I find this system useful as a additional knowledge base, but deficient in that it does not take into account elevation differences within a given area, as well as our topography. We've got 2 mountain ranges, lakes, Puget Sound, maritime, typical shallow snow cover & a variety of rain patterns in the mix.
Example: Sammamish is included in the same area as Seattle. These two places have very different weather patterns & while the average low temperatures might hover around 27-30 degrees in Seattle, travel 8 miles east & you've got temps in the teens. I watched this all play out while living many years in Sammamish. I found that I learned the most useful information about plant hardiness by communicating with other gardeners throughout the region.
Most gardening books rate Seattle as zone 8, Sammamish/Issaquah as zone 7. At the south end of the Sammamish plateau it was closer to zone 5or 6 as far as plant hardiness is concerned. It really all depends on siting.
The other thing not addressed is how our climate seesaws from year to year. I kept records of summer weather from 1985 on, based on Tomato & pepper growth/production, as well as the Mediterranean woody herbs. Most years June shows cool, July warms up to the upper 70s to low 80s & August would have a heat push for a couple weeks. Then it all slides downhill!
It might be beneficial to have a thread devoted to what each person has found successful for their location.
