Wyoming · zones 4a–5a
When to plant onions in Wyoming.
Sow or transplant between Mar 20 to May 3 in a typical year, keyed to Wyoming's median last frost around May 28. A second, fall sowing fits Jul 23 to Sep 3. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Wyoming season.
1 · Start indoors
Feb 6 to Mar 22
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Mar 20 to May 3
Keyed to the ~May 28 last frost. Sow 0.5" deep, 4" apart.
Fall round
Jul 23 to Sep 3
Counted back from the ~Sep 17 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Wyoming spans more than one climate: Zone 5 is the most common zone, but the state runs 4a to 5a. 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.
In Wyoming: watch for
The pests that find onions here.
The short season here, about 112 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: Wyoming gives onions about 112 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 Wyoming: questions
When should I plant onions in Wyoming?+
In a typical Wyoming year, sow onions or set out transplants between Mar 20 to May 3, keyed to the state's median last frost around May 28. There's also a fall window: Jul 23 to Sep 3. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Wyoming?+
Start seeds inside about 6 weeks before set-out: roughly Feb 6 to Mar 22 for a typical Wyoming season.
What USDA zone is Wyoming?+
Wyoming spans zones 4a–5a, mostly zone 5. 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.