When to Plant Tomatoes in California

California's large size makes planning vegetable gardens challenging when you have only statewide date recommendations for sowing and transplanting warm-weather crops such as tomatoes (Solanum lycopersicum). Along with acquiring temperature timetables for your part of the state, keep in mind the specific habits of tomato plants. This knowledge comes in handy if the micro-climate of your neighborhood, for example, tends to experience more cold snaps or heat spikes than the rest of the region. If you plant your tomatoes too early, they might succumb to frost or not fruit properly. Start them too late, on the other hand, and they will be reluctant to bear the flowers needed for good fruit set.

Tomato seeds are more likely to sprout when the air temperatures hover at 70 to 80 degrees Fahrenheit. This germination requirement is true of either garden-sown or indoor-started tomato seeds.  

Tomato plant flowers don't emerge once the temperature reaches 100 F or higher. That's important because the more profusely your tomato plants bloom, the heavier your tomato fruit crop will be.

During the fruiting period, tomatoes need air temperatures that don't drop below 50 to 55 F.

During the plants' heaviest growth period, the preferred temperature is 80 to 90 F. 

California's warm climate allows many of its home gardeners to sow seeds directly into the ground outdoors. Gardeners in interior portions of the state, such as the Sacramento and San Joaquin valleys, can sow tomato seeds outdoors as early as March. Southern portions of the state along the coastline normally allow for direct seeding in April or May and Northern California in May. In the warmest parts of the state -- generally the desert valleys, however, direct seeding can happen as early as December but should be completed in March before the heat rises.

The same timetable that applies for when tomato seeds can first go in the ground outdoors also applies to transplanting seedlings. Whether you start your own tomato plants or get them from a nursery, set them in the ground at the times recommended for direct seeding in your area.

If you start your own tomato plants indoors to get a jump on the growing season, then time your seed sowing so that the resulting plants will be about 6 to 8 weeks old when seedlings and seeds normally go into the ground outdoors in your area. For Sacramento and San Joaquin valleys' growers, this means the seeds should be sown indoors from mid-January to mid-February. Southwestern California growers should plan to start indoor plants from March in warm areas all the way up to mid-June in cooler portions. If you're along the state's northern coast or interior northern sections, then sow tomato seeds indoors in March and April. In desert regions, November to February is the best time to get sowing indoors for the early planting those hot climates require.