Idaho · zones 5b–7b
When to plant broccoli in Idaho.
Sow or transplant between Mar 20 to Apr 17 in a typical year, keyed to Idaho's median last frost around May 11. A second, fall sowing fits Jun 14 to Jul 24. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Broccoli dates for a typical Idaho season.
1 · Start indoors
Feb 6 to Mar 6
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Mar 20 to Apr 17
Keyed to the ~May 11 last frost. Sow 0.5" deep, 18" apart.
Fall round
Jun 14 to Jul 24
Counted back from the ~Sep 28 first frost.
Harvest
~60 days later
~1.2 lb per plant over ~3 weeks.
Idaho spans more than one climate: Zone 6 is the most common zone, but the state runs 5b to 7b. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
Idaho growing notes
Broccoli's spring planting is racing the calendar here, and its fall planting almost never has to.
Spring is a sprint, fall is a walk
Broccoli goes in as a transplant, started about a half inch deep and set out 18 inches apart, needing roughly 60 days to head up. A spring planting has to reach that head-forming stage before the valley's early summer heat arrives, and a transplant that sits stalled for even a couple of weeks in cold soil right after going in eats into a margin that wasn't large to begin with inside the state's 140-day season. The test for a stalled head beats guessing at the calendar: a plant that has sized up nicely in leaf but shows no sign of a central bud tightening once it is that big is running behind, and pushing it with more nitrogen at that point tends to grow more leaf, not a faster head.
The fall planting carries none of that pressure. Set out into the heat of summer to head up as the weather cools, it finishes into conditions this crop actually prefers, sizing up over extra weeks without racing anything. Between the two windows here, fall is the one that consistently rewards patience, and spring is better treated as a bonus crop some years won't finish as cleanly.
Two cultivars that follow from the season, and the head that quits early
Waltham 29 is worth knowing for the fall planting specifically. It is a cold-hardy type bred to hold up and keep heading through the cooler weather a fall crop finishes into. De Cicco, a quicker Italian heirloom that also throws a run of side shoots after the main head is cut, suits the tighter spring window better, since a faster finish leaves less time exposed to the season's early heat.
A transplant held too long in its pot before going into the ground, or one that gets checked hard by a cold snap right after transplanting, sometimes responds by throwing a head the size of a coin and quitting instead of growing on to a full one, a problem growers call buttoning. There is no recovering a plant once it does this. The fix lives entirely upstream: not holding transplants past their prime, and not setting them into ground that is still too cold, not in anything done afterward.
The head hides what the leaf doesn't
Cabbage worms and loopers are a different problem on this crop than on a leafy green: the concern is a caterpillar riding inside the tight florets of the head itself, where a rinse under the tap does not reach it. Cutting heads and soaking them in salted water for a few minutes before cooking drives out anything hiding in the clusters. Aphids settle into those same tight florets as a grayish, waxy-looking cluster easy to miss until the head gets turned over, and the same salted soak that clears out caterpillars clears these out too.
Flea beetles chew new transplants hardest in the first couple of weeks, exactly when a spring planting can least spare the leaf area it needs to reach that racing head-forming stage on schedule, making row cover worth more on the spring round than the fall one. Cabbage root maggot does its damage below ground, the larvae feeding on roots and the stem base of a young transplant while spring soil is still cool and moist; a stiff paper or cardboard ring pressed into the earth around each stem at planting time blocks the egg-laying fly, cheap insurance that matters more on the spring round than the fall. Black rot shows as yellow, V-shaped patches at the leaf margin that point toward the vein and darken it from within, worse on foliage that stays wet into the evening. Pull any leaf that shows it and bin it rather than compost it, since the bacteria overwinters on old debris and returns the next season if given the chance.
In Idaho: watch for
The pests that find broccoli here.
The short season here, about 140 frost-free days, keeps broccoli in front of cabbage worms & loopers only briefly. One early row cover usually carries a planting through the worst of it.
Green caterpillars chewing ragged holes; green droppings.
Organic fix: Handpick, cover with netting, spray Bt.
Gray-green clusters tucked into the leaf folds.
Organic fix: Blast with water, then insecticidal soap.
Tiny holes peppering the young leaves.
Organic fix: Row cover from day one; diatomaceous earth.
Black rot
Yellow V-shaped wedges cutting in from the leaf edge, veins blackening inside.
Organic fix: Pull infected plants, water at the base, keep brassicas out of that bed 3 years.
Seedlings wilt, yellow and stall for no reason; white maggots tunnel the roots.
Organic fix: Row cover before the flies are up, when yellow rocket blooms; rotate the bed.
Your season math: Idaho gives broccoli about 140 frost-free days, and it needs roughly 60 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting broccoli in Idaho: questions
When should I plant broccoli in Idaho?+
In a typical Idaho year, sow broccoli or set out transplants between Mar 20 to Apr 17, keyed to the state's median last frost around May 11. There's also a fall window: Jun 14 to Jul 24. Your exact dates shift with your ZIP code and microclimate.
When do I start broccoli seeds indoors in Idaho?+
Start seeds inside about 6 weeks before set-out: roughly Feb 6 to Mar 6 for a typical Idaho season.
What USDA zone is Idaho?+
Idaho spans zones 5b–7b, mostly zone 6. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many broccoli plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.