Idaho · zones 5b–7b
When to plant onions in Idaho.
Sow or transplant between Mar 3 to Apr 16 in a typical year, keyed to Idaho's median last frost around May 11. A second, fall sowing fits Aug 3 to Sep 14. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Idaho season.
1 · Start indoors
Jan 20 to Mar 5
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Mar 3 to Apr 16
Keyed to the ~May 11 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 3 to Sep 14
Counted back from the ~Sep 28 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Idaho spans more than one climate: Zone 6 is the most common zone, but the state runs 5b to 7b. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
Idaho growing notes
Bulb onions answer to day length before they answer to anything else here, and Idaho's northern latitude puts every part of the state, valley to high country, in long-day territory.
Bulbing runs on daylight, not on the calendar
A long-day onion is bred for exactly the daylight stretch that a far-north latitude like Idaho's produces as the growing season moves toward summer. Bulb onions form their bulb once daylight reaches a length tied to the variety, not once the plant reaches a certain size or age. A short-day type, bred for southern latitudes, would start bulbing here too, only much earlier in its life, while the plant is still small, and quit putting on leaf right when it needed more. Choosing a long-day type is not a matter of preference in this state; it is the one decision this crop does not forgive getting backwards.
The numbers otherwise are plain: transplants go in about half an inch deep, spaced roughly 4 inches apart down the row, and the crop takes about 95 days to bulb up and cure to harvest size, a big share of the state's 140-day frost-free run. Because the trigger is day length rather than plant size, the transplant has to go in early enough to build real leaf growth before that trigger arrives. A plant barely rooted in when the days hit that mark bulbs anyway, just small, using only the leaf it had managed by then, with no second chance that season to grow bigger.
Storage versus fresh eating, and the fall option most gardens skip
The choice left to the grower is what the bulb is for. At around half a pound of onion per plant, a row grown for winter storage needs real length in the ground and a thicker skin to be worth the pantry space, and a storage type needs to cure fully before coming out of the ground to keep well. A sweeter, thinner-skinned type is worth growing instead for eating fresh through late summer, since those bulbs were never going to hold past a few weeks regardless of how well they cured.
A fall planting is also on the table, mainly in the milder pockets of the Treasure Valley floor, setting out small transplants to overwinter for a jump on the following spring. That asks more of the plant than a spring transplant does, and is not the safer bet on the state's colder ground. The reliable version of this crop, valley or high country, stays a spring transplant set out early enough to build leaf before the days get long.
Two insects, two diseases, and which end of the state favors which
Onion thrips are tiny enough to miss until the damage is obvious: pale streaking and a silvered look across the leaf. They favor hot, dry conditions and build up fastest in the valley's low-humidity heat rather than in cooler ground. A plant that is otherwise strong outgrows light thrips damage, but a heavily silvered leaf stops feeding the bulb underneath, so a bad thrips year is worth actually treating rather than waiting out. Onion maggot works below ground instead, the larvae attacking the base of young transplants while spring soil is still cool and wet. A transplant that suddenly wilts and pulls up with a rotted, hollowed base did not fail from transplant shock; that is the maggot at work.
Downy mildew shows as a grayish, fuzzy coating on the leaves, worse on the humid nights the northern panhandle sees more of than the valley floor does. White rot is the one worth taking most seriously over time: it is soil-borne, persists for years once it shows up, and a bed that has grown it needs a different spot for onions next time, not the same ground again, and not another allium put there in its place. Of the four, thrips and maggot are managed season to season; white rot is the one that changes where a bed gets planted for good.
In Idaho: watch for
The pests that find onions here.
The short season here, about 140 frost-free days, keeps onion, bulb in front of onion thrips only briefly. One early row cover usually carries a planting through the worst of it.
Silvery streaks and curled leaf tips in hot spells.
Organic fix: Insecticidal soap or neem; hang sticky traps.
Seedlings wilt; white maggots in the bulb base.
Organic fix: Float a row cover over the row; rotate beds each year.
Pale patches with fuzzy gray growth in damp weather.
Organic fix: Space for airflow, water early in the day, spray neem.
White rot
Yellowing and dieback, then wilt; fluffy white mold at the bulb base and roots so rotted the plant lifts straight out.
Organic fix: No cure once it is in the soil. Plant clean stock, never follow onions with onions, and keep infested soil off tools and boots.
Your season math: Idaho gives onions about 140 frost-free days, and it needs roughly 95 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting onions in Idaho: questions
When should I plant onions in Idaho?+
In a typical Idaho year, sow onions or set out transplants between Mar 3 to Apr 16, keyed to the state's median last frost around May 11. There's also a fall window: Aug 3 to Sep 14. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Idaho?+
Start seeds inside about 6 weeks before set-out: roughly Jan 20 to Mar 5 for a typical Idaho season.
What USDA zone is Idaho?+
Idaho spans zones 5b–7b, mostly zone 6. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many onions plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.