Plant Radishes in Zone 10b

Zone 10b averages 350 frost-free days, running from a last spring frost around January 15 to a first fall frost around December 31. Radishes needs roughly 28 days from planting out to first harvest, so it uses about 8 percent of that window. Radishes is a hardy crop, which is what decides where in the window it belongs.

Radishes handles cold soil, so there is no reason to wait for the frost date. Direct sow around Dec 18, roughly 4 weeks before the Zone 10b 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 28 days to harvest, a 350-day Zone 10b season has room for about 8 sowings, so stagger them rather than planting everything at once.

Yes. Sow radishes by December 10 in Zone 10b. That is 21 days before the average first frost, which covers the 28 days to maturity plus about two weeks for the slower growth that comes with shortening days, minus the roughly 21 days radishes keeps going once light frost arrives.

Radishes are the fastest fall insurance crop. Fall roots stay milder and split less than spring ones because the soil is cooling rather than heating.