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Barbara Koh

Plant Zones in the San Francisco Bay Area

By April 17, 2013

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If you're starting a garden and deciding what to grow in it, you may want to check on your plant zone. (Unlike most zoning in San Francisco, this, fortunately, does not require dealing with the bureaucracy at City Hall). Plant zones are standards to help determine which plants are most likely to flourish in your neck of the woods.

The U.S. Department of Agriculture's zoning is based on the average lowest winter temperatures in an area over a 30-year period. The USDA has defined 13 different zones for the country, with each zone spanning a 10-degree F difference in the average annual minimum temperature. (Zones are further sub-divided into 5-degree F differences, indicated with "a" and "b.")

The latest USDA map, the 2012 USDA Plant Hardiness Zone Map, is based on temperatures recorded from 1976-2005, and considers temperature changes due to elevation, topography and proximity to large bodies of water like oceans and lakes. On the interactive map, find your location (which you can do by entering your ZIP code) and click on it. Your zone number, the zone's average temperature and temperature range, and your longitude and latitude will pop up. San Francisco is in zones 10a (with a range of 35 to 40 F) and 10b (range of 30 to 35F).

Sunset magazine's climate zones were designed to take into account temperature and factors like rainfall, air flow, wind and humidity. "The USDA maps tell you only where a plant may survive the winter; our climate zone maps let you see where that plant will thrive year-round," says the magazine, which focuses on the U.S. West.

On Sunset's map of the San Francisco Bay Area, the city is in zone 17 (a "heat-starved climate" where fog tends to smother light and sunshine). Surrounding areas are in zones 16 and 15. Zone 16, which includes northern Marin County, is praised as "benign" and "one of Northern California's finest horticultural climates."

But finding your zone by your ZIP code requires going to a different page, where you can also search for suitable plants based on your climate zone and other criteria.

Note that neither the USDA nor the Sunset zone map can factor in micro-climates (which can exist even within your backyard), drastic swings in temperature, soil type and moisture, pollution and other environmental variables.

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