When to Plant Onions in Utah
Published: April 21, 2026
Utah gardeners in Zone 6a have a 31-week growing season for onion (March 30 to November 1). Utah'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 Utah based on your local frost calendar.
Onion Planting Dates for Utah
| Start seeds indoors | January 19–26 |
| Last frost (average) | March 30 |
| Transplant outdoors | March 2–12 |
| Direct sow outdoors | March 2–12 |
| Expect first harvest | May 31 – June 30 |
| First fall frost (average) | November 1 |
Dates above assume Zone 6a, the most common zone in Utah. Gardeners in cooler Ogden (Zone 5b) plant 1–2 weeks later; warmer St. George (Zone 6b) 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 Utah's Climate
Utah's moderate Zone 6a 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 Utah. 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 Utah 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 Utah
| Month | Task | Notes |
|---|---|---|
| January | Start seeds indoors | Use grow lights; warm soil to 70–75°F for best germination |
| February | — | Dormant season |
| March | Transplant outdoors | Soil temp must be 35°F+; nighttime lows above 50°F |
| 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 Utah 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.
- •Utah'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 Utah
In Utah'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 Utah
Top cities in Utah — select for city-level frost dates and planting calendars.
Frequently Asked Questions
When should I plant onion in Utah?
In Utah (Zone 6a), start onion seeds indoors around January 19–26 and transplant outdoors around March 2–12 — 4 weeks before the state's average last frost of March 30.
What is the last frost date in Utah?
Utah's average last frost date is March 30 in the state's most common Zone 6a. 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 Utah?
onion is a cool-season crop in Utah, 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 Utah?
For Utah's Zone 6a, Intermediate-day varieties (Candy, Super Star) work best in the 35–38°N band.
When do I start onion seeds indoors in Utah?
Start onion seeds indoors in Utah around January 19–26, which is 10 weeks before the state's average last frost of March 30. Use grow lights and keep soil temperatures at 70–75°F for fastest germination.