Sunset Western Garden Book Zone Map
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 March to late November—but fairly mild winters (lows of 28 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 March to early December, 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, the colder air drains to the valleys.
Zone 22. Colder-winter Parts of Southern California's Coastal Region
Growing season: March to early December. 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 January). 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.