When to Plant Onions in Colorado
The backbone of the kitchen garden. Choose short-day, intermediate, or long-day varieties based on your latitude.
The Short Answer
Colorado Frost Dates
Your planting dates depend on which part of Colorado you're in. Here are the frost date ranges by region:
| Region | Zones | Last Frost (Spring) | First Frost (Fall) |
|---|---|---|---|
| Front Range (Denver) | 5a, 5b, 6a | May 1 - May 15 | Sep 25 - Oct 10 |
| Western Slope | 4b, 5a, 5b, 6a | May 10 - Jun 1 | Sep 15 - Oct 5 |
| Mountain Regions | 3a, 3b, 4a, 4b | Jun 1 - Jun 20 | Aug 25 - Sep 15 |
Onions Planting Schedule for Colorado
Front Range (Denver) (Zones 5a, 5b, 6a)
Average last frost: May 1 - May 15 · Average first frost: Sep 25 - Oct 10
Western Slope (Zones 4b, 5a, 5b, 6a)
Average last frost: May 10 - Jun 1 · Average first frost: Sep 15 - Oct 5
Mountain Regions (Zones 3a, 3b, 4a, 4b)
Average last frost: Jun 1 - Jun 20 · Average first frost: Aug 25 - Sep 15
Growing Onions in Colorado
Onions in Colorado's Climate
You're in long-day onion territory — plant varieties that bulb when days exceed 14-16 hours. This means Copra, Walla Walla, and Yellow Globe Danvers. Plant sets or transplants from mid-April. The critical distinction: short-day onions from southern catalogs will NOT form bulbs in your zone because your long summer days trigger bulbing too early, before the plant is large enough.
Soil Considerations for Colorado
Alkaline clay soils common along Front Range. Rocky, thin soils in mountains. Amend heavily with compost and sulfur to lower pH. Loose, well-drained soil is especially important for onions since the edible portion grows underground. If your soil is heavy clay, consider raised beds.
Colorado Climate & Growing Season
High altitude means intense sun but cool nights. Low humidity. Hail risk in late spring. Short but intense growing season at elevation. Onions can handle frost well, which is an advantage in Colorado's climate. You can push planting dates earlier in spring and extend into fall.
Growing season length varies across Colorado: Front Range (Denver) (5a, 5b, 6a) has a last frost around May 1 - May 15, while Mountain Regions (3a, 3b, 4a, 4b) sees frost end around Jun 1 - Jun 20. This difference matters for onions — but onions handle frost well, so the timing difference is less critical.
Growing Tips
Day length triggers bulbing. Northern gardeners need long-day varieties. Southern gardeners need short-day varieties.
Companion Planting
Plant onions alongside these companions for better growth:
Keep onions away from:
The Bottom Line
Last reviewed: March 29, 2026