Warming Earth Has Changed U.S. Hardiness Zones
“Plant hardiness zone designations represent what’s known as the ‘average annual extreme minimum temperature’ at a given location during a particular time period (30 years, in this instance). Put another way, the designations 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
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