When Can You Start Tomato Seeds in Southern California?

Calculating When to Start Seeds

• Tomatoes start best when their seeds are sown indoors. In coastal areas of Southern California, you can plant tomatoes outdoors as early as mid-March, but wait until April to avoid a late freeze if you are in an inland area. When starting the seeds indoors, calculate six to eight weeks before you plan to plant tomato seedlings outdoors, and use that date to start the seeds indoors. Plan to plant tomatoes outdoors after the last frost date has passed and night temperatures are consistently above 45 degrees Fahrenheit.

Starting Seeds

• Germinating tomato seeds requires heat and light. Use a seed flat with drainage holes and filled with a seed-starting mix designed to drain well. Tomato seeds are planted 1/4 inch deep in soil and spaced 1 inch apart. Place the seed flat in a bright, warm area where the temperature is between 75 and 85 degrees Fahrenheit. Keep the seeds moist by misting their soil with a light spray of water whenever the soil starts to feel dry. Only dampening the soil in that way doesn't disturb the seeds. After tomato seeds germinate, they need at least eight hours of bright sunlight each day.

Heating and Lighting

• Artificial light and a heat source help to stimulate tomato seed germination. Using a horticultural heating pad is an optional but good practice for maintaining a consistent source of bottom heat. Grow lights also help speed germination. If you have access to florescent lights, place them 2 inches above the seed flat, and provide 14 to 16 hours of light per day. As the seedlings grow, raise the lights so they are consistently 2 inches above the plants.

Transplanting and Planting

• Tomato seedlings can be transplanted when they are 1 1/2 inches tall. Move each seedling into its own 4-inch diameter pot filled with potting soil. After transplanted, the seedlings require a slightly cooler temperature range of 60 to 70 degrees Fahrenheit. When the seedlings are 4 to 8 inches tall and night temperatures are consistently above 45 degrees Fahrenheit, plant the tomato seedlings in an outdoor garden.

Direct Seeding and Testing Soil Temperature

• Tomato seeds germinate quickly and reliably when planted in soil that is 75 to 85 degrees Fahrenheit. The seeds also germinate in soil that is 59 to 60 degrees Fahrenheit, but their germination time is extended at that lower temperature range, taking up to two weeks compared to one week at higher temperatures. Plant tomato seeds directly in an outdoor garden's soil when the soil temperature is at or above 60 degrees Fahrenheit. Inserting a soil thermometer into the top 4 inches of the garden bed soil is a way to test the soil's temperature. Plant tomato seeds 1 inch apart and 1/4 inch deep in the outdoor garden.

Selecting Cultivars

• Several tomato varieties grow well in Southern California. In coastal areas, where summers are cooler than inland, varieties that grow well include "Better Boy," "Early Girl" and the large, beefsteak tomato variety "Supersteak." "Celebrity" and "Champion" are two tomato varieties that grow well in coastal areas and in hotter inland climates. Other varieties suitable for inland areas include "Big Pick," "Ace Hybrid" and "Royal Flush."