The USDA Zone map puts me on the line between 6b - 7a, but arbor day says I am zone 8? I am inclined to think arbor day is correct, because we have not seen 0F - 5F since I was a child, unless that is including windchill?

I just changed mine to 9A since the we moved. In SW California, there are so many "microclimates" that it seems like the zone could shift from year to year. I'm on a western facing slope so I tend to get more rain than others. I think I might get more heat than where I used to live because I previously was at the base of a eastern slope of a mountain. After the sun went over the mountain, we got a late afternoon shade well before a lot of other people did.

Phillip

5F - 10F would be a zone 7b

The most recent USDA hardiness zone map was published in 1990 and is based on the annual average coldest temperature from 1974 to 1986. The arbor day foundation map was published in 2006 and is based on the average annual coldest temperature of 15 years of data from about 1991 to 2005. I'm not sure of the year range for the arbor day map. The hardiness zone where I live can range from 3a to 5a depending on what range of years you look at. We've gone from 4 to 5 and now back to 4 over the past 70 years with some short periods of zone 3 in there. Because of the inherent climate variabilty from year to year, any hardiness map has to be treated as a guideline only.