When to Plant Onions in Missouri
Published: April 21, 2026
Missouri gardeners in Zone 6b have a 33-week growing season for onion (March 22 to November 10). Missouri's spring and fall windows both work well for onion — some gardeners plant both successions for a long harvest. This guide gives exact dates for Missouri based on your local frost calendar.
Onion Planting Dates for Missouri
| Start seeds indoors | January 11–18 |
| Last frost (average) | March 22 |
| Transplant outdoors | February 22–March 4 |
| Direct sow outdoors | February 22–March 4 |
| Expect first harvest | May 23 – June 22 |
| First fall frost (average) | November 10 |
Dates above assume Zone 6b, the most common zone in Missouri. Gardeners in cooler St. Louis (Zone 6b) plant 1–2 weeks later; warmer Kansas City (Zone 6a) can plant 1–2 weeks earlier.
⚠ CRITICAL: Choose variety based on your latitude. Short-day varieties for south of 35°N (Texas, Florida, California). Long-day varieties for north of 35°N (Ohio, Minnesota, New York). Intermediate varieties work in the middle band.
Growing Onions in Missouri's Climate
Missouri's moderate Zone 6b climate is well-suited to onion with minimal special accommodation. The standard transplant calendar works as written, and harvest windows line up with what seed packets recommend without adjustment.
Succession planting extends your onion harvest significantly in Missouri. A second planting 3 to 4 weeks after the first gives you a backup in case of pest pressure and extends the total harvest window into early fall.
Common onion pests to watch for in Missouri include Onion Thrips and Onion Maggot. The first line of defense is companion planting: Carrot and Tomato planted nearby discourage several of these pests by confusing host-plant identification or repelling adults before they can lay eggs. Weekly garden walks in the early morning catch problems when they're still manageable.
Onion Garden Calendar for Missouri
| Month | Task | Notes |
|---|---|---|
| January | Start seeds indoors | Use grow lights; warm soil to 70–75°F for best germination |
| February | Transplant outdoors | Soil temp must be 35°F+; nighttime lows above 50°F |
| March | Harden off seedlings | Set seedlings outdoors 1–2 hours daily, increasing gradually |
| April | — | Dormant season |
| May ← | Watch for bolting | Harvest promptly; spring heat arrives quickly |
| June | — | Dormant season |
| July | — | Dormant season |
| August | — | Dormant season |
| September | — | Dormant season |
| October | — | Dormant season |
| November | First frost approaches | Harvest remaining onion; cover plants on frost nights |
| December | — | Dormant season |
Onion Growing Tips for Missouri Gardeners
- •Match variety to latitude: short-day south of 35°N, long-day north of 35°N, intermediate in between.
- •Growing from sets (small bulbs) is fastest; growing from transplants gives the most variety options.
- •Missouri's moderate climate supports succession planting — sow new transplants or seeds every 3 weeks during spring to extend the harvest window into fall.
- •Reduce watering as tops begin to fall over — dry conditions encourage tight, firm bulbs that store longer.
Companion Planting for Onions in Missouri
In Missouri's moderate climate, planting onion with Carrot and Tomato helps deter pests and improve soil. Avoid planting near Bean and Pea, which inhibit onion growth through root chemistry or shared pest pressure. See the full onion companion planting guide for the complete list.
Onions Planting Dates by City in Missouri
Top cities in Missouri — select for city-level frost dates and planting calendars.
Frequently Asked Questions
When should I plant onion in Missouri?
In Missouri (Zone 6b), start onion seeds indoors around January 11–18 and transplant outdoors around February 22–March 4 — 4 weeks before the state's average last frost of March 22.
What is the last frost date in Missouri?
Missouri's average last frost date is March 22 in the state's most common Zone 6b. Frost dates vary by city — higher elevations and northern counties run 1 to 3 weeks later than the state average. See the city list below for city-level frost dates.
Can I grow onion year-round in Missouri?
onion is a cool-season crop in Missouri, with spring and fall windows. Summer heat bolts the plants before they can produce a harvestable crop. Succession planting every 2–3 weeks stretches the harvest window.
What onion varieties grow best in Missouri?
For Missouri's Zone 6b, Intermediate-day varieties (Candy, Super Star) work best in the 35–38°N band.
When do I start onion seeds indoors in Missouri?
Start onion seeds indoors in Missouri around January 11–18, which is 10 weeks before the state's average last frost of March 22. Use grow lights and keep soil temperatures at 70–75°F for fastest germination.