Among the most important factors determining which plants can survive where is how cold the winter is. If you’re planting a garden this spring, this set of maps can help you see how planting zones across the country have shifted ever so subtly over the past few decades in response to warming climate.

Click titles below maps to switch map view. The maps at right show average minimum winter temperatures across the contiguous United States divided into 10-degree planting zones for the most recent U.S. Climate Normals (1981-2010), the old Normals (1971-2000), and how the margins of the planting zones shifted between the two time periods as a result of warmer winter minimum temperatures. In these areas, warmer nights are allowing plant species to shift their range northward.

Updated each decade, the U.S. Climate Normals from NOAA's National Climatic Data Center are 30-year averages of many pieces of weather information collected from thousands of weather stations nationwide. Each time they are updated, an old decade is dropped, and a new one is added. The last update was in July 2011: the decade 1971-1980 was dropped, 2001-2010 was added, and the new 30-year window for the U.S. Climate Normals became 1981-2010.

Since the '70s was an unusually cool decade, while 2001-2010 was the warmest ever recorded, it is not surprising that the average temperature rose for most locations. For the United States as a whole, though, it was not daytime highs (maximum temperatures) but overnight lows (minimum temperatures) that rose the most compared with the 1970s.

Climate normals can help people understand what conditions they can expect wherever they may live… and plant. A previous article on Climate.gov explored the implications of the new climate normals for gardeners and landscapers. Not only can plants can generally survive farther north than they used to, but the fire season is longer and pests are able to thrive and spread in forests and other natural landscapes. Pollination patterns may also be changing.

The Third National Climate Assessment confirms that landscapes across the country are changing rapidly.  The report predicts that “species, including many iconic species, may disappear from regions where they have been prevalent or become extinct, altering some regions so much that their mix of plant and animal life will become almost unrecognizable.”

These maps of climate-related planting zones were created by the National Oceanic and Atmospheric Administration (NOAA) in 2011 as a special service to the American Public Garden Association. In 2012, the U.S. Department of Agriculture (USDA), using data collected by NOAA, updated their official Plant Hardiness Zone Map, the standard by which gardeners and growers can determine which plants are most likely to thrive at a location.

NOAA Climate.gov map, based on climate normals data from the National Climatic Data Center.

Related Links

The New Climate Normals: Gardeners Expect Warmer Nights. NOAA Climate.gov.

References

Groffman, P. M., P. Kareiva, S. Carter, N. B. Grimm, J. Lawler, M. Mack, V. Matzek, and H. Tallis, 2014: Ch. 8: Ecosystems, Biodiversity, and Ecosystem Services. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 195-219.