your garden, maybe now is the time. Because the times, they are a changin’!
The new USDA map is interactive. You can check out your hardiness zone here. For the NOAA State of the Climate report, click here.
Finally, here is a 27 second video from NASA illustrating global average surface temperatures from 1880 to 2011.
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