The 2012 USDA Plant Hardiness Zone Map, released last week, is the new standard by which gardeners and growers can determine which plants are most likely to thrive in all 50 states plus Puerto Rico. Divided into 13 zones, the map is based on the average annual minimum winter temperature, divided by 10 degrees. Each zone is halved into "a" and "b" by 5 degrees.
Sacramento moved from Zone 9a to Zone 9b, reflecting slightly warmer winter lows – with a minimum above 25 degrees instead of the former standard of 20 degrees. This doesn't mean that Sacramento nights can't dip below 25, but it's become much rarer in the past four decades.
The USDA used average temperatures from 1976 to 2005 to determine the new zones. The former map was based on temperatures from 1974 to 1986.
Last revised in 1990, the zone map used to be distributed primarily as oversize posters for USDA offices, university extensions and nurseries. It became familiar to gardeners on the backs of seed packets. Not yet available in poster form, the new map is online and can be downloaded by home gardeners and professionals.
To find the hardiness zone for a specific location, go to http://planthardiness.ars.usda.gov and enter the ZIP code. With two clicks, the map automatically generates the answer.
The zones serve as a general guide to help determine not only frost tolerance, but seed germination. Temperatures need to be above a certain level for seeds to sprout. In that way, the zones also are used as guidelines for planting dates.
Some plants are strong indicators of particular zones. They'll do well in Zone 9, for example, but can't take the chill of Zone 8. Zone 9 indicators include: asparagus fern, Tasmanian blue gum (Eucalyptus globulus), Australian bush cherry, fuchsia, silk oak (Grevillea robusta) and California pepper tree.
In Zone 10 and warmer parts of Zone 9b, more plants are possible including: avocado, Meyer lemon, Mexican lime, bougainvillea, cherimoya, oak-leafed papaya, Dutchman's pipe, guava, red banana, lemon eucalyptus, rubber plant and royal palm.
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