Kansas · zones 5b–7a
When to plant onions in Kansas.
Sow or transplant between Feb 4 to Mar 20 in a typical year, keyed to Kansas's median last frost around Apr 14. A second, fall sowing fits Aug 27 to Oct 8. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Kansas season.
1 · Start indoors
Dec 24 to Feb 6
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 4 to Mar 20
Keyed to the ~Apr 14 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 27 to Oct 8
Counted back from the ~Oct 22 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Kansas spans more than one climate: Zone 6 is the most common zone, but the state runs 5b to 7a. 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.
Kansas growing notes
Kansas onions live or die on heat, not just day length.
Heat decides more than the day-length label
Kansas sits at roughly 37 to 40 degrees north, comfortably inside the intermediate-day band, so varieties like Red Candy Apple and Candy, both bred to trigger bulbing at 12 to 14 hours of daylight, are the safe picks off the seed rack. Talon is a long-day type bred for higher latitudes, and pushed into bulbing before it has built enough leaf, it comes up small; save it for the state's far northern tier, if at all, rather than counting on it statewide. But the bigger local variable is heat, not light: Kansas summers regularly cross 95 degrees while bulbs are still sizing up, and onion bulb growth slows sharply once soil temperature near the root zone holds above about 85 degrees for more than a couple of weeks running.
That means the real skill in growing onions here is not choosing a day-length category, it is getting the plant's leaf growth finished before peak heat arrives, because a plant still building leaves when the heat clamps down simply stops early with a smaller bulb than it should have made. Onion transplants shrug off a light freeze, so set them out 2 to 4 weeks ahead of the last frost rather than waiting for it, and feed nitrogen through that early leafy stage, not later, to bank growth while temperatures still cooperate.
Curing before a hot, dry October catches you
Once tops fall over on their own, cure bulbs somewhere shaded and airy, never in direct sun. Kansas autumn sun on exposed bulbs can push their internal temperature well past 100 degrees on a 90 degree afternoon, cooking the outer rings into a soft, cooked-looking layer that will not store. A tarp-covered drying rack under a porch roof, or simply spreading bulbs in a single layer in a garage with the door cracked for airflow, both keep bulbs out of that direct heat.
The test for done curing is the neck: squeeze it after 2 weeks, and a spongy feel means it needs more time, while a papery, dry snap means it is ready to trim and store. Store cured bulbs at 32 to 40 degrees somewhere ventilated; a hot, dry Kansas fall means moving the crop indoors well before the first hard freeze rather than leaving mesh bags out on a porch overnight.
Thrips are the insect that actually threatens a Kansas onion bed
Onion maggot favors a cold, soggy spring to build real numbers, weather that a Kansas spring usually burns off within a couple of weeks before turning warm and dry, so a wilted, mushy-rooted onion here is worth checking for standing water and bacterial soft rot before assuming maggot damage. Thrips are the insect that actually decides a Kansas onion crop's yield most years, since heat and dry stretches, not cool wet ones, are what builds a damaging population in this state.
Thrips take over once summer heat sets in, leaving silvery streaks on the leaves during hot, dry stretches. Rap a few tops sharply against a plain paper plate and watch what falls: several specks skittering across the surface is the cue to treat with insecticidal soap, aimed at the crease where each leaf meets the neck, since that crease is exactly where thrips tuck out of reach of a direct-contact spray.
In Kansas: watch for
The pests that find onions here.
A middle-length season, about 191 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: Kansas gives onions about 191 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 Kansas: questions
When should I plant onions in Kansas?+
In a typical Kansas year, sow onions or set out transplants between Feb 4 to Mar 20, keyed to the state's median last frost around Apr 14. There's also a fall window: Aug 27 to Oct 8. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Kansas?+
Start seeds inside about 6 weeks before set-out: roughly Dec 24 to Feb 6 for a typical Kansas season.
What USDA zone is Kansas?+
Kansas spans zones 5b–7a, 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.