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

Planting zone for Casper, WY

Casper, WY is in USDA zone 5a across its 5 ZIP codes. The median last spring frost is May 26 and the first fall frost September 22.

Casper on the zone map

Outlined: its 3 ZIP code areas · 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
ZIP codes5
Last frostMay 26
First frostSep 22
Growing season117 days

5a 5 ZIPs

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Casper, 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 Casper, WY

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMay 26median — 50% chance of a later frost
Safe for tender plantsJune 10only 10% chance of frost after
First fall frostSeptember 22median — 10% chance by Sep 5
Growing season117 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 11 and returns around October 2.

From the NOAA station Casper Natrona Co. Airport, WY, elevation 5,338 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 Casper

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Apr 14 – Apr 28
SpinachSow outdoors · Apr 14 – May 5
Broccoli, cabbageStart indoors · Mar 17 – Mar 31
Broccoli, cabbageTransplant · May 5 – May 19
Onion setsPlant outdoors · Apr 28 – May 12
PotatoesPlant outdoors · May 5 – May 19
LettuceSow outdoors · Apr 28 – May 12
Carrots, beetsSow outdoors · May 5 – May 19
TomatoesStart indoors · Mar 31 – Apr 14
TomatoesTransplant · Jun 2 – Jun 16
Peppers, eggplantStart indoors · Mar 17 – Mar 31
Peppers, eggplantTransplant · Jun 9 – Jun 23
BeansSow outdoors · Jun 2 – Jun 16
Sweet cornSow outdoors · Jun 2 – Jun 16
Cucumbers, squashSow outdoors · Jun 2 – Jun 16
Fall lettuce, spinachSow outdoors · Jul 28 – Aug 11
GarlicPlant cloves · Sep 8 – Oct 13
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 26) and first fall frost (Sep 22) 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 82602 came on Mar 3 — 37 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 Casper, WY.

Soil in Casper, WY

The soil mapped at the center of ZIP 82602 is Hiland — loamy sand, well drained.

TextureLoamy sanddrains and warms fast; dries out and needs more feeding
pH7.0neutral; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter1.5%low — add compost
Hydrologic groupBwater soaks in moderately
Farmland classNot prime farmlandUSDA classification

Map unit: Hiland loamy sand, 2 to 15 percent slopes.

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 82602; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Casper, WY

Summer high88 °Faverage high, July
Winter low17 °Faverage low, January
Hot days36days a year at 90 °F or more
Freezing nights163nights a year at 32 °F or less
Growing degree days2,405base 50 °F — the season’s usable heat
Longest day15 h 21 mindaylight on June 21
Shortest day9 h 01 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation13.4 inper year, rain and melted snow

Average high and low, by month

10°20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 38 °F, low 17 °F 38°17° JanFebruary: average high 40 °F, low 18 °F 40°18° FebMarch: average high 49 °F, low 25 °F 49°25° MarApril: average high 57 °F, low 31 °F 57°31° AprMay: average high 67 °F, low 41 °F 67°41° MayJune: average high 79 °F, low 50 °F 79°50° JunJuly: average high 88 °F, low 56 °F 88°56° JulAugust: average high 87 °F, low 55 °F 87°55° AugSeptember: average high 76 °F, low 44 °F 76°44° SepOctober: average high 61 °F, low 33 °F 61°33° OctNovember: average high 48 °F, low 25 °F 48°25° NovDecember: average high 38 °F, low 17 °F 38°17° Dec

Precipitation by month inches

0″1″2″3″January: 0.44 in precipitation 0.4 JanFebruary: 0.63 in precipitation 0.6 FebMarch: 0.97 in precipitation 1.0 MarApril: 1.47 in precipitation 1.5 AprMay: 2.74 in precipitation 2.7 MayJune: 1.55 in precipitation 1.6 JunJuly: 1.21 in precipitation 1.2 JulAugust: 1.02 in precipitation 1.0 AugSeptember: 1.05 in precipitation 1.1 SepOctober: 1.25 in precipitation 1.3 OctNovember: 0.45 in precipitation 0.5 NovDecember: 0.65 in precipitation 0.7 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Casper Wwtp, WY (4 mi away); precipitation: Casper Wwtp, WY (4 mi away). Daylight is calculated from latitude.

ZIP codes in Casper

826015a
−20 to −15 °F
826025a
−20 to −15 °F
826045a
−20 to −15 °F
826055a
−20 to −15 °F
826095a
−20 to −15 °F

Wyoming planting zones →

Questions about Casper

What planting zone is Casper, WY?

Casper, WY is in USDA hardiness zone 5a on the 2023 map (5a: −20 to −15 °F).

When is the last frost in Casper, WY?

The median last spring frost is May 26 and the median first fall frost is September 22 (NOAA 1991–2020 normals, Casper Natrona Co. Airport station).

When can I plant a garden in Casper, WY?

Cool-season crops like peas and spinach go in around Apr 14 – May 5; tomatoes, peppers, beans and squash after about Jun 2, once frost risk has passed.

What kind of onions grow in Casper, WY?

Long-day onions: the longest day is 15 h 21 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

How much rain and snow does Casper, WY get?

About 13.4 inches of precipitation a year (NOAA 1991–2020 normals).