For the first time since 1990, the USDA has revamped the zones in its plant hardiness map.
While this is big news for America’s 80 million gardeners, it’s also important for farmers, reports agriculture.com.
USDA’s Risk Management Agency uses the zone designations to set some crop insurance standards, and scientists use the plant hardiness zones as a data layer in many research models such as modeling the spread of exotic weeds and insects.
“This is the most sophisticated Plant Hardiness Zone Map yet for the United States,” said Dr. Catherine Woteki, USDA’s Under Secretary for Research, Education and Economics. “The increases in accuracy and detail that this map represents will be extremely useful for gardeners and researchers.”
Plant hardiness zone designations represent the average annual extreme minimum temperatures at a given location during a particular time period. They do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants at specific locations.
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.
Zone boundaries in the new map have shifted in many areas compared to the 1990 version. 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 from 1976 to 2005, while the 1990 map was based on data from only 1974 to 1986. The new version uses data from many more weather stations than the previous version.
The map now takes into account factors as changes in elevation, nearness to large bodies of water, and position on the terrain, such as valley bottoms and ridge tops.