Green Circle Growers Blog

USDA Recalculates Plant Hardiness Zones

Green Circle Growers - Thursday, April 26, 2012

Indicative of changing weather and temperature patterns, March set high temperature records across much of the U.S. Chicago, where early spring temperatures rarely climb above 50, saw the mercury rise to 80 degrees F. an unprecedented eight times during March. From America’s eastern plains, across the Midwest and South and into New England, unseasonably warm temperatures caused bulbs to bloom and perennial plants to start growing a full month ahead of schedule.

USDA Recalculates Plant Hardiness ZonesWhile the temperature increase was particularly noticeable this spring, average daily temperatures across the U.S. have been rising slowly but steadily, forcing the U.S. Department of Agriculture to revise the national plant hardiness map for the first time since 1990. The map divides the country into 26 zones based on increments of the average lowest temperature. Plant growers, farmers and gardeners use the map to determine which plants will grow successfully in each zone.

For example, the 1990 plant hardiness map, which was based on temperatures between 1974 and 1986, split Ohio between zones 5 and 6. The higher the zone number, the higher the temperature required for plant survival with zone numbers climbing as you move south. The new map is based on a longer temperature span, 30 years compared to the previous map’s 12 years. According to the National Climatic Data Center, during the 30-year period between 1976 and 2005, the average national temperature increased by two-thirds of a degree. That might not sound like a lot, but it was enough to push plant zones north, putting nearly all of Ohio in zone 6.

To make the most of your outdoor greenery, Green Circle Growers recommends purchasing outdoor plants zoned for your location