When to Plant Onions in Colorado
Published: April 21, 2026
Colorado gardeners in Zone 6a have a 31-week growing season for onion (March 30 to November 1). Colorado'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 Colorado based on your local frost calendar.
Onion Planting Dates for Colorado
| 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 Colorado. Gardeners in cooler Denver (Zone 6a) plant 1–2 weeks later; warmer Colorado Springs (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 Colorado's Climate
Colorado'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 Colorado. 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 Colorado 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 Colorado
| 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 Colorado 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.
- •Colorado'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 Colorado
In Colorado'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 Colorado
Top cities in Colorado — select for city-level frost dates and planting calendars.
Frequently Asked Questions
When should I plant onion in Colorado?
In Colorado (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 Colorado?
Colorado'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 Colorado?
onion is a cool-season crop in Colorado, 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 Colorado?
For Colorado'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 Colorado?
Start onion seeds indoors in Colorado 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.