Yes — Strong Match
onion is grown in zones 2-11, which covers every zone Wyoming contains (3b-6a). Whether the season is long enough is the question that remains.
Onion in Wyoming — the short answer
Onion in Wyoming: grown in zones 2–11, full sun.
- Plant zones
- 2-11
- Not verified — NC State Extension Gardener Plant Toolbox lists zones 5a–10b
- Wyoming zones
- 3b-6a
- USDA PHZM 2023
- Frost window
- May 4 - May 13 – Sep 30 - Oct 7
- NOAA RCC ACIS 1991-2020, 28°F
- Shortest county season
- 79 days
- NOAA Regional Climate Centers ACIS, 1991-2020, county 28°F killing freeze
- Days to maturity
- 110 days
- Growable Ground Plant Database
Plant data: Growable Ground Plant Database, compiled from FAO ECOCROP, USDA PLANTS and university Extension sources — per-field provenance varies and is published with the dataset.
Your yard isn't the whole zone.
Wyoming spans zones 3b-6a, but your yard sits in exactly one — and slope, tree cover, and cold-air pockets nudge it further. Enter your address and we'll score onion against your parcel's actual hardiness, soil, and sun.
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Wyoming is not one climate
A single Wyoming answer averages 23 counties whose growing seasons are 87 days apart — from 79 frost-free days in Teton County to 166 in Big Horn County. Which end of that your land sits nearer changes what onion can finish before the first hard freeze.
The middle half of 23 counties runs 136–155 days, with a median of 151 days — a 87-day span end to end.
Measured across 23 Wyoming counties. NOAA Regional Climate Centers ACIS, 1991-2020, county 28°F killing freeze. Quartiles are order statistics — every figure shown is a value a real county recorded, not an interpolation. Your own site sits somewhere inside this range; a parcel report resolves where.
Where in Wyoming Fits Best
Even within Wyoming's zones 3b-6a, county microclimates differ enough to change what thrives. These counties carry the closest zone match for onion (USDA PHZM 2023):
Zone Comparison
Onion Needs
- USDA Zones: 2-11
- Sun: Full Sun
- Mature size: 1.5 ft tallWe haven't confirmed this against a published source yet.
- Picking window: ~7 days of harvestWe haven't confirmed this against a published source yet.
- Drought tolerance: MediumWe haven't confirmed this against a published source yet.
Wyoming Has
- USDA Zones: 3b-6a
- Last Hard Freeze (28°F): May 4 - May 13
- First Hard Freeze (28°F): Sep 30 - Oct 7
- Annual Rainfall: 6-20 inches
- Common Soils: Sandy loam, Clay, Alkaline
Plant Zone Range (zones 2-11)
Preferred Soil pH
Plant data: Growable Ground Plant Database, compiled from FAO ECOCROP, USDA PLANTS, and university Extension sources — per-field provenance varies and is published with the dataset. State data: USDA ARS PHZM 2023, NOAA Climate Normals, NRCS SSURGO.
When to Plant Onion in Wyoming
The frost window
Across Wyoming, the last spring hard freeze (28°F) clears between May 4 and May 13, and the first fall hard freeze lands between Sep 30 and Oct 7 (NOAA Regional Climate Centers ACIS, 1991-2020). Counting from the latest last freeze to the earliest first freeze, that's a 140-day window you can count on — up to 156 days on a mild site in a kind year. That marks the hard freeze, not the last light frost, so tender transplants usually wait another 2-3 weeks.
When to start the seed
Those dates are when onion goes out, not when it starts. It is sown under cover about 9 weeks ahead, so counting back from Wyoming's late-side last freeze of May 13 is what fixes your sowing date — and a yard that clears frost earlier than the state's late date starts earlier still.
Grown as an annual here
Onion is a true perennial in its home range — it overwinters from USDA zone 5 and warmer (NC State Extension Gardener Plant Toolbox) — but Wyoming runs as cold as zone 3, so it is killed back each winter and replanted each spring. Treat it on the season schedule above, not as a permanent planting: there is no fall establishment window for it here.
Frost window: NOAA Climate Normals 1991-2020. Plant timing fields: Growable Ground Plant Database — frost-free days from FAO ECOCROP, days to harvest from university Extension production guides, and growing-degree-day and chill-hour minimums that are compilation estimates rather than published figures. Your site's own frost dates can run earlier or later than the state range — a parcel report pins them down.
Growing Season Fit
Onion is grown in zones 2–11, which covers every zone Wyoming contains (3b-6a). It overwinters in the ground only in zone 5 and warmer (NC State Extension Gardener Plant Toolbox). Wyoming's typical season runs ~151 frost-free days against the ~85 onion needs.
Growing degree days
Onion needs ~1700 GDD (base 50°F) to ripen. We haven't confirmed this against a published source yet. The state median runs ~1866 GDD (from PRISM 1991-2020 climate normals), so Wyoming sits right at the threshold — pay attention to siting and microclimate.
GDD and chill-hour figures are computed from PRISM 1991-2020 monthly climate normals; frost-free days from NOAA Regional Climate Centers ACIS, 1991-2020. Each is the state median across its counties (P25/P75 span the within-state spread). Hardiness-zone range is USDA PHZM 2023.
Soil + Drainage Fit
Onion tolerates a wide pH range (pH 4.3-8.3) — 4 units, acid ground through alkaline — so soil acidity is rarely what limits it in Wyoming. A soil test is still worth having, but for what else it tells you about this site, not to clear this plant. It will grow across that whole span, but it does best between pH 6 and 7 — worth knowing before you decide whether a Wyoming soil test result is a problem or just a number. Drainage matters: this plant wants well (dry spells). If your Wyoming site is heavier clay or sits in a low spot, a raised bed lifts the roots above wet ground, or work in compost.
Plant pH requirements: FAO ECOCROP and USDA PLANTS, with the remaining rows from other published sources or Growable Ground compilation. Drainage: FAO ECOCROP, with the remaining rows from other published sources or Growable Ground compilation. Wyoming soil profile from USDA NRCS SSURGO. Site-specific verification: a 30-minute soil test from your local Extension lab.
Before you plant onion for food
Anything you grow to eat takes up what is in the ground it grows in, so what is on record near you is worth knowing before onion goes in. What follows is Wyoming’s documented federal record at state scale — a screening signal, not a soil test, and not a statement about any one lot.
Safe to Grow Here?
What the federal record shows across Wyoming — and how to grow with it.
We checked the federal record across Wyoming — 4,526 documented sites across 7 of the 8 source types we track.
The most significant on record: 64 Superfund sites. Sites tracked in EPA's Superfund program — from assessment-stage CERCLIS entries to confirmed National Priorities List cleanup sites.
Wyoming carries one of the heavier federal records we track — and that's not a verdict on your yard. Proximity to a documented site is information, not a diagnosis: nothing here says any particular parcel is affected. It does earn one concrete step — before food beds go in the ground, a professional soil test tells you exactly what you're working with, and raised beds of imported, lab-tested soil grow well almost anywhere in the meantime.
Sources Checked
across Wyoming
Severity Distribution
across Wyoming
Highest-Severity Sites
Know Before You Grow
- •Underground tanks leak petroleum as they age — near a former filling station, test the soil.
- •Mine waste leaches heavy metals. Test for lead, arsenic and cadmium before growing food here.
- •TRI facilities report what they release. Check the wind — downwind beds catch more of it.
Sources: EPA, USGS — 950K documented sites tracked nationwide across 8 federal source types.
What Else to Consider for Onion in Wyoming
The zone range tells you where onion is grown — but Wyoming growers also need to think about:
Extremely short growing season (60-90 frost-free days)
Very low rainfall requires irrigation
Persistent high winds desiccate and damage plants
What to actually do about these — and the constraints that apply to every Wyoming grower, not just onion — is covered on the Wyoming growing guide.
Where in Wyoming Onion Is Zone-Hardy
Onion is rated for USDA zones 2–11. Every one of Wyoming’s 23 counties sits inside that band — from Albany County at zone 4b to Uinta County at zone 5b — so winter cold is not what decides this plant anywhere in the state. What is left to vary is your own parcel: slope, shelter and cold-air drainage routinely move a site half a zone either way from the county figure.
Across those 23 counties Wyoming’s winter-low ladder runs from zone 4b in Albany County to zone 5b in Uinta County — 1 USDA zone of spread, which is 10°F between the two counties’ average coldest night of the year. That is why a statewide hardiness answer is where you start and not where you finish.
The planting window moves with it. Teton County holds its last hard freeze (28°F) until about Jun 16, while Big Horn County is clear by Apr 26 — 51 days apart. Set onion out on your county’s date, not the state’s.
County hardiness zones: USDA Plant Hardiness Zone Map (PHZM) 2023, per county. County last-freeze dates: NOAA Regional Climate Centers ACIS (1991-2020), county 28°F killing freeze — earlier than the folk 32°F "last frost". Onion’s zone 2–11 band: Not verified — NC State Extension Gardener Plant Toolbox lists zones 5a–10b. A parcel report resolves your address’s own zone and frost dates rather than its county’s.
Pollinator + Wildlife Value
Onion draws honeybees, native bees, and butterflies (moderate value). Planting it near vegetable beds can lift fruit set on neighboring crops. Outdoors, onion can only work that bloom inside Wyoming's frost window — roughly 151 days, opening after the last 28°F freeze (May 4 - May 13) and closing at the first (Sep 30 - Oct 7), per NOAA Regional Climate Centers ACIS, 1991-2020. Pollinator value: We haven't confirmed this against a published source yet.
Wyoming Cooperative Extension
For Wyoming-specific cultivar recommendations, planting calendars, and pest pressure for onion, the canonical source is University of Wyoming Extension. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Onion native to Wyoming?
No — the USDA PLANTS Database lists Onion as introduced rather than native in the Lower 48, so it is not part of Wyoming's native flora. It grows here as a garden plant; pairing it with a few Wyoming natives keeps local pollinators fed too.
Looking for plants that belong here? The Wyoming growing guide lists plants that USDA PLANTS records growing in Wyoming and that Kew and NatureServe Explorer both list in its native range, leaving out any that NatureServe flags as exotic there or that our audit holds.
State records: USDA PLANTS Database state-distribution data (presence, not state-level nativity), accessed 2026-10-19.
Common Questions About Growing Onion in Wyoming
When can I plant Onion in Wyoming?
Wyoming's last spring hard freeze (28°F) clears between May 4 and May 13, and the first fall hard freeze lands between Sep 30 and Oct 7 (NOAA Regional Climate Centers ACIS, 1991-2020). Onion overwinters only from USDA zone 5 and warmer, and Wyoming runs as cold as zone 3, so it is grown as an annual here — plant it after the last hard freeze, two to three weeks later for tender stock, and expect to replant next spring rather than establishing it for the long term.
What hardiness zone is Onion grown in across Wyoming?
Wyoming spans USDA hardiness zones 3b-6a (USDA ARS PHZM 2023). Onion carries a range of zones 2-11, so the overlap zones are where outdoor growing is most reliable.
How many frost-free days does a typical Wyoming site have?
A typical Wyoming site sees ~151 frost-free days per year (derived from NOAA Regional Climate Centers ACIS, 1991-2020). Onion needs 85+ frost-free days, so check whether your local microclimate runs above or below the state average before settling on a planting date.
Is Onion native to Wyoming?
No — the USDA PLANTS Database lists Onion as introduced rather than native in the Lower 48, so it is not part of Wyoming's native flora. It grows here as a garden plant; pairing it with a few Wyoming natives keeps local pollinators fed too.
How should I amend the soil for Onion in Wyoming?
Onion tolerates pH 4.3-8.3 and well (dry spells) drainage (FAO ECOCROP). That is a 4-unit span, acid ground through alkaline, so pH is rarely what limits this plant on a Wyoming site — amend for drainage and organic matter first.
Will Onion actually grow on my specific land in Wyoming?
State-level zone + climate data is a sketch. A Growable Ground parcel report scores onion against your address's mapped soil pH, drainage, sun, and frost-date data drawn from USDA SSURGO, NOAA, and PRISM — not state averages.
Check your specific parcel in Wyoming
State-level data is a sketch. Your Growable Ground report scores onion against your land's mapped soil, modelled sun, drainage, and frost data — not zone averages.
Three things about your lot that Wyoming onion averages can't show:
We read public map data around Wyoming — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Wyoming onion average
Explore More
Growing guides for onion
Explore Wyoming county by county
Wyoming carries 23 counties with their own USDA zone — a sample of the growing pages for onion there:
Where else onion grows
Onion's zone band crosses state lines — these are the states closest to Wyoming in winter climate where it also has a page, plus the far ends of its range. Each one re-answers the question with that state's own record.
More to grow in Wyoming
Wyoming has a growing page for 807 plants — a rotating sample of the rest of the shelf.
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