Rose,
I don't know much about it personally but based on what I've read posted on various blogs and gardening websites, many Californian gardeners swear by the 'Sunset Zones' because they take into consideration many of the factors that create the large variety of micro-climates in their state. Below are a couple of links to check out.
Sunset zone info:
http://www.sunset.com/garden/c...
Plant Finder with Sunset zone finder:
http://plantfinder.sunset.com/...
I definitely understand the importance of knowing the heat tolerance of plants since we have very hot temperatures here too. It's so disappointing to see a new plant that was gorgeous in spring suddenly melt into a moldy puddle when the summer temperatures arrive. Hopefully someone from California will check in here soon and give you some specialized advice.
