Plant Potatoes 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. Potatoes needs roughly 90 days from planting out to first harvest, so it uses about 30 percent of that window. Potatoes is a half-hardy crop, which is what decides where in the window it belongs.

Potatoes handles cold soil, so there is no reason to wait for the frost date. Direct sow around Feb 1, roughly 2 weeks before the Zone 9b last frost, as soon as the ground is workable. Sowing at the last frost instead costs you the coolest and best part of the season. At 90 days to harvest, a 298-day Zone 9b season has room for about 2 sowings, so stagger them rather than planting everything at once.

Yes. Sow potatoes by September 7 in Zone 9b. That is 94 days before the average first frost, which covers the 90 days to maturity plus about two weeks for the slower growth that comes with shortening days, minus the roughly 10 days potatoes keeps going once light frost arrives.

A fall potato crop works in zone 8 and warmer, planted in late summer from certified seed pieces that have broken dormancy.