A plant's performance is governed by the total climate: length of growing season, timing and amount of rainfall, winter lows, summer highs, humidity. Sunset's climate zone maps take all these factors into account--unlike the familiar hardiness zone maps devised by the U.S. Department of Agriculture, which divide the U.S. and Canada into zones based strictly on winter lows. The U.S.D.A. 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. For more information, consult Sunset's National Garden Book and Western Garden Book.
ZONE 16. Northern and Central California Coast Range Thermal Belts
Growing season: late Feb. to late Nov. With cold air draining to lower elevations, winter lows typically run from 32 degrees to 19 degrees F/0 degrees to -7 degrees C. Like Zone 15, this region is dominated by maritime air, but its winters are milder on average.
ZONE 17. Oceanside Northern and Central California and Southernmost Oregon
Growing season: late Feb. to early Dec. Coolness and fog are hallmarks; summer highs seldom top 75 degrees F/24 degrees C, while winter lows run from 36 degrees to 23 degrees F/2 degrees to -5 degrees C. Heat-loving plants disappoint or dwindle here.
ZONE 18. Hilltops and Valley Floors of Interior Southern California
Growing season: mid-Mar. through late Nov. Summers are hot and dry; rain comes in winter, when lows reach 28 degrees to 10 degrees F/-2 degrees to -12 degrees C. Plants from the Mediterranean and Near Eastern regions thrive here.
ZONE 19. Thermal Belts around Southern California's Interior Valleys
Growing season: early Mar. through Nov. As in Zone 18, rainy winters and hot, dry summers are the norm--but here, winter lows dip only to 27 degrees to 22 degrees F/-3 degrees to -6 degrees C, allowing some tender evergreen plants to grow outdoors with protection.
ZONE 20. Hilltops and Valley Floors of Ocean-influenced Inland Southern California
Growing season: late Mar. to late Nov.--but fairly mild winters (lows of 28 degrees to 23 degrees F/-2 degrees to -5 degrees C) allow gardening through much of the year. Cool and moist maritime influence alternates with hot, dry interior air.
ZONE 21. Thermal Belts around Southern California's Ocean-influenced Interior Valleys
Growing season: early Mar. to early Dec., with the same tradeoff of oceanic and interior influence as in Zone 20. During the winter rainy season, lows range from 36 degrees to 23 degrees F/2 degrees to -5 degrees C--warmer than in Zone 20, since the colder air drains to the valleys.
ZONE 22. Colder-winter Parts of Southern California's Coastal Region
Growing season: Mar. to early Dec. Winter lows seldom fall below 28 degrees F/-2 degrees C (records are around 21 degrees F/-6 degrees C), though colder air sinks to this zone from Zone 23. Summers are warm; rain comes in winter. Climate here is largely oceanic.
ZONE 23. Thermal Belts of Southern California's Coastal Region
Growing season: almost year-round (all but first half of Jan.). Rain comes in winter. Reliable ocean influence keeps summers mild (except when hot Santa Ana winds come from inland), frosts negligible; 23 degrees F/-5 degrees C is the record low.
ZONE 24. Marine-dominated Southern California Coast
Growing season: all year, but periodic freezes have dramatic effects (record lows are 33 degrees to 20 degrees F/1 degree to -7 degrees C). Climate here is oceanic (but warmer than oceanic Zone 17), with cool summers, mild winters. Subtropical plants thrive.
ZONE 25. South Florida and the Keys
Growing season: all year. Add ample year-round rainfall (least in Dec. through Mar.), high humidity, and overall warmth, and you have a near-tropical climate. The Keys are frost-free; winter lows elsewhere run from 40 degrees to 25 degrees F/4 degrees to -4 degrees C.
ZONE 26. Central and Interior Florida
Growing season: early Feb. to late Dec., with typically humid, warm to hot weather. Rain is plentiful all year, heaviest in summer and early fall. Lows range from 15 degrees F/-9 degrees C in the north to 27 degrees F/-3 degrees C in the south; arctic air brings periodic hard freezes.
ZONE 27. Lower Rio Grande Valley
Growing season: early Mar. to mid-Dec.. Summers are hot and humid; winter lows only rarely dip below freezing. Many plants from tropical and subtropical Africa and South America are well adapted here.
ZONE 28. Gulf Coast, North Florida, Atlantic Coast to Charleston
Growing season: mid-Mar. to early Dec. Humidity and rainfall are year-round phenomena; summers are hot, winters virtually frostless but subject to periodic invasions by frigid arctic air. Azaleas, camellias, many subtropicals flourish.
ZONE 29. Interior Plains of South Texas
Growing season: mid-Mar. through Nov. Moderate rainfall (to 25" annually) comes year-round. Summers are hot. Winter lows can dip to 26 degrees F/-3 degrees C, with occasional arctic freezes bringing much lower readings.
ZONE 30. Hill Country of Central Texas
Growing season: mid-Mar. through Nov. Zone 30 has higher annual rainfall than Zone 29 (to 35") and lower winter temperatures, normally to around 20 degrees F/-7 degrees C. Seasonal variations favor many fruit crops, perennials.