Missouri · zones 5b–7a
When to plant onions in Missouri.
Sow or transplant between Feb 5 to Mar 21 in a typical year, keyed to Missouri's median last frost around Apr 15. A second, fall sowing fits Aug 25 to Oct 6. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Missouri season.
1 · Start indoors
Dec 25 to Feb 7
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 5 to Mar 21
Keyed to the ~Apr 15 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 25 to Oct 6
Counted back from the ~Oct 20 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Missouri 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.
Missouri growing notes
Missouri sits on the onion day-length line. Use it.
Why day length decides your variety
Onions bulb on a light timer, not a calendar. Long-day types wait for 14 to 16 hours of daylight, short-day types fire at 10 to 12, and Missouri, sitting between 36 and 40 degrees north, is squarely in intermediate-day territory: varieties that bulb at 12 to 14 hours. Candy and Superstar are the proven intermediate performers here and routinely make softball-size bulbs.
Plant a short-day onion in Missouri and it bulbs in April at golf-ball size. Plant a long-day type, especially in the southern half of the state, and it may run out of summer before it finishes. The variety tag matters more than any other decision you make on this crop.
Transplants for bulbs, sets for green onions
For big storage bulbs, start from transplants, the pencil-size bare-root seedlings sold in bundles, and set them out weeks before the last frost; onions shrug off freezes that would kill a tomato. Sets, the marble-size dry bulbs, are 2 weeks faster to green onions but bolt to seed far more often when they try to make a large bulb, because a set is already a year old.
Space transplants 4 inches apart for full bulbs, and remember the leaf rule: every leaf on the plant becomes one ring on the bulb. That is why early planting and steady nitrogen through May matter; the plant must build its leaf count before the day-length trigger flips it from growing leaves to fattening the bulb.
Thrips, blotch, and the fall-over signal
Missouri's muggy summers bring the two standard onion problems. Thrips are the first: silvery streaks on leaves in hot dry stretches. The test is to thump a few tops over white paper and count the specks that crawl; more than a few per plant earns a spray of insecticidal soap. Purple blotch is the second: bull's-eye purple lesions in humid weather, managed by watering at the base in the morning and never over the top in the evening.
Harvest starts when the tops fall over on their own; when about half the row is down, stop watering, wait a week, and lift the bulbs. Cure them 2 to 3 weeks somewhere shaded and breezy until the necks are paper-dry, then store. A well-cured Candy keeps about 3 months; pungent long-keepers trade sweetness for 6 or more.
In Missouri: watch for
The pests that find onions here.
A middle-length season, about 188 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: Missouri gives onions about 188 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 Missouri: questions
When should I plant onions in Missouri?+
In a typical Missouri year, sow onions or set out transplants between Feb 5 to Mar 21, keyed to the state's median last frost around Apr 15. There's also a fall window: Aug 25 to Oct 6. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Missouri?+
Start seeds inside about 6 weeks before set-out: roughly Dec 25 to Feb 7 for a typical Missouri season.
What USDA zone is Missouri?+
Missouri 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.