Iowa · zones 4b–6a
When to plant onions in Iowa.
Sow or transplant between Feb 21 to Apr 6 in a typical year, keyed to Iowa's median last frost around May 1. A second, fall sowing fits Aug 12 to Sep 23. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Iowa season.
1 · Start indoors
Jan 10 to Feb 23
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 21 to Apr 6
Keyed to the ~May 1 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 12 to Sep 23
Counted back from the ~Oct 7 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Iowa spans more than one climate: Zone 5 is the most common zone, but the state runs 4b to 6a. 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.
Iowa growing notes
Iowa wind can knock an onion top over before it means anything.
Deep glacial loam does half the work
Iowa's soil is largely deep glacial till loam, built up under prairie grass for thousands of years, and that structure is close to ideal for a bulb that needs to expand sideways without resistance: friable, well-drained ground that a compacted clay-state gardener has to build with years of compost, Iowa mostly starts with already. The remaining prep is straightforward: work in an inch of compost at planting and keep the bed weed-free, since onions have shallow roots and lose more to weed competition than almost any other garden crop.
Set transplants 4 inches apart, and choose a long-day variety, one that needs 14 to 16 hours of summer daylight to trigger bulbing, since Iowa's latitude, roughly 40 to 43 degrees north, delivers exactly that stretch of daylight by early summer. Patterson has become the standard long-day storage pick for this latitude now that Copra, the old workhorse variety gardeners still ask for by name, has been discontinued by its breeder and grown scarce on seed racks. Redwing, a red long-day type with similar keeping ability, rounds out a proven pairing for Iowa's daylight window.
Wind breaks the harvest signal every other state relies on
Most onion guides tell you to wait for the tops to fall over on their own as the harvest signal, but Iowa's open, largely treeless fields see straight-line wind events strong enough to flatten a cornfield, the kind of derecho-line storm that swept across the state in August 2020, and a wind gust like that can knock onion tops flat weeks before the bulbs are actually finished. Treat a wind-flattened top with suspicion rather than as the green light.
The more reliable check in Iowa is the neck itself: squeeze it gently just above the bulb, and if it is still firm and thick rather than soft and thin, the bulb has more growing to do regardless of what the foliage looks like after a storm. Once roughly half the necks in the bed have gone soft on their own, stop watering, wait about a week, and then lift the crop.
Curing needs a fan more than it needs sunshine
Iowa falls stay humid often enough that outdoor curing, spreading bulbs on a tarp the way a drier climate can, risks neck rot before the wrappers finish tightening; a damp autumn is common enough here that it should be the default assumption, not the exception. Cure indoors instead, in a garage or shed with a box fan running and the bulbs spread one layer deep, never piled, for 2 to 3 weeks.
Onion thrips and downy mildew, the two pests that show up most on Iowa onion pages, both favor the same humid stretches that complicate curing, so scout for thrips, tiny silvery streaks on the leaves, through late summer and treat before harvest rather than after. A properly cured Patterson keeps 6 months or more in a cool basement, which is most of the reason to fight through the humid curing conditions in the first place.
In Iowa: watch for
The pests that find onions here.
A middle-length season, about 159 frost-free days, means onion, bulb overlaps onion thrips for a stretch each summer. Check plants every week or two and treat early.
Silvery streaks and curled leaf tips in hot spells.
Organic fix: Insecticidal soap or neem; hang sticky traps.
Seedlings wilt; white maggots in the bulb base.
Organic fix: Float a row cover over the row; rotate beds each year.
Pale patches with fuzzy gray growth in damp weather.
Organic fix: Space for airflow, water early in the day, spray neem.
White rot
Yellowing and dieback, then wilt; fluffy white mold at the bulb base and roots so rotted the plant lifts straight out.
Organic fix: No cure once it is in the soil. Plant clean stock, never follow onions with onions, and keep infested soil off tools and boots.
Your season math: Iowa gives onions about 159 frost-free days, and it needs roughly 95 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting onions in Iowa: questions
When should I plant onions in Iowa?+
In a typical Iowa year, sow onions or set out transplants between Feb 21 to Apr 6, keyed to the state's median last frost around May 1. There's also a fall window: Aug 12 to Sep 23. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Iowa?+
Start seeds inside about 6 weeks before set-out: roughly Jan 10 to Feb 23 for a typical Iowa season.
What USDA zone is Iowa?+
Iowa spans zones 4b–6a, mostly zone 5. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many onions 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.