Plant Broccoli 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. Broccoli needs roughly 65 days from planting out to first harvest, so it uses about 45 percent of that window. Broccoli is a half-hardy crop, which is what decides where in the window it belongs.

Because broccoli needs a head start, sow seed indoors around Mar 24, about 6 weeks ahead of the Zone 4b last frost. Transplants go out around Apr 21. Starting earlier than that in Zone 4b usually backfires: plants outgrow their cells and stall when they finally hit cold soil.

Yes. Sow broccoli by August 1 in Zone 4b. That is 58 days before the average first frost, which covers the 65 days to maturity plus about two weeks for the slower growth that comes with shortening days, minus the roughly 21 days broccoli keeps going once light frost arrives.

Fall broccoli avoids the spring problem of heads buttoning up when temperatures spike. Start transplants indoors in midsummer rather than direct sowing into hot soil.