Utah · zones 6a–8b
When to plant onions in Utah.
Sow or transplant between Feb 21 to Apr 6 in a typical year, keyed to Utah's median last frost around May 1. A second, fall sowing fits Aug 10 to Sep 21. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Utah season.
1 · Start indoors
Jan 10 to Feb 23
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 21 to Apr 6
Keyed to the ~May 1 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 10 to Sep 21
Counted back from the ~Oct 5 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Utah spans more than one climate: Zone 6 is the most common zone, but the state runs 6a to 8b. 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.
Utah growing notes
Utah's canal water decides your onion crop before the seed does.
Irrigation scheduling, not the calendar, drives bulb size
Most Utah gardens run on delivered water, either culinary tap or a shared irrigation turn, and onions are exceptionally sensitive to how that water gets rationed. The bulbing trigger is fixed by day length, but bulb size is decided by how many leaves the plant grew beforehand, and each leaf needs steady moisture to form; a garden on a once-a-week turn needs a deep soaking, 1.5 inches at a time, rather than the light daily splash that suits shallow-rooted greens.
Once bulbing starts and the base of the plant visibly begins to swell, watering can taper toward 1 inch a week without hurting final size, since the plant is now converting stored leaf growth rather than building new roots. Gardeners on a fixed irrigation schedule should move their onion bed to the front of the rotation in the 6 weeks before that swell begins, the stretch when the crop is least forgiving of a skipped turn.
Soil sulfur, not soil pH, sets the bite
Utah's high pH soil, commonly 7.5 to 8.5 across the valleys, gets blamed for onion flavor it does not actually control. The real lever is sulfur, the nutrient onions convert into the compounds behind pungency: more available sulfur in the root zone makes a sharper bulb, less makes a milder one, and pH itself barely moves that needle. A gardener who works elemental sulfur into the row at planting, about a tablespoon per foot, hoping to soften an onion, gets the opposite result, since sulfur additions raise the bite rather than bring it down.
Utah's calcareous valley ground typically already runs high in soil sulfur, which is one reason home-grown onions here trend toward the assertive side regardless of variety. The soils behind the country's mildest commercial onions, Georgia's Vidalia district and Washington's Walla Walla valley, are defined by sandy, well-leached, genuinely low-sulfur ground, not by acidity. A gardener chasing a milder Utah onion should ask the county extension office for a soil sulfur reading and skip sulfur fertilizer rather than fight the valley's pH; the tradeoff runs the other way too, since a sharper, higher-sulfur bulb also cures and stores several weeks longer than a mild one.
Salt buildup is the quiet yield killer
Ditch and canal water across much of Utah carries more dissolved mineral salt than municipal tap water, and onions rank among the more salt-sensitive garden crops; roots struggle to pull in moisture once salts concentrate in the top few inches of soil, even when the bed looks adequately watered from the surface. A white, crusty film on the soil between waterings is the visible warning sign, not a coincidence of dry weather.
Deep, infrequent watering flushes salts down past the root zone instead of concentrating them near the surface the way frequent shallow watering does, so shift any onion bed on canal water toward fewer, longer soakings, roughly once every 4 to 5 days rather than daily. A 2-inch mulch layer also slows the surface evaporation that pulls those salts back upward in the first place.
In Utah: watch for
The pests that find onions here.
A middle-length season, about 157 frost-free days, means onion, bulb overlaps onion thrips for a stretch each summer. Check plants every week or two and treat early.
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: Utah gives onions about 157 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 Utah: questions
When should I plant onions in Utah?+
In a typical Utah year, sow onions or set out transplants between Feb 21 to Apr 6, keyed to the state's median last frost around May 1. There's also a fall window: Aug 10 to Sep 21. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Utah?+
Start seeds inside about 6 weeks before set-out: roughly Jan 10 to Feb 23 for a typical Utah season.
What USDA zone is Utah?+
Utah spans zones 6a–8b, 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.