Wisconsin · zones 4a–5b
When to plant onions in Wisconsin.
Sow or transplant between Feb 25 to Apr 10 in a typical year, keyed to Wisconsin's median last frost around May 5. A second, fall sowing fits Aug 8 to Sep 19. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Wisconsin season.
1 · Start indoors
Jan 14 to Feb 27
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 25 to Apr 10
Keyed to the ~May 5 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 8 to Sep 19
Counted back from the ~Oct 3 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Wisconsin spans more than one climate: Zone 5 is the most common zone, but the state runs 4a to 5b. 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.
Wisconsin growing notes
Wisconsin's muck fields are the best onion ground in the state.
Black dirt farming, not garden soil
Drive through the muck fields of south central Wisconsin, soil the color of coffee grounds from generations of drained lakebed and marsh bottom, and you are looking at some of the best onion and carrot ground in the state: loose, moisture holding, and nearly pure organic matter, letting a bulb swell without ever fighting compaction. A home gardener without muck soil can copy the effect with 3 inches of finished compost worked into the top 8 inches every year; it will never be true muck, but it moves the ground in that direction.
Muck soil holds water almost too well, which changes the watering math: check moisture 2 inches down before adding more, since soggy organic soil suffocates onion roots faster than sandy loam ever does. If your garden runs sandy instead, expect to water more often in smaller doses, because sand cannot store the reserve muck holds for free. Either way, onions want steady moisture through bulb swell and almost none the last 2 weeks before harvest, when drying down concentrates flavor and toughens the neck for storage.
Long days, and the leaf count that decides bulb size
Wisconsin sits between roughly 42 and 47 degrees north, solidly inside long day onion territory, where midsummer daylight stretches past 15 hours and triggers bulbing regardless of variety. Patterson has become the storage standard here and keeps 6 months or better in a cool dry spot; Redwing supplies a red keeper with similar staying power. Copra was the old benchmark for a generation of Wisconsin growers, but the breeder discontinued that F1 cross, and seed for it has grown scarce even where established plants still turn up. Anyone chasing sweetness over storage should grow Walla Walla, understanding the trade: it softens within weeks and needs eating, not shelving.
Bulb size traces straight back to leaf count: each leaf standing when the plant switches over to bulbing turns into one more ring, and no leaf grown after that switch counts. That is the whole argument for starting from transplants rather than sets: a transplant already carries several weeks of leaf growth into the garden, while a set spends its first weeks catching up. Feed nitrogen steadily through late spring while leaves are still forming, and stop once the tops begin to lean, since fertilizer after that point feeds the leaves instead of the bulb.
Thrips in a dry stretch, maggots in a wet one
Onion thrips favor Wisconsin's hot dry weeks in July, rasping silvery streaks into the leaves until a bad infestation can stunt bulb size outright. Check by tapping a few leaves over a white index card: more than 5 or 6 moving specks per plant justifies a spray of insecticidal soap, timed for evening so it does not scorch leaves in full sun.
Onion maggot works the opposite conditions, thriving in the cool wet springs Wisconsin serves up most years, and it is the reason floating row cover on planting day matters more here than in drier states. Its larvae hollow out the stem below the soil line, so the first symptom you see, a seedling wilting and lifting out soft, means that plant is already finished. Downy mildew follows the same wet weather pattern, showing as pale patches with gray fuzz on the underside of leaves; thin the planting for airflow and it rarely gets a foothold.
In Wisconsin: watch for
The pests that find onions here.
A middle-length season, about 151 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: Wisconsin gives onions about 151 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 Wisconsin: questions
When should I plant onions in Wisconsin?+
In a typical Wisconsin year, sow onions or set out transplants between Feb 25 to Apr 10, keyed to the state's median last frost around May 5. There's also a fall window: Aug 8 to Sep 19. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Wisconsin?+
Start seeds inside about 6 weeks before set-out: roughly Jan 14 to Feb 27 for a typical Wisconsin season.
What USDA zone is Wisconsin?+
Wisconsin spans zones 4a–5b, 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.