When to Plant Onions in Colorado
In Colorado, the time to plant onions outside runs from Mar 17 – Mar 31 in Grand Junction to Apr 23 – May 7 in Durango. In Denver, it's Apr 6 – Apr 20. The average last spring frost differs by about 5 weeks across the state (Apr 14 in Grand Junction, May 21 in Durango), so use the dates for the town nearest you.
Onions are cold-hardy. Plant sets or transplants early so they grow leaves before long summer days trigger bulbing.
- Start indoors: 10–12 weeks before the last frost.
- Plant outside: 2–4 weeks before the last frost.
Onions planting dates in Colorado by city
| City | Zone | Start indoors | Plant outside |
|---|---|---|---|
| Arvada | 6a | Feb 10 – Feb 24 | Apr 7 – Apr 21 |
| Aurora | 6a | Feb 12 – Feb 26 | Apr 9 – Apr 23 |
| Boulder | 6a | Feb 11 – Feb 25 | Apr 8 – Apr 22 |
| Brighton | 5b | Feb 8 – Feb 22 | Apr 5 – Apr 19 |
| Broomfield | 6a | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Castle Pines | 5b | Feb 14 – Feb 28 | Apr 11 – Apr 25 |
| Castle Rock | 5b | Feb 14 – Feb 28 | Apr 11 – Apr 25 |
| Cañon City | 6a | Feb 9 – Feb 23 | Apr 6 – Apr 20 |
| Centennial | 6a | Feb 10 – Feb 24 | Apr 7 – Apr 21 |
| Colorado Springs | 6a | Feb 8 – Feb 22 | Apr 5 – Apr 19 |
| Commerce City | 6a | Feb 8 – Feb 22 | Apr 5 – Apr 19 |
| Denver | 6a | Feb 9 – Feb 23 | Apr 6 – Apr 20 |
| Durango | 6b | Feb 26 – Mar 12 | Apr 23 – May 7 |
| Englewood | 6a | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Erie | 5b | Feb 12 – Feb 26 | Apr 9 – Apr 23 |
| Evans | 5b | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Firestone | 5b | Feb 12 – Feb 26 | Apr 9 – Apr 23 |
| Fort Collins | 5b | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Fountain | 6a | Feb 8 – Feb 22 | Apr 5 – Apr 19 |
| Frederick | 5b | Feb 12 – Feb 26 | Apr 9 – Apr 23 |
| Golden | 5b | Feb 8 – Feb 22 | Apr 5 – Apr 19 |
| Grand Junction | 7a | Jan 20 – Feb 3 | Mar 17 – Mar 31 |
| Greeley | 5b | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Greenwood Village | 6a | Feb 12 – Feb 26 | Apr 9 – Apr 23 |
| Johnstown | 5b | Feb 15 – Mar 1 | Apr 12 – Apr 26 |
| Lafayette | 6a | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Lakewood | 6a | Feb 8 – Feb 22 | Apr 5 – Apr 19 |
| Littleton | 6a | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Lone Tree | 6a | Feb 10 – Feb 24 | Apr 7 – Apr 21 |
| Longmont | 5b | Feb 12 – Feb 26 | Apr 9 – Apr 23 |
| Louisville | 6a | Feb 11 – Feb 25 | Apr 8 – Apr 22 |
| Loveland | 5b | Feb 15 – Mar 1 | Apr 12 – Apr 26 |
| Montrose | 6b | Feb 3 – Feb 17 | Mar 31 – Apr 14 |
| Northglenn | 6a | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Parker | 5b | Feb 14 – Feb 28 | Apr 11 – Apr 25 |
| Pueblo | 6a | Feb 5 – Feb 19 | Apr 2 – Apr 16 |
| Thornton | 6a | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Westminster | 6a | Feb 7 – Feb 21 | Apr 4 – Apr 18 |
| Wheat Ridge | 6a | Feb 10 – Feb 24 | Apr 7 – Apr 21 |
| Windsor | 5b | Feb 8 – Feb 22 | Apr 5 – Apr 19 |
Calculated from each town's average (50%) frost dates, from the nearest NOAA station (1991–2020). Typical dates for the 2027 planting season, based on NOAA's 1991–2020 averages. They are long-term odds, not a forecast: always check your local forecast before planting. Zones: 2023 USDA Plant Hardiness Zone Map (USDA-ARS and Oregon State University). Not the official map; see planthardiness.ars.usda.gov.
See all frost dates in Colorado, or the general guide to when to plant onions.
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