Farmers and gardeners alike are familiar with the USDA plant hardiness zone map, which divides the country into growing regions based on their typical winter minimums. Simply put, if you live in a zone where the temperature is likely to hit zero some nights, you can’t expect the same plants to survive as you could in places where nighttime lows never dip below freezing. The map considers topography as well as geography. Depending on where you live in the New York area, for example, you may be in zones 5, 6, or 7.
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Zone maps have been around since at least the 1930s, and the USDA published its first edition in 1960. The American Horticultural Society publishes an alternate zone map that takes maximum temperatures into consideration as well. That makes sense, since plants that need cooler temperatures to thrive won’t do well in intense heat. There’s also the National Gardening Zone Map, which takes rainfall and humidity into consideration. For many, though, the USDA zone map has become the standard.
There were significant updates to the USDA map in 1990 and again in 2012, adding more details and additional sub-zones, and reflecting some changes in weather conditions. Hardiness zones have generally shifted northward, reflecting warmer temperatures across the country.
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