Indiana · zones 5b–6b
When to plant onions in Indiana.
Sow or transplant between Feb 12 to Mar 28 in a typical year, keyed to Indiana's median last frost around Apr 22. A second, fall sowing fits Aug 22 to Oct 3. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Indiana season.
1 · Start indoors
Jan 1 to Feb 14
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 12 to Mar 28
Keyed to the ~Apr 22 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 22 to Oct 3
Counted back from the ~Oct 17 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Indiana 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.
Indiana growing notes
An onion variety bred for southern gardens can hit its bulbing trigger within weeks in Indiana ground and swell early into an undersized bulb, because bulbing here answers to the length of the day, not the heat of summer.
The variety decision that happens before anything gets planted
Bulb onions here are set as transplants, spaced about 4 inches apart, and need around 95 days to size up a bulb worth pulling. None of that matters if the wrong type goes in: onions start swelling a bulb once daylight crosses a variety's trigger length, not once the weather warms up. Indiana sits far enough north that its long summer days blow past the low trigger a short-day type needs, while long-day and many intermediate-day varieties are matched to those same hours. A short-day type, bred for the Gulf Coast and Deep South, hits its trigger within weeks of going into Indiana ground, while the plant is still small, so it stops building leaf and bulbs early instead of sizing up first.
That means an undersized bulb, not a missing one. The test is on the seed packet or the sets bin, not in the ground: look for the words long-day or intermediate-day before buying. A grower unsure about a bed can pull one bulb in early summer and slice it in half: a long-day type is still mostly leaf base with no bulb layering yet, where a mismatched short-day type already shows a small, finished bulb that stopped growing weeks earlier.
Spring is the standard window, fall is the specialty one
A spring-set transplant is the standard path here and lines up with the long-day trigger without extra planning: leaves build through late spring, daylight stretches into early summer, and the bulb finishes on schedule. Indiana's growing season also allows a fall planting, using hardy sets put in late enough to root but not push much top growth before winter, aiming to bulb early the following summer. That route trades a slightly earlier harvest for a plant that must survive an Indiana winter first, and it only pays off with a genuinely cold-hardy overwintering type, not an ordinary spring set held back and planted late.
Either window ends with the same finishing signal: tops falling over on their own means the bulb has stopped pulling energy from the leaves, not that something has gone wrong. Pulling before that happens, or chasing extra size by leaving bulbs in once most tops have gone down, mostly risks the bulb resprouting or rotting in wet ground.
Thrips and maggot are the two to watch; the diseases follow wet weather
Onion thrips are easy to miss until the damage is obvious: silvery, papery streaks on the leaves where they have been rasping, worst during a hot, dry stretch when their numbers build fast. Pull a leaf apart at the base, right in the tight fold where new growth emerges; that fold is where thrips hide during the day, making it the spot to check before calling a plant clean. Onion maggot works the opposite end of the plant: its larvae tunnel into the bulb and roots below the soil line, usually on young transplants, and a plant that suddenly yellows and wilts for no visible reason above ground is worth digging up before blaming dry weather.
Downy mildew shows up as pale, sometimes purplish fuzzy patches on the leaves after a run of humid, overcast weather, the kind Indiana summers deliver in stretches, and it spreads fastest where air sits still between crowded rows. White rot is the one to take most seriously: it lives on in the soil around bulb crops for years once established, so a bed that develops the disease's fluffy white mat and pinhead-sized black flecks at harvest should skip onions, garlic, and any other allium for a good stretch of seasons, not just the next one.
In Indiana: watch for
The pests that find onions here.
A middle-length season, about 178 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: Indiana gives onions about 178 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 Indiana: questions
When should I plant onions in Indiana?+
In a typical Indiana year, sow onions or set out transplants between Feb 12 to Mar 28, keyed to the state's median last frost around Apr 22. There's also a fall window: Aug 22 to Oct 3. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Indiana?+
Start seeds inside about 6 weeks before set-out: roughly Jan 1 to Feb 14 for a typical Indiana season.
What USDA zone is Indiana?+
Indiana 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.