By Dr. Susan Hamilton
If you haven’t heard, one of the biggest items in gardening news lately is the unveiling of the new U.S. Department of Agriculture (USDA) Plant Hardiness Zone Map. The map is the standard by which gardeners and growers can determine what plants are most likely to endure winter temperatures at a given location.
Even though 2012 was an unusually warm winter for much of the United States, low temperature during the winter is a crucial factor in the survival of plants at specific locations. The USDA Plant Hardiness Zone Map is based on the average annual minimum winter temperature. While the zones do represent the average annual extreme minimum temperatures at a given location during a 30-year period in the past, they do not reflect the coldest it has ever been or ever will be at a specific location.
The USDA zone map had not been updated since 1990, and the new version of the map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees Fahrenheit) and 13 (60-70 degrees Fahrenheit). Each zone is a 10-degree Fahrenheit band, further divided into 5-degree Fahrenheit zones “A” and “B.” The new map – jointly developed by USDA’s Agricultural Research Service (ARS) and Oregon State University’s (OSU) PRISM Climate Group – is available online at www.planthardiness.ars.usda.gov.
For the first time, the USDA zone map offers a Geographic Information System (GIS)-based interactive format and is specifically designed to be Internet-friendly. The map website also incorporates a function that allows gardeners to find their zone by ZIP code.
Zone hardiness is a handy thing to know, but don’t be frustrated when a plant dies, and certainly don’t give up trying to grow that particular plant again. Growing plants can be very complex, and many environmental factors can impact the winter hardiness of a plant – wind, soil type, soil moisture, humidity, pollution, snow and winter sunshine to name a few. Individual gardens may even have localized microclimates that may be warmer or cooler than the general zone for your area, so no hardiness zone map can take the place of the detailed knowledge that gardeners pick up about their own gardens through hands-on experience.
Keep in mind, too, that Tennessee is in the Mid-South, meaning the “transition zone” between the North and the deep South. For Tennessee gardeners, this means our plants may not always acquire a gradual transitioning from fall to winter or winter to spring. We aren’t surprised by a bout of extremely cold weather early in the fall that may injure plants even though the temperatures may not reach the average lowest temperature for our zone. Similarly, exceptionally warm weather in midwinter followed by a sharp change to seasonably cold weather may injure plants as well.
Be aware that the hardiness zone map is a great guide, but only a guide – and only when the zones assigned to plants by producers are accurate.
If you are interested in learning more about the history and development of the map, noted horticulturist Tony Avent of Plant Delights Nursery offers a wonderful in-depth and thorough article.
About the Author
Dr. Sue Hamilton is director of the UT Gardens and an associate professor on the faculty of the University of Tennessee Department of Plant Sciences. The Gardens are a project of the UT Institute of Agriculture, with locations in Knoxville and Jackson. Visit utgardens.tennessee.edu to learn more.