With the crazy weather that the United States has experienced lately—snow on Halloween or Snow-tober and then record heat in the winter—it's hard to deny that there is some sort of climate change. For the first time it seems that whatever changes in the weather have occurred, you'll find them reflected in planting recommendations. That is, the U.S. Department of Agriculture (USDA) plant hardiness zones map has changed in 2012.
In January the USDA released this new map, which is available online as an interactive tool. You can search for your planting zone by using the site's "find your zone by ZIP code" function.
Not quite sure why you need to know what hardiness zones are? Well, according to the USDA, these zones represent the average annual extreme minimum temperatures at a given location during a particular time period. They 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.
The new version of the map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees F) and 13 (60-70 degrees F). Each zone is a 10-degree Fahrenheit band.
Not only are there two new zones on these maps but also each of the existing zones is slightly warmer than the last time the USDA made the maps. Compared to the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. I was able to compare the old map with the new one on the National Wildlife Federation website.
For example, where I live in the Philadelphia suburbs used to be squarely in the "green" in Zone 6 on the USDA map. In the 2012 version of the map, my area has shifted to "yellow-green" in Zone 6b.
While some might point to global warming for this difference—it's possible, after all—the USDA says that more sophisticated mapping and temperature records are more responsible for the shift than anything else. These advances greatly improved the accuracy and detail of the map, especially in mountainous regions of the western United States. In some cases, advances resulted in changes to cooler, rather than warmer, zones.
What does this mean for the 80 million or so American gardeners? When it comes to picking plants for their garden, they should be sure to reference the new zones on the maps, rather than going by old standards, to have a better chance of choosing appropriate plants for a garden that will survive and thrive.