What does USDA hardiness zone 5a mean in Wyoming?

Zone 5a means an average annual extreme minimum winter temperature of −20 to −15 °F — the standard USDA measure of which perennials survive winter. 61 locations in Wyoming fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in Wyoming's zone 5a is the growing season longest?

Owl Creek runs the longest season on this page at about 152 days; Kelly is shortest at about 63 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.

When should I plant tomatoes in Wyoming's zone 5a?

Using Gillette's frost dates (this page's median-season location): start seeds indoors about Mar 27 – Apr 10, then transplant outside about May 29 – Jun 5. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.

Does every location in Wyoming's zone 5a have the same frost dates?

No — they do not all share a weather station, so frost dates vary by a week or more across the 61 locations listed (see the table above for each location's own dates).

USDA zone 5a in Wyoming

−20 to −15 °F avg. annual extreme minimum · 61 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 5a covers 61 locations across Wyoming (average annual extreme minimum −20 to −15 °F): Alcova, WY, Alta, WY, Atlantic, WY, Beulah, WY, Big Horn, WY, Buffalo, WY, and 55 more. Average last spring frost across this group ranges May 5 (Owl Creek) to June 24 (Kelly), and growing seasons run 63–152 days (Kelly to Owl Creek) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Wyoming location's own frost dates; the printable planting calendar uses Gillette, this group's median-season location.

USDA zone: 5a−20 to −15 °F
Last frost range: May 5–Jun 24avg, 32°F
First frost range: Aug 28–Oct 7avg, 32°F
Growing season range: 63–152days

Locations in this group

Every location below shares this zone and a nearby weather station, but keeps its own frost dates — find your town for its own numbers, or use the ZIP planner on the home page for an instant personalized answer.

Own frost dates per location.
Location	Also covers	Nearest station	Last frost	First frost	Season (days)
Alcova	—	Pathfinder Dam	May 9	Oct 7	148
Alta	Driggs, Victor	Alta 1 Nnw	Jun 14	Sep 11	85
Atlantic	—	Lander 11 Sse	May 13	Oct 2	140
Beulah	—	Bear Ridge	May 17	Sep 27	132
Big Horn	Ranchester, Dayton	Sheridan Co Ap	May 21	Sep 23	123
Buffalo	—	Buffalo	May 24	Sep 21	118
Burlington	—	Emblem	May 12	Sep 28	136
Carpenter	—	Carpenter 3n	May 14	Sep 26	133
Casper	Antelope Hills, Powder River, Vista West	Casper Natrona Co Ap	May 26	Sep 22	117
Casper Mountain	Meadow Acres	Casper Wwtp	May 10	Sep 29	138
Centennial	—	Centennial 1ne	Jun 2	Sep 19	106
Chugwater	—	Chugwater	May 18	Sep 24	129
Clearmont	—	Leiter 9n	May 14	Sep 30	137
Cody	—	Wapiti 1wsw	May 27	Sep 25	121
Deaver	Byron, Frannie	Deaver	May 6	Oct 1	148
Douglas	Glenrock, Lost Springs	Douglas Converse Co Ap	May 23	Sep 22	122
Elk Mountain	—	Elk Mtn	May 28	Sep 13	108
Esterbrook	—	Double Four Rch	Jun 2	Sep 9	99
Ethete	—	Riverton	May 16	Sep 28	133
Fairview	Smoot, Alpine	Afton	Jun 20	Sep 1	71
Fort Washakie	Pavillion	Diversion Dam	May 21	Sep 20	119
Gillette *	—	Gillette Campbell Ap	May 22	Sep 25	125
Glendo	—	Glendo Rsvr Usbr	May 9	Oct 5	148
Guernsey	Lingle, Hartville, Fort Laramie	Old Ft Laramie	May 14	Sep 26	134
Hanna	—	Leo 6 Sw	May 20	Sep 28	128
Hillsdale	—	Archer	May 21	Sep 24	124
Hudson	—	Lander Hunt Fld Ap	May 18	Sep 27	131
Hulett	—	Hulett	May 17	Sep 23	128
Hyattville	Shell	Shell	May 16	Sep 24	130
James	—	Green Rvr	May 24	Sep 22	120
Kaycee	—	Kaycee	May 18	Sep 25	129
Kelly	—	Moose	Jun 20	Aug 31	70
Kelly	—	Moose 1 Nne	Jun 24	Aug 28	63
Kemmerer	—	La Barge	Jun 9	Sep 4	84
LaGrange	—	Phillips	May 21	Sep 21	121
Little America	—	Church Buttes Gas Plt	May 25	Sep 27	124
Lovell	Cowley	Lovell	May 9	Sep 29	142
Lusk	—	Redbird	May 16	Sep 24	130
Manville	—	Lusk 2 Sw	May 23	Sep 20	119
Midwest	Edgerton	Midwest	May 22	Sep 22	121
Owl Creek	—	Thermopolis	May 5	Oct 5	152
Parkman	—	Dayton	May 24	Sep 18	114
Pine Bluffs	Burns	Pine Bluffs 5w	May 16	Sep 27	132
Powder River	—	Powder Rvr (school)	May 30	Sep 22	115
Powder River	Shoshoni	Black Mtn	May 14	Oct 3	140
Powder River	—	Powder River #2	May 23	Sep 20	121
Ralston	Belfry	Clark 3ne	May 18	Sep 22	125
Ralston	—	Powell Fld Stn	May 19	Sep 20	123
Reliance	Point of Rocks, Superior	Rock Springs Ap	May 22	Sep 28	126
Riverton	—	Riverton	May 12	Sep 24	132
Robertson	Lyman	Mtn View	Jun 14	Sep 9	84
Saratoga	Encampment	Saratoga	Jun 2	Sep 12	101
Sheridan	—	Sheridan Fld Stn	May 18	Sep 24	127
Sinclair	Bairoil	Rawlins Muni Ap	Jun 1	Sep 15	105
Story	—	Story	May 24	Sep 23	120
Ten Sleep	—	Tensleep 16sse	May 22	Sep 18	117
Torrington	Henry, Huntley	Torrington Muni Ap	May 11	Sep 29	139
Van Tassell	Hartville	Torrington 29n	May 17	Sep 28	133
Veteran	—	Yoder 5 W	May 15	Sep 27	132
Westview Circle	—	Wheatland 4 N	May 19	Sep 22	125
Wright	—	Wright 12w	May 25	Sep 23	120

* Gillette is this group's median-season location — the calendar below is computed from its data.

Data: NOAA NCEI U.S. Climate Normals 1991–2020 (public domain) and USDA Plant Hardiness Zone Map 2023. Planting windows synthesized from U.S. Cooperative Extension guides.

Planting calendar (Gillette, representative)

Computed from Gillette's average frost dates — the median-season location in this Wyomingzone 5a group. hatched = start seeds indoors, solid green = plant out, teal = a fall sowing, and the terracotta dot marks the estimated first harvest. Ranges are extension-guide planning guidance, not guarantees — your own location's dates may run a week or two earlier or later (see the table above).

Crop

JanFebMarAprMayJunJulAugSepOctNovDec

Tomato

Pepper

Cucumber

Summer squash / zucchini

Start indoors
Plant out
Fall sowing
First harvest

Planting windows for Gillette (Wyoming, zone 5a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 27 – Apr 10	May 29 – Jun 5	Jul 28 – Aug 17	—	matures comfortably
Pepper	Very tender	Mar 13 – Mar 27	Jun 5 – Jun 12	Aug 4 – Sep 3	—	matures comfortably
Cucumber	Tender	Apr 24 – May 1	May 29 – Jun 5	Jul 18 – Aug 7	—	matures comfortably
Summer squash / zucchini	Tender	—	May 29 – Jun 5	Jul 13 – Jul 28	—	matures comfortably
Bush bean	Tender	—	May 29 – Jun 5	Jul 18 – Jul 28	Jul 27 – Aug 6	matures comfortably
Sweet corn	Tender	—	May 22 – Jun 5	Jul 21 – Aug 20	—	matures comfortably
Basil	Very tender	Apr 10 – Apr 24	May 29 – Jun 5	Jun 28 – Jul 13	—	matures comfortably
Lettuce	Half-hardy	Apr 10 – Apr 24	Apr 24 – May 8	Jun 8 – Jun 23	Jul 13 – Jul 28	matures comfortably
Pea	Hardy	—	Apr 10 – Apr 24	Jun 4 – Jun 19	Jul 3 – Jul 18	matures comfortably
Spinach	Hardy	—	Apr 10 – Apr 24	May 20 – May 30	Jul 23 – Aug 2	matures comfortably
Carrot	Half-hardy	—	May 1 – May 8	Jun 30 – Jul 20	Jun 23 – Jul 13	matures comfortably
Broccoli	Half-hardy	Mar 27 – Apr 10	Apr 24 – May 8	Jun 18 – Jul 8	Jun 28 – Jul 18	matures comfortably

Which crops fit this growing season

Using Gillette's own 125-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 12 of 12 crops mature here with comfortable margin. A different location in the table above with a longer or shorter season will get a different mix — the ZIP planner on the home page gives your own.

Frost & freeze dates (Gillette)

From NOAA's 1991–2020 Climate Normals at station USW00094023. The median (p50) is the average date; the 90%-safe column is the date the freeze has passed in about 9 years out of 10 — the conservative date to plant after or harvest before.

Freeze probabilities by temperature threshold (MM/DD, NOAA 1991–2020), Gillette.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 3	Jun 18	Sep 15	Sep 29	101
32°F (freeze)	May 22	Jun 4	Sep 25	Oct 8	125
28°F	May 9	May 22	Oct 4	Oct 18	148
24°F	Apr 28	May 11	Oct 13	Oct 26	169

Growing degree days

Growing degree days (GDD) accumulate warmth above a base temperature over the year — a better predictor of crop development than the calendar alone. This group's median GDD (base 50°F) across all 61 locations is 2,156; Gillette's own reading is shown below.

Annual growing degree days for Gillette (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	2,232	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	4,103	cool-season crops (brassicas, greens)

Hardiness zone 5a in Wyoming

Zone 5a means an average annual extreme minimum winter temperature of about −20 to −15 °F on the USDA's 2023 map — the number that tells you which perennials, shrubs, and trees reliably survive an average winter here. It does not set your planting dates, which come from the frost calendar above. Wyoming spans 4 hardiness zones in this dataset — see all Wyoming locations for the full range, including zones4a, 4b, 5b.

Explore zone 5a in other states at zone 5a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 5a mean in Wyoming?: Zone 5a means an average annual extreme minimum winter temperature of −20 to −15 °F — the standard USDA measure of which perennials survive winter. 61 locations in Wyoming fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in Wyoming's zone 5a is the growing season longest?: Owl Creek runs the longest season on this page at about 152 days; Kelly is shortest at about 63 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.
When should I plant tomatoes in Wyoming's zone 5a?: Using Gillette's frost dates (this page's median-season location): start seeds indoors about Mar 27 – Apr 10, then transplant outside about May 29 – Jun 5. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.
Does every location in Wyoming's zone 5a have the same frost dates?: No — they do not all share a weather station, so frost dates vary by a week or more across the 61 locations listed (see the table above for each location's own dates).

Sources & method

Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020, one reading per location's own nearest station (see the table above). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to each location's ZIP. Planting windows are computed by counting from the average last and first frost using per-crop offsets synthesized from U.S. Cooperative Extension guides — the full method and citations are on the methodology page.