What does USDA hardiness zone 4b mean in Montana?

Zone 4b means an average annual extreme minimum winter temperature of −25 to −20 °F — the standard USDA measure of which perennials survive winter. 73 locations in Montana fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in Montana's zone 4b is the growing season longest?

Melstone runs the longest season on this page at about 151 days; Big Sky is shortest at about 78 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 Montana's zone 4b?

Using Highwood's frost dates (this page's median-season location): start seeds indoors about Mar 29 – Apr 12, then transplant outside about May 31 – Jun 7. 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 Montana's zone 4b 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 73 locations listed (see the table above for each location's own dates).

USDA zone 4b in Montana

−25 to −20 °F avg. annual extreme minimum · 73 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 4b covers 73 locations across Montana (average annual extreme minimum −25 to −20 °F): Alzada, MT, Alzada, MT, Ashland, MT, Augusta, MT, Avon, MT, Ayers Ranch Colony, MT, and 67 more. Average last spring frost across this group ranges May 5 (Melstone) to June 16 (Big Sky), and growing seasons run 78–151 days (Big Sky to Melstone) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Montana location's own frost dates; the printable planting calendar uses Highwood, this group's median-season location.

USDA zone: 4b−25 to −20 °F
Last frost range: May 5–Jun 16avg, 32°F
First frost range: Sep 2–Oct 4avg, 32°F
Growing season range: 78–151days

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)
Alzada	—	Albion 1 N	Jun 2	Sep 6	95
Alzada	—	Ridgeway 1 S	May 17	Sep 22	127
Ashland	—	Sonnette 7 Sw	May 25	Sep 19	115
Augusta	—	Gibson Dam	Jun 12	Sep 8	88
Avon	Canyon Creek, Marysville	Austin 1 W	Jun 4	Sep 14	100
Ayers Ranch Colony	Grass Range	Grass Range	May 20	Sep 24	126
Babb	St. Mary	St. Mary 1 Ssw	Jun 3	Sep 8	94
Baker	—	Baker Muni Ap	May 16	Sep 26	132
Biddle	—	Biddle 8 Sw	May 14	Sep 28	134
Biddle	—	Weston 1 E	May 23	Sep 18	118
Big Sky	—	Ennis	Jun 16	Sep 4	78
Big Timber	Twodot	Melville 4 W	Jun 9	Sep 7	88
Birney	Crow Agency	Busby	May 23	Sep 15	113
Broadus	—	Broadus	May 16	Sep 28	135
Broadus	—	Powderville 8 Nne	May 13	Sep 25	132
Broadus	—	Brandenberg	May 18	Sep 22	126
Butte-Silver Bow	—	Butte Bert Mooney Ap	Jun 13	Sep 5	83
Bynum	Miller Colony	Choteau	May 28	Sep 12	104
Camp Three	—	Roundup	May 20	Sep 21	124
Crow Agency	Hardin, Pompeys Pillar	Hardin	May 13	Sep 29	135
Custer	—	Hysham 25 Sse	May 26	Sep 16	111
Custer	Nibbe	Pompeys Pillar 15n	May 22	Sep 22	121
Deer Lodge	Racetrack	Anaconda	Jun 1	Sep 14	105
Denton	Coffee Creek	Denton	Jun 1	Sep 11	101
Dupuyer	Kingsbury Colony	Valier	May 17	Sep 24	129
Ekalaka	—	Ekalaka	May 20	Sep 23	126
Fairfield	—	Fairfield	May 13	Sep 27	135
Floweree	—	Carter 14 W	May 23	Sep 23	120
Forsyth	Rosebud	Forsyth	May 9	Sep 29	142
Garrison	Elliston, Goldcreek	Elliston	Jun 9	Sep 8	91
Geraldine	—	Geraldine	May 14	Sep 26	134
Geyser	Riceville, Raynesford	Raynesford 2 Nnw	May 22	Sep 23	123
Glen	—	Twin Bridges	Jun 8	Sep 2	82
Great Falls	Black Eagle	Great Falls Intl Ap	May 19	Sep 24	126
Hardy	—	Cascade 5 S	May 21	Sep 19	120
Harlowton	—	Harlowton	May 23	Sep 16	113
Hays	Zortman	Zortman	May 18	Sep 23	126
Heart Butte	—	E Glacier	Jun 8	Sep 5	88
Hebgen Lake Estates	—	Hebgen Dam	Jun 15	Sep 10	87
Helena Valley Northeast	Clancy, Helena, Helena West Side, Rimini	Helena Rgnl Ap	May 9	Sep 30	142
Highwood *	—	Shonkin 7 S	May 24	Sep 21	119
Hysham	—	Hysham	May 10	Sep 29	141
Hysham	—	Ingomar 9 E	May 21	Sep 19	120
Jardine	—	Gardiner	May 21	Sep 27	128
Jefferson	—	Boulder	Jun 7	Sep 8	90
Jordan	—	Jordan Ap	May 18	Sep 20	123
Judith Gap	—	Judith Gap	Jun 1	Sep 15	104
Judith Gap	Moccasin, Utica	Moccasin Exp Stn	May 28	Sep 20	112
Lavina	Ryegate	Ryegate	May 17	Sep 23	127
Lincoln	Wolf Creek	Rogers Pass 9 Nne	May 21	Sep 15	115
Lindsay	—	Circle	May 17	Sep 22	126
Marion	—	Kalispell Glacier Ap	May 31	Sep 10	100
Martinsdale	—	Martinsdale 3 Nnw	Jun 3	Sep 8	94
Melstone	Jordan	Mosby 4 Ene	May 5	Oct 4	151
Miles	—	Miles City F Wiley Fld	May 8	Sep 30	144
Musselshell	Melstone	Melstone	May 10	Sep 29	141
Neihart	Monarch	Neihart 8 Nnw	Jun 6	Sep 12	99
Rapelje	Broadview, Acton	Rapelje	May 24	Sep 21	116
Rockport Colony	—	Blackleaf	Jun 9	Sep 3	86
Roy	Winnett	Valentine	May 18	Sep 21	124
Shawmut	—	Ryegate 18 Nnw	May 26	Sep 18	112
Shepherd	Huntley, Ballantine	Huntley Exp Stn	May 12	Sep 25	136
Silver Gate	—	Mystic Lake	Jun 5	Sep 14	99
Simms	Fort Shaw, Power	Sun Rvr 4 S	Jun 2	Sep 10	96
Stockett	—	Great Falls Wfo	May 12	Sep 28	138
The Silos	—	Townsend 3se	May 19	Sep 19	122
Warm Spring Creek	Moore	Lewistown Muni Ap	May 23	Sep 21	119
West Yellowstone	—	Yellowstone Park Mammoth	Jun 3	Sep 14	102
White Sulphur Springs	Springdale Colony	White Sulphur Springs #2	Jun 4	Sep 11	97
Wilsall	—	Wilsall 8 Ene	Jun 8	Sep 7	89
Windham	—	Stanford	May 25	Sep 21	118
Winifred	Hilger	Winifred	May 16	Sep 23	127
Wyola	—	Wyola 1 Sw	May 21	Sep 17	117

* Highwood 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 (Highwood, representative)

Computed from Highwood's average frost dates — the median-season location in this Montanazone 4b 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 Highwood (Montana, zone 4b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 29 – Apr 12	May 31 – Jun 7	Jul 30 – Aug 19	—	matures comfortably
Pepper	Very tender	Mar 15 – Mar 29	Jun 7 – Jun 14	Aug 6 – Sep 5	—	matures comfortably
Cucumber	Tender	Apr 26 – May 3	May 31 – Jun 7	Jul 20 – Aug 9	—	matures comfortably
Summer squash / zucchini	Tender	—	May 31 – Jun 7	Jul 15 – Jul 30	—	matures comfortably
Bush bean	Tender	—	May 31 – Jun 7	Jul 20 – Jul 30	Jul 23 – Aug 2	matures comfortably
Sweet corn	Tender	—	May 24 – Jun 7	Jul 23 – Aug 22	—	matures comfortably
Basil	Very tender	Apr 12 – Apr 26	May 31 – Jun 7	Jun 30 – Jul 15	—	matures comfortably
Lettuce	Half-hardy	Apr 12 – Apr 26	Apr 26 – May 10	Jun 10 – Jun 25	Jul 9 – Jul 24	matures comfortably
Pea	Hardy	—	Apr 12 – Apr 26	Jun 6 – Jun 21	Jun 29 – Jul 14	matures comfortably
Spinach	Hardy	—	Apr 12 – Apr 26	May 22 – Jun 1	Jul 19 – Jul 29	matures comfortably
Carrot	Half-hardy	—	May 3 – May 10	Jul 2 – Jul 22	Jun 19 – Jul 9	matures comfortably
Broccoli	Half-hardy	Mar 29 – Apr 12	Apr 26 – May 10	Jun 20 – Jul 10	Jun 24 – Jul 14	matures comfortably

Which crops fit this growing season

Using Highwood's own 119-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 (Highwood)

From NOAA's 1991–2020 Climate Normals at station USC00247540. 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), Highwood.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	—	—	—	—	90
32°F (freeze)	May 24	Jun 10	Sep 21	Oct 5	119
28°F	May 7	May 27	Oct 1	Oct 19	147
24°F	Apr 25	May 10	Oct 12	Nov 1	171

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 73 locations is 1,824; Highwood's own reading is shown below.

Annual growing degree days for Highwood (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,622	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,446	cool-season crops (brassicas, greens)

Hardiness zone 4b in Montana

Zone 4b means an average annual extreme minimum winter temperature of about −25 to −20 °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. Montana spans 7 hardiness zones in this dataset — see all Montana locations for the full range, including zones3b, 4a, 5a, 5b, 6a, 6b.

Explore zone 4b in other states at zone 4b, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 4b mean in Montana?: Zone 4b means an average annual extreme minimum winter temperature of −25 to −20 °F — the standard USDA measure of which perennials survive winter. 73 locations in Montana fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in Montana's zone 4b is the growing season longest?: Melstone runs the longest season on this page at about 151 days; Big Sky is shortest at about 78 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 Montana's zone 4b?: Using Highwood's frost dates (this page's median-season location): start seeds indoors about Mar 29 – Apr 12, then transplant outside about May 31 – Jun 7. 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 Montana's zone 4b 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 73 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.