Plant Tomatoes in Zone 10a

Zone 10a averages 323 frost-free days, running from a last spring frost around January 31 to a first fall frost around December 20. Tomatoes needs roughly 75 days from planting out to first harvest, so it uses about 23 percent of that window. Tomatoes is a tender crop, which is what decides where in the window it belongs.

Because tomatoes needs a head start, sow seed indoors around Dec 13, about 7 weeks ahead of the Zone 10a last frost. Transplants go out around Feb 14. Starting earlier than that in Zone 10a usually backfires: plants outgrow their cells and stall when they finally hit cold soil. At 75 days to harvest, a 323-day Zone 10a season has room for about 3 sowings, so stagger them rather than planting everything at once.

Yes. Sow tomatoes by September 8 in Zone 10a. That is 103 days before the average first frost, which covers the 75 days to maturity plus about two weeks for the slower growth that comes with shortening days, plus a two week margin because frost kills tomatoes outright.

A second tomato crop only works where the first fall frost is late enough to finish it, which in practice means zone 9 and warmer.