Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated

Planting zone for 82420 — Cowley, WY

ZIP code 82420 (Cowley, Big Horn County) is in USDA zone 5a: the coldest winter night averages −20 to −15 °F. The median last spring frost is May 9 and the first fall frost September 29.

Where ZIP 82420 is

Outlined: the ZIP code area · tap the map for the zone at any spot · scroll or pinch to zoom

Not the official USDA Plant Hardiness Zone Map. Zones come from the 2023 USDA hardiness-zone data by the PRISM Group, Oregon State University; we re-mapped and recolored them. The official map is at planthardiness.ars.usda.gov.

Zone5a−20 to −15 °F
Coldest night−28.9 to −26.1 °C
Last frostMay 9
First frostSep 29
2012 zone4b

Was 4b on the 2012 map — half a zone warmer now. The zone is the one at the center of the ZIP code; hilly ZIPs can span two zones. What zone 5a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Cowley, WY…

Forecast data: National Weather Service (api.weather.gov), public domain. This is not a warning service — for official frost and freeze warnings see weather.gov.

Frost dates in Cowley, WY

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMay 9median — 50% chance of a later frost
Safe for tender plantsMay 25only 10% chance of frost after
First fall frostSeptember 29median — 10% chance by Sep 16
Growing season142 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 29 and returns around October 9.

From the NOAA station Lovell, WY — 6 miles away, elevation 3,849 ft. NOAA NCEI U.S. Climate Normals 1991–2020, 32 °F threshold. Low spots and valleys can freeze later in spring than the station.

When to plant vegetables in 82420

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 28 – Apr 11
SpinachSow outdoors · Mar 28 – Apr 18
Broccoli, cabbageStart indoors · Feb 28 – Mar 14
Broccoli, cabbageTransplant · Apr 18 – May 2
Onion setsPlant outdoors · Apr 11 – Apr 25
PotatoesPlant outdoors · Apr 18 – May 2
LettuceSow outdoors · Apr 11 – Apr 25
Carrots, beetsSow outdoors · Apr 18 – May 2
TomatoesStart indoors · Mar 14 – Mar 28
TomatoesTransplant · May 16 – May 30
Peppers, eggplantStart indoors · Feb 28 – Mar 14
Peppers, eggplantTransplant · May 23 – Jun 6
BeansSow outdoors · May 16 – May 30
Sweet cornSow outdoors · May 16 – May 30
Cucumbers, squashSow outdoors · May 16 – May 30
Fall lettuce, spinachSow outdoors · Aug 4 – Aug 18
GarlicPlant cloves · Sep 15 – Oct 20
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 9) and first fall frost (Sep 29) with the rules of thumb U.S. extension services publish. Cool-season crops take a light frost; warm-season crops do best once the soil is above about 60 °F.

When spring arrived in 2026

In 2026, spring leaf-out near 82420 came on Mar 22 — 18 days earlier than the 1991–2020 average (Apr 9).

JanFebMarAprMayJun

The Spring Index marks the day when accumulated warmth is enough for lilac and honeysuckle to leaf out — a widely used signal that the growing season has started. Source: USA National Phenology Network, www.usanpn.org — Spring Indices, first leaf, 2026 (data as of 2026-09-08) and 30-year average 1991–2020 (CC BY 4.0).

Native plants for zone 5a in Wyoming

542 plants native to the lower 48 states, recorded in Wyoming, and rated by the USDA to survive −20 °F or colder — the bottom of zone 5a. Each links to its USDA PLANTS profile.

Trees 22

American elm120 ft · blooms early spring · medium shade tolerance · to −46 °Fbalsam poplar80 ft · blooms late spring · high shade tolerance · to −79 °Fblack locust60 ft · blooms spring · high shade tolerance · to −37 °Fblue spruce100 ft · blooms late spring · medium shade tolerance · to −38 °Fboxelder60 ft · blooms early spring · low shade tolerance · to −46 °FDouglas-fir200 ft · blooms mid spring · medium shade tolerance · to −33 °F

Shrubs 98

alderleaf buckthorn3 ft · blooms late spring · low shade tolerance · to −33 °Falderleaf mountain mahogany15 ft · blooms late spring · medium shade tolerance · to −41 °Falpine laurel1 ft · blooms late spring · low shade tolerance · to −28 °FAmerican black currant3 ft · blooms early spring · to −46 °FAmerican plum24 ft · blooms mid spring · high shade tolerance · to −38 °Fantelope bitterbrush6 ft · blooms early spring · medium shade tolerance · to −36 °F

Wildflowers & perennials 249

alkali buttercup1 ft · blooms late spring · low shade tolerance · to −38 °Falpine bistort1 ft · blooms mid spring · high shade tolerance · to −38 °Falpine mountainsorrel1 ft · blooms mid summer · medium shade tolerance · to −43 °Falpine sweetvetch2 ft · blooms early summer · high shade tolerance · to −62 °FAmerican alpine speedwell1 ft · blooms summer · medium shade tolerance · to −28 °FAmerican bistort1.5 ft · blooms mid spring · high shade tolerance · to −38 °F

Grasses 161

Alaska oniongrass3 ft · blooms mid spring · low shade tolerance · to −33 °Falkali cordgrass3 ft · blooms late summer · high shade tolerance · to −38 °Falkali sacaton3 ft · blooms mid summer · high shade tolerance · to −38 °FAlpine Meadow-Foxtail0.5 ft · blooms mid spring · medium shade tolerance · to −33 °Falpine timothy1.4 ft · blooms late spring · medium shade tolerance · to −43 °Fanalogue sedge1.5 ft · blooms late spring · medium shade tolerance · to −38 °F

Vines 3

American bittersweet15 ft · blooms spring · low shade tolerance · to −38 °Friverbank grape45 ft · blooms late spring · medium shade tolerance · to −33 °Fwestern white clematis1 ft · blooms mid summer · medium shade tolerance · to −38 °F

Source: USDA, NRCS. 2026. The PLANTS Database (plants.usda.gov), National Plant Data Team — state distribution, native status and the “Temperature, Minimum” characteristic. Check light, moisture and soil needs before buying; local nurseries and your state native plant society can confirm what thrives in Cowley, WY.

Soil in Cowley, WY

The soil mapped at the center of ZIP 82420 is Stutzman — silty clay loam, well drained.

TextureSilty clay loamholds water and nutrients; slow to drain and warm up
pH8.6alkaline; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter0.75%low — add compost
Hydrologic groupCwater soaks in slowly
Farmland classNot prime farmlandUSDA classification

Map unit: Stutzman silty clay loam, 0 to 10 percent slopes -- Draft.

Soil Survey Staff, Natural Resources Conservation Service, USDA. Soil Survey Geographic (SSURGO) Database, via Soil Data Access, accessed 2026-10-29. The soil at the center point of ZIP 82420; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Cowley, WY

Summer high87 °Faverage high, July
Winter low7 °Faverage low, January
Hot days25days a year at 90 °F or more
Freezing nights182nights a year at 32 °F or less
Growing degree days2,207base 50 °F — the season’s usable heat
Longest day15 h 36 mindaylight on June 21
Shortest day8 h 46 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation6.5 inper year, rain and melted snow
Snowfall13 inper year; 18 days with snow on the ground

Average high and low, by month

0°10°20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 31 °F, low 7 °F 31°7° JanFebruary: average high 36 °F, low 12 °F 36°12° FebMarch: average high 50 °F, low 23 °F 50°23° MarApril: average high 58 °F, low 31 °F 58°31° AprMay: average high 67 °F, low 42 °F 67°42° MayJune: average high 78 °F, low 50 °F 78°50° JunJuly: average high 87 °F, low 56 °F 87°56° JulAugust: average high 85 °F, low 52 °F 85°52° AugSeptember: average high 74 °F, low 43 °F 74°43° SepOctober: average high 59 °F, low 32 °F 59°32° OctNovember: average high 44 °F, low 20 °F 44°20° NovDecember: average high 33 °F, low 10 °F 33°10° Dec

Precipitation by month inches · ❄ = snowfall

0″1″2″January: 0.19 in precipitation, 3.5 in snowfall 0.2❄4 JanFebruary: 0.27 in precipitation, 2.8 in snowfall 0.3❄3 FebMarch: 0.23 in precipitation, 1.2 in snowfall 0.2❄1 MarApril: 0.67 in precipitation, 0.7 in snowfall 0.7❄1 AprMay: 1.12 in precipitation, 0.1 in snowfall 1.1 MayJune: 1.19 in precipitation 1.2 JunJuly: 0.56 in precipitation 0.6 JulAugust: 0.29 in precipitation 0.3 AugSeptember: 0.79 in precipitation, 0.1 in snowfall 0.8 SepOctober: 0.74 in precipitation, 0.8 in snowfall 0.7❄1 OctNovember: 0.20 in precipitation, 1.5 in snowfall 0.2❄2 NovDecember: 0.20 in precipitation, 2.6 in snowfall 0.2❄3 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Lovell, WY (6 mi away); precipitation: Lovell, WY (6 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 5a.

824215a
Deaver, WY · 7 mi
824235a
Frannie, WY · 8 mi
824125a
Byron, WY · 10 mi
824315a
Lovell, WY · 15 mi
590145a
Bridger, MT · 19 mi
824405a
Ralston, WY · 24 mi
590665a
Pryor, MT · 29 mi
824225a
Emblem, WY · 29 mi

Wyoming planting zones →

Questions about 82420

What planting zone is ZIP code 82420?

ZIP code 82420 (Cowley, WY) is in USDA hardiness zone 5a on the 2023 map: the coldest night of an average winter reaches −20 to −15 °F (−28.9 to −26.1 °C).

When is the last frost in Cowley, WY?

The median last spring frost (32 °F) near 82420 is May 9; after May 25 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Lovell station.

When is the first frost in Cowley, WY?

The median first fall frost near 82420 is September 29, giving a median frost-free growing season of 142 days.

When should I plant tomatoes in 82420?

Start tomato seeds indoors around Mar 14 – Mar 28 and set transplants outside around May 16 – May 30, one to three weeks after the median last frost.

How much rain and snow does 82420 get?

About 6.5 inches of precipitation a year and 13 inches of snowfall (NOAA 1991–2020 normals, Lovell station).

How hot do summers get in Cowley, WY?

The average high peaks at 87 °F in July, with about 25 days a year at 90 °F or more.

What kind of soil does 82420 have?

The USDA soil survey maps Stutzman at the center of ZIP 82420: silty clay loam, well drained, pH about 8.6 (alkaline).

Did spring come early in Cowley, WY in 2026?

Spring leaf-out near 82420 came on Mar 22 in 2026, 18 days earlier than the 1991–2020 average (Apr 9), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Cowley, WY?

Long-day onions: the longest day near 82420 is 15 h 36 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 82420 change?

Yes. The 2012 USDA map put 82420 in zone 4b; the 2023 map puts it in zone 5a. The newer map uses 1991–2020 data and more weather stations.