When to plant onions in Idaho
Idaho’s average last spring frost is May 10, and the first fall frost typically arrives around September 25 — about 138 frost-free days (USDA zones 3b, 4b, 5b, 6a, 6b, 7a). Onion needs 90–120 days to mature, which fits comfortably within Idaho's growing season.
Idaho covers a wide range of climates, so your exact frost dates may differ from the state average by two weeks or more. For dates tuned to your town, use the calendar generator.
When to plant onions in Idaho: the short answer
For a spring planting, plant out Apr 12 – Apr 29, with seeds started indoors Feb 22 – Mar 8. Seeds germinate in soil from 40 to 90°F.
Windows are ~1–2 week bands around each date. Frost dates are a statewide average from NOAA/NCEI normals; your town may differ by two weeks or more. Calendar year 2026.
How to grow onions in Idaho, step by step
- Start seeds indoors (Feb 22 – Mar 8). Sow seeds ½ inch deep in seed-starting mix under bright light. They take 7-14 days to sprout, fastest in soil around 70°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. They need at least six to eight hours of direct sun a day and loose soil at least 12 inches deep, with a pH between 6.0 and 7.0. Dig in compost before planting.
- Plant out (Apr 12 – Apr 29). Space plants about 4 inches apart in rows 12 inches apart. It handles frost well, so it can go out before the last frost date. As a cool-season crop it does best before summer heat arrives in Idaho.
- Water and feed. Give it steady, even moisture, about an inch of water a week from rain or the hose.
- Watch for pests and problems. Pests: onion maggot, onion thrips. Diseases and disorders: downy mildew, white rot, neck rot, purple blotch.
- Harvest (from about Jul 18 – Aug 17). Onion takes 90-120 days to reach harvest. Match the day-length type to your latitude: long-day onions for the north, short-day for the south, day-neutral anywhere. Bulbing is triggered by day length, not calendar date.
Figures come from our crop data; dates use Idaho’s average frost dates. More in the full onion growing guide.
Onion planting dates by USDA zone in Idaho
Idaho spans USDA zones 3b, 4b, 5b, 6a, 6b, 7a. Colder zones, usually higher or further inland, have a later last spring frost and an earlier first fall frost than the state average, so the onion dates shift with them.
| Zone | Last frost | First frost | Season | Start indoors | Plant onion |
|---|---|---|---|---|---|
| Zone 3 | May 15 | Sep 15 | 120 days | Feb 27 – Mar 13 | Apr 17 – May 4 |
| Zone 4 | May 10 | Sep 25 | 135 days | Feb 22 – Mar 8 | Apr 12 – Apr 29 |
| Zone 5 | Apr 30 | Oct 5 | 160 days | Feb 12 – Feb 26 | Apr 2 – Apr 19 |
| Zone 6 | Apr 15 | Oct 15 | 185 days | Jan 28 – Feb 11 | Mar 18 – Apr 4 |
| Zone 7 | Apr 5 | Oct 25 | 200 days | Jan 18 – Feb 1 | Mar 8 – Mar 25 |
When to plant onions in nearby states
Questions gardeners in Idaho ask
When is the best time to plant onions in Idaho?
Plant onion transplants outdoors in Idaho between April 12 and April 29. That timing is set by Idaho's average last spring frost of May 10; Idaho spans several climates, so cooler and higher parts of the state run later.
When should I start onion seeds indoors in Idaho?
Start seeds indoors between February 22 and March 8, about 10 weeks before the last frost, so the seedlings are ready to plant out from April 12.
When will onions planted in Idaho be ready to harvest?
Planted on time, onions are usually ready from about July 18 to August 17, 90-120 days after transplanting.
When is the last frost in Idaho?
On average around May 10, with the first fall frost around September 25 - about 138 frost-free days. Your town may differ by two weeks or more.