|I am having a difficult time determining what zones I should be looking for when buying outdoor plants- we live in Concord, CA. I am also wondering if everyone uses the same zoning system.
|You're not alone in your confusion and pinpointing your gardening zone. According to the USDA, you're in Zone 9 with minimum temperatures of 20 to 30F. But that's a pretty general statement which is qualified by the following:
One of the smallest regions in the plant hardiness zone map, Zone 9 includes central Florida, the Gulf coast of Louisiana and Texas, much of California, and the southern Oregon coast. This mild-winter region is characterized by a long growing season and almost no winter freeze. Summers are either hot and dry or hot and humid. Although it might seem that just about any plant could do well in such a mild area, Zone 9 (and warmer) is not good for plants that require a winter chill. Those that flourish include Asparagus setaceus (Asparagus fern), Eucalyptus globulus (Tasmanian blue gum), Syzygium paniculatum (Australian bush cherry), Fuchsia hybrids (Fuchsia), and Schinus molle (California pepper tree).
Microclimates develop within cities, neighborhoods, and even your own yard due to elevation (including low spots), influence of bodies of water, and protection from tall trees or buildings. One of the best sources I have found for determining at least a reasonable assessment of gardening zones is through Sunset Western Garden Book. The publishers have taken into account the major elements (elevation and proximity to water) to further divide the USDA zones into mini-zones. You're in their zone 15. The book also lists plants that are likely to thrive in all gardening zones. Your library should have a copy of the book (ISBN# 0-376-03851-9)
Hope this helps clarify things!