Pretty, huh? I'm such that's not what they had in mind, but it is. I really like the color gradient on the map.Wouldn't it make great gardening inspired wall art? I totally think so.
Here's a close-up of Kansas.
What does that actually mean? It means that based on weather data for the last 30 years (1976-2005 to be exact, the average minimum temperature in my area is slightly warmer than it was from 1974-1986 (the dates they used to produce the 1990 map).
It also means that I could theoretically plant plants which can handle temperatures down to -10 F, but can't quite hack -15 with a reasonable expectation of their survival. So far, I've found that means I now have the opportunity to plant lesser Calamint, Colewort, Crocosmia, Lavender Cotton, and Leadwort. There are also some plants that need the amount of cold weather expected in Zone 5 to thrive. Those might not do so well. It could also mean that it gets a little too hot for some plants that are zone 5 although hardiness zones are only reflective of cold tolerance so they don't necessarily tell you about heat tolerance.