Plant Zucchini in Zone 9b

Zone 9b averages 298 frost-free days, running from a last spring frost around February 15 to a first fall frost around December 10. Zucchini needs roughly 55 days from planting out to first harvest, so it uses about 18 percent of that window. Zucchini is a tender crop, which is what decides where in the window it belongs.

Because zucchini needs a head start, sow seed indoors around Jan 25, about 3 weeks ahead of the Zone 9b last frost. Transplants go out around Mar 1. Starting earlier than that in Zone 9b usually backfires: plants outgrow their cells and stall when they finally hit cold soil. At 55 days to harvest, a 298-day Zone 9b season has room for about 4 sowings, so stagger them rather than planting everything at once.

Yes. Sow zucchini by September 18 in Zone 9b. That is 83 days before the average first frost, which covers the 55 days to maturity plus about two weeks for the slower growth that comes with shortening days, plus a two week margin because frost kills zucchini outright.

One midsummer sowing of zucchini replaces the spring plants right as squash vine borer takes them out.