Plant Radishes in Zone 4b

Zone 4b averages 146 frost-free days, running from a last spring frost around May 5 to a first fall frost around September 28. Radishes needs roughly 28 days from planting out to first harvest, so it uses about 19 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 Apr 7, roughly 4 weeks before the Zone 4b 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 146-day Zone 4b season has room for about 3 sowings, so stagger them rather than planting everything at once.

Yes. Sow radishes by September 7 in Zone 4b. 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.