What are the plant hardy zones and how do they relate to climate?

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What are the plant hardy zones and how do they relate to climate?

If you are involved with gardening, you probably are aware of the hardy zones listed on seed packets. The U.S. Department of Agriculture developed the zones and first published them in 1960.

A hardiness zone provides information on the type of plants capable of surviving certain climatic conditions. These conditions include the ability of a plant to survive a minimum temperature range. Regions with hardiness zone 4b have an average annual minimum temperature range between -25° and -20°F, while zone 5a has a range between -20 to -15°F. The climate zones are determined from temperature records kept by the National Climatic Data Center, or NCDC.

Recognizing that climate varies over long time periods, every ten years the NCDC computes a revised 30-year average temperature and extreme temperatures for the U.S. Between 1961-1990 and 1971-2000, the 30-year average minimum winter temperatures increased at nearly all locations in the continental U.S. This suggests that when the USDA next revises the hardy zone map, the zones in Wisconsin may shift northward.

While the hardiness zones are very useful, they cannot account for all climate and weather conditions such as snowfall, which can insulate the plants during a cold winter, or severe summer time heat. In addition to knowing your plant hardiness zone, it is also very useful to talk with local master gardeners and nurseries as you plan your plantings this season.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.