Ohio · zones 5b–6b
When to plant onions in Ohio.
Sow or transplant between Feb 15 to Mar 31 in a typical year, keyed to Ohio's median last frost around Apr 25. A second, fall sowing fits Aug 20 to Oct 1. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Ohio season.
1 · Start indoors
Jan 4 to Feb 17
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 15 to Mar 31
Keyed to the ~Apr 25 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 20 to Oct 1
Counted back from the ~Oct 15 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Ohio spans more than one climate: Zone 6 is the most common zone, but the state runs 5b to 6b. 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.
Ohio growing notes
Ohio is long-day onion country.
Pick a long-day variety or fight the sun
At Ohio's latitude, roughly 39 to 42 degrees north, midsummer days stretch past 15 hours, which is exactly the trigger long-day onions wait for. Patterson is the storage standard and holds 6 months or more in a cool basement. Ailsa Craig grows the show-off sweet giants, and Walla Walla makes a mild sweet bulb that must be eaten within about 6 weeks, because sweetness and storage life pull in opposite directions.
Short-day varieties from southern catalogs bulb in Ohio's April at golf-ball size, no matter how well you grow them. Read the day-length line on the variety description before anything else; it is the one spec that cannot be fixed with better care.
The February head start
Big Ohio onions are born indoors in February. Seed started 10 to 12 weeks before set-out, trimmed with scissors to 4 inches tall whenever it flops, goes out in April as pencil-thick transplants that laugh at frost. Purchased transplant bundles work the same if starting seed is not your thing.
The biology behind the head start is the leaf rule: each leaf becomes one ring on the finished bulb, and the plant stops making leaves the moment day length flips it into bulbing in late June. Every week of leaf growth you bank before the solstice converts directly into bulb diameter after it.
Onion maggot is the Ohio ambush
Cool wet springs, which Ohio supplies most years, are onion maggot weather. The adult fly lays eggs at the base of young plants, and the maggots hollow out seedlings, which yellow, wilt, and pull up rotted. Once the maggots are in a plant, nothing rescues it, so the whole defense is up front: floating row cover over the row on planting day denies the fly a landing site, and a 3-year rotation away from the whole onion family starves the population.
One more Ohio habit worth building: pull and bin every culled or rotting onion instead of tossing it near the garden, because those culls carry the maggots through winter. Clean beds this fall are the pest control for next spring.
In Ohio: watch for
The pests that find onions here.
A middle-length season, about 173 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: Ohio gives onions about 173 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 Ohio: questions
When should I plant onions in Ohio?+
In a typical Ohio year, sow onions or set out transplants between Feb 15 to Mar 31, keyed to the state's median last frost around Apr 25. There's also a fall window: Aug 20 to Oct 1. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Ohio?+
Start seeds inside about 6 weeks before set-out: roughly Jan 4 to Feb 17 for a typical Ohio season.
What USDA zone is Ohio?+
Ohio spans zones 5b–6b, mostly zone 6. 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.