Back in 2003 I was on the cover of the Grand Rapids Press when a draft version and rumblings of a new plant hardiness zone map were being discussed. The map was pulled from circulation which showed dramatic northward movement of hardiness zones. Some of this vacillation may have been over disagreements on the political hot button of global warming.
In 2006 the Arbor Day Foundation issued a map showing climate zones had shifted. The USDA with their new map release confirms these trends today. http://planthardiness.ars.usda.gov/PHZMWeb/#
Plant hardiness zones represent the average extreme minimum temperatures at a location. They don’t reflect the coldest a location has gotten or will get, but rather the average lowest winter temperature. The updated map, the first update since 1990, moves from a static map to an interactive map allowing the user to zero in on their neighborhood. This is very similar to what you would do when using Google maps. This is far better than the bottom half of Michigan being painted with a broad brush stroke and considered to be “zone 5” in the old static map. The new interactive map gives credit for pockets or “micro-climates” due to unique circumstances. For example in West Michigan closer to the lake shore the lake “warms” the average minimum temperature and lake effect snows provide plant insulation.
As you “zone” in it’s not unusual to find different hardiness zones within a community. In my area we have micro-climate variation from 6B (minimum of -5 to 0), 6A (minimum of -10 to -5), 5 B (minimum of -15 t0 -10) and 5A (minimum of -20 to -15) all in the same county.
As an “entre-manure” I’ve always pushed the envelope and tried plants not considered hardy to my zone. For years I’ve planted zone 6 plants even though my area was considered zone 5. With the new map and my move to zone 6 A, don’t be surprised if I’m tempted to experiment with overwintering some zone 7 plants!