Hardiness zones are geographic areas with the same range of low temperatures, say 30 – 40 F, well, most winters it never gets below freezing here in Berkeley.
The National Arbor Day Foundation has posted a map showing the shift of hardiness zones between 1990 and 2006, and another showing how many zones your area has changed in that time.
Thanks to We Support Lee for the pointer.
Have you been noticing differences in minimum temperatures in that time?
Here in Berkeley, the warm September/October winds blow in October/November, but minimum temperatures seem to be the same.
An interesting set of charts on min and max temperatures for Australia is here.
I’ve noticed higher minimum temperatures this December than last year.