Onion
What Plant Yet? knows about onion, and where each number comes from. For today's answer, pick your town.
Planting numbers
- Frost category
- Hardy Source: Colorado State University Extension
- Plant outdoors
- 2–4 weeks before typical last frost Given for hardy crops as a group. Source: Colorado State University Extension
- Start seeds indoors
- 10 weeks before typical last frost Source: Colorado State University Extension
- Cover at or below
- 28°F Source: Oregon State University Extension
- Grows once daytime temperatures reach
- 40°F Given for hardy crops as a group. Source: Colorado State University Extension
- Days to harvest
- 100–120 Source: Utah State University Extension
Left out because we found no university extension source for it: minimum night temperature. We don't estimate.
Growing tips
- In Colorado, choose long-day or day-neutral varieties. (Colorado State University Extension)
- Set out transplants or sets between mid-April and mid-May. (Colorado State University Extension)
- Sets or plants are the easiest way to grow onions in much of Colorado. (Colorado State University Extension)
- Snap off seed heads; they steal energy from the bulb. (Colorado State University Extension)
- Mulch with dry grass clippings to cool soil, hold moisture and block weeds. (Colorado State University Extension)
How the answer uses this
The planting window opens 2–4 weeks before typical last frost in your town and stays open while a planting still has 120 days to mature before the typical first fall frost. Inside the window the answer is Plant when the coldest of the next 7 nights stays above 28°F (no source gives a minimum night temperature, so the cold-damage temperature is used). Cover tonight shows when tonight's low is 28°F or colder while onion is normally in the ground.
Sources
- Colorado State University Extension — checked Nov 11, 2026
- Colorado State University Extension — checked Nov 11, 2026
- Oregon State University Extension — checked Nov 11, 2026
- Utah State University Extension — checked Nov 11, 2026
Guidance only. Extension advice is written for typical conditions; your soil, variety and microclimate matter.