Gardening in California: zones, frost dates and when to plant

Four growing regions

The UC Master Gardener Program’s planting table, adapted from the California Master Gardener Handbook, divides the state into four broad vegetable regions:

• North and North Coast: Monterey County northward.
• South Coast: San Luis Obispo County southward.
• Interior Valleys: the Sacramento and San Joaquin valleys and others like them.
• Desert Valleys: the Imperial and Coachella valleys.

The NOAA state climate summary explains why they differ so much. The Pacific Ocean keeps coastal temperatures mild year-round, while inland areas swing more widely. The southern deserts are among the hottest and driest places in the country, and the high mountains get cold winters and heavy snow. UC cautions that each region is large and local climates vary within it, so use the frost dates for your city above along with what experienced local gardeners do.

Planting times shift by months, not weeks

The same crop can have very different planting windows in different parts of the state. In the UC table, tomatoes are set out in May on the North Coast, in April and May on the South Coast and in the interior valleys, and from December through March in the desert valleys. Lettuce runs from August to April on the South Coast but only from September to December in the deserts. Sweet corn spans March to July in the interior valleys and only February to March in the deserts, where summer heat ends the season early.

The handbook separates cool-season crops, which do best at average temperatures of 55°F to 75°F and tolerate light frost, from warm-season crops, which want long, hot days between 65°F and 95°F. In mild coastal and southern areas, cool-season crops fill the garden from fall through spring. The San Diego County planting guide from UC Cooperative Extension sets fall and winter windows for broccoli, cabbage, carrots, lettuce and spinach on both the coast and inland. It also notes that coastal gardeners can start warm-season crops such as beans, squash and tomatoes a few weeks earlier than inland gardeners.

That San Diego guide splits even one county into four bioclimates: coastal maritime, inland hill and mesa, inland valley and canyon, and inland high elevation. It reminds gardeners that the start and end of a planting window can move by several weeks from year to year, and that average temperatures are now warmer than when its dates were first worked out.

County Master Gardener programs publish their own schedules. The Sacramento County Vegetable Planting Schedule (EHN 11), based on UC Davis Vegetable Research and Information Center data, marks for each crop whether to start it in a sheltered spot, direct-seed it or set out transplants, month by month.

Dry summers make irrigation essential

California’s rain falls mostly in winter. The NOAA summary explains that the North Pacific High moves north in summer and keeps storms away, which makes summer the dry season. UC Master Gardeners of Alameda County point out that in the Bay Area, vegetables need watering all summer, through much of spring and fall, and sometimes in winter.

Their guidance for summer irrigation:

• Plan on one to three waterings a week, and more in heat waves or wind.
• Water sandy soil lightly and often; water clay, common in the Bay Area, deeply and less often.
• Keep seedbeds moist until seeds sprout. Beans, cucumbers, squash, tomatoes, peppers and eggplant need water most while flowering and setting fruit.
• Corn and most beans use the most water of common summer crops, while deep-rooted tomatoes need less.
• Check moisture 2 inches down between waterings, and avoid overwatering, which suffocates roots and invites disease.

Drought years compound the problem. The climate summary describes 2014 and 2015 as one of the state’s most severe droughts, when record warmth combined with several dry years.

Using summer heat against soil pests

Long, hot, cloudless summers allow a technique that works better in much of California than in most states: soil solarization. UC IPM’s Pest Notes describe covering moist, smooth soil with tightly anchored clear plastic for four to six weeks. That heats the soil enough to kill many disease organisms, weed seeds and nematodes, mostly in the top 12 inches. It works best in the Central Valley, the desert valleys and other inland areas from June through August, with July the most reliable month. Cooler coastal gardens may need to wait for warm, fog-free spells in August or September and leave the plastic on up to eight weeks. After the plastic comes off, the bed can go straight into a fall or winter crop.

For crop-by-crop timing, check the planting schedule from your county’s UC Master Gardener program, since even neighboring areas can differ by weeks.