When to plant onions in Kansas
Kansas’s average last spring frost is April 15, and the first fall frost typically arrives around October 20 — about 188 frost-free days (USDA zones 5b, 6a, 6b, 7a). Onion needs 90–120 days to mature, which fits comfortably within Kansas's growing season.
When to plant onions in Kansas: the short answer
For a spring planting, plant out Mar 18 – Apr 4, with seeds started indoors Jan 28 – Feb 11. Seeds germinate in soil from 40 to 90°F.
Windows are ~1–2 week bands around each date. Frost dates are a statewide average from NOAA/NCEI normals; your town may differ by two weeks or more. Calendar year 2026.
How to grow onions in Kansas, step by step
- Start seeds indoors (Jan 28 – Feb 11). Sow seeds ½ inch deep in seed-starting mix under bright light. They take 7-14 days to sprout, fastest in soil around 70°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. They need at least six to eight hours of direct sun a day and loose soil at least 12 inches deep, with a pH between 6.0 and 7.0. Dig in compost before planting.
- Plant out (Mar 18 – Apr 4). Space plants about 4 inches apart in rows 12 inches apart. It handles frost well, so it can go out before the last frost date. As a cool-season crop it does best before summer heat arrives in Kansas.
- Water and feed. Give it steady, even moisture, about an inch of water a week from rain or the hose.
- Watch for pests and problems. Pests: onion maggot, onion thrips. Diseases and disorders: downy mildew, white rot, neck rot, purple blotch.
- Harvest (from about Jun 23 – Jul 23). Onion takes 90-120 days to reach harvest. Match the day-length type to your latitude: long-day onions for the north, short-day for the south, day-neutral anywhere. Bulbing is triggered by day length, not calendar date.
Figures come from our crop data; dates use Kansas’s average frost dates. More in the full onion growing guide.
Onion planting dates by USDA zone in Kansas
Kansas spans USDA zones 5b, 6a, 6b, 7a. Colder zones, usually higher or further inland, have a later last spring frost and an earlier first fall frost than the state average, so the onion dates shift with them.
When to plant onions in nearby states
| State | Last frost | Plant out |
|---|---|---|
| Kansas (this page) | April 15 | Mar 18 – Apr 4 |
| North Dakota | May 15 | Apr 17 – May 4 |
| South Dakota | May 10 | Apr 12 – Apr 29 |
| Nebraska | April 30 | Apr 2 – Apr 19 |
| Oklahoma | April 1 | Mar 4 – Mar 21 |
Questions gardeners in Kansas ask
When is the best time to plant onions in Kansas?
Plant onion transplants outdoors in Kansas between March 18 and April 4. That timing is set by Kansas's average last spring frost of April 15.
When should I start onion seeds indoors in Kansas?
Start seeds indoors between January 28 and February 11, about 10 weeks before the last frost, so the seedlings are ready to plant out from March 18.
When will onions planted in Kansas be ready to harvest?
Planted on time, onions are usually ready from about June 23 to July 23, 90-120 days after transplanting.
When is the last frost in Kansas?
On average around April 15, with the first fall frost around October 20 - about 188 frost-free days. Your town may differ by two weeks or more.