Too early to plant — last frost in San Mateo, CA is Dec 29

San Mateo's average last spring frost is around Dec 29, and its first fall frost around Dec 26 — a growing season of roughly 362 days. These figures come from the nearest NOAA station with long-term records (San Francisco Intl Ap, CA).

These are 1991–2020 averages, not guarantees: about half of years see a frost a little later in spring or earlier in fall. Cautious gardeners wait a week or two past the last-frost date before setting out frost-tender plants, or keep row cover handy for surprise cold snaps.

To build a planting calendar, count backward from the last spring frost for indoor seed-starting, and forward from the first fall frost for crops that need time to mature before cold returns.