Nebraska · zones 4b–5b
When to plant onions in Nebraska.
Sow or transplant between Feb 18 to Apr 3 in a typical year, keyed to Nebraska's median last frost around Apr 28. A second, fall sowing fits Aug 11 to Sep 22. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Nebraska season.
1 · Start indoors
Jan 7 to Feb 20
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 18 to Apr 3
Keyed to the ~Apr 28 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 11 to Sep 22
Counted back from the ~Oct 6 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Nebraska spans more than one climate: Zone 5 is the most common zone, but the state runs 4b 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.
Nebraska growing notes
An onion here doesn't bulb because the calendar says July, it bulbs because the days have gotten long enough to trigger it, and the wrong variety hits that trigger too early, before it has grown enough leaf to size up.
The Clock This Plant Actually Runs On
Most vegetables size up on a countdown of days and warmth. An onion works differently: it bulbs in response to how many hours of daylight it gets each day, a photoperiod switch rather than a calendar one. Once day length reaches whatever threshold the variety was bred for, the plant stops pouring its energy into new leaves and starts swelling a bulb at the base instead. Nebraska's latitude sits well up into what seed catalogs sell as long-day territory, and that classification decides the whole growing plan.
Plant a short-day type bred for a southern garden here and it reaches that trigger far too early: day length crosses its low threshold before the plant has built much leaf, so it starts swelling a bulb within weeks of germinating instead of near the solstice. It still bulbs, just small and premature, because the leaf count a plant puts on before bulbing starts is what sets its final size. The fix lives in the catalog listing before a set or transplant goes into the ground: buy stock sold as long-day, not the short-day or intermediate types racked for warmer regions. The test in the row is simple. A plant that holds off swelling until the year's longest days, after putting on a full run of leaves, is on schedule for a full-size bulb. One that is already swelling a small bulge weeks earlier, with only a handful of leaves behind it, has the wrong genetics, not a feeding problem.
A Long Commitment From a Short Root System
Onion goes in here as a transplant, set shallow at a half inch deep and spaced a tight 4 inches apart, and it takes about 95 days to turn that seedling-sized start into a full bulb. The roots underneath stay shallow and sparse the whole run, nothing like the taproot a carrot sends into the same ground, so a transplant that dries out even briefly in its first weeks stalls instead of recovering.
Most of the work happens in the spring window here. A transplant set as soon as the ground can be worked gets the full run of lengthening days ahead of it before bulbing takes over, and a full-size bulb at harvest, running close to half a pound, is what that stretch buys. A planting held back for later starts that same clock with far fewer long days left to work with.
What Shows Up on the Leaf, and What Ends the Bulb
Onion thrips are tiny enough that most growers spot the damage before the insect: silvery, papery streaks rasped into the leaf surface, worse through a hot, dry, windy stretch than after a rain. Onion maggot works underground instead, the larvae of a fly that lays eggs at the base of a young plant. A transplant that yellows and topples with no obvious cause above ground is worth lifting out of Nebraska's loose loess soil to check underground, since maggot damage looks nothing like the leaf scarring thrips leave behind.
Downy mildew shows as a grayish-purple fuzz on the leaves during a humid, overcast stretch, and because a bulb depends on healthy top growth to keep filling out, losing leaves to it late in the season caps how big that bulb finishes. White rot is the one that matters most. Unlike the other three, it lives in the soil itself for years once established, entering through the roots and rotting the bulb from the base up until the whole plant collapses. The only real defense is keeping infected sets, transplants, or soil out of clean ground, and pulling and destroying, not composting, any plant that collapses with a soft, watery rot at the base.
In Nebraska: watch for
The pests that find onions here.
A middle-length season, about 161 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: Nebraska gives onions about 161 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 Nebraska: questions
When should I plant onions in Nebraska?+
In a typical Nebraska year, sow onions or set out transplants between Feb 18 to Apr 3, keyed to the state's median last frost around Apr 28. There's also a fall window: Aug 11 to Sep 22. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Nebraska?+
Start seeds inside about 6 weeks before set-out: roughly Jan 7 to Feb 20 for a typical Nebraska season.
What USDA zone is Nebraska?+
Nebraska spans zones 4b–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.