What does USDA hardiness zone 3b mean in Montana?

Zone 3b means an average annual extreme minimum winter temperature of −35 to −30 °F — the standard USDA measure of which perennials survive winter. 13 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 3b is the growing season longest?

Hinsdale runs the longest season on this page at about 132 days; Opheim is shortest at about 89 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 3b?

Using Rudyard's frost dates (this page's median-season location): start seeds indoors about Mar 26 – Apr 9, then transplant outside about May 28 – Jun 4. 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 3b 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 13 locations listed (see the table above for each location's own dates).

USDA zone 3b in Montana

−35 to −30 °F avg. annual extreme minimum · 13 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 3b covers 13 locations across Montana (average annual extreme minimum −35 to −30 °F): Gildford, MT, Hilldale Colony, MT, Hinsdale, MT, Opheim, MT, Opheim, MT, Outlook, MT, and 7 more. Average last spring frost across this group ranges May 14 (Zurich) to June 5 (Opheim), and growing seasons run 89–132 days (Opheim to Hinsdale) — 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 Rudyard, this group's median-season location.

USDA zone: 3b−35 to −30 °F
Last frost range: May 14–Jun 5avg, 32°F
First frost range: Sep 2–Sep 26avg, 32°F
Growing season range: 89–132days

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)
Gildford	—	Gildford	May 24	Sep 15	114
Hilldale Colony	—	Havre City Co Ap	May 15	Sep 23	129
Hinsdale	—	Hinsdale 4 Sw	May 15	Sep 26	132
Opheim	—	Opheim 12 Sse	Jun 5	Sep 2	89
Opheim	—	Opheim 10 N	Jun 1	Sep 7	99
Outlook	—	Raymond Border Stn	May 19	Sep 23	125
Plentywood	Reserve, Antelope	Plentywood	May 25	Sep 14	111
Redstone	—	Redstone	May 28	Sep 7	101
Rudyard *	—	Joplin	May 21	Sep 19	119
Scobey	Peerless, Whitetail	Scobey 4 Nw	May 28	Sep 6	99
Sleeping Buffalo	—	Saco 1 Nnw	May 15	Sep 23	130
Whitewater	Loring Colony	Port Of Morgan	May 19	Sep 22	124
Zurich	Chinook	Chinook	May 14	Sep 23	130

* Rudyard 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 (Rudyard, representative)

Computed from Rudyard's average frost dates — the median-season location in this Montanazone 3b 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 Rudyard (Montana, zone 3b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 26 – Apr 9	May 28 – Jun 4	Jul 27 – Aug 16	—	matures comfortably
Pepper	Very tender	Mar 12 – Mar 26	Jun 4 – Jun 11	Aug 3 – Sep 2	—	matures comfortably
Cucumber	Tender	Apr 23 – Apr 30	May 28 – Jun 4	Jul 17 – Aug 6	—	matures comfortably
Summer squash / zucchini	Tender	—	May 28 – Jun 4	Jul 12 – Jul 27	—	matures comfortably
Bush bean	Tender	—	May 28 – Jun 4	Jul 17 – Jul 27	Jul 21 – Jul 31	matures comfortably
Sweet corn	Tender	—	May 21 – Jun 4	Jul 20 – Aug 19	—	matures comfortably
Basil	Very tender	Apr 9 – Apr 23	May 28 – Jun 4	Jun 27 – Jul 12	—	matures comfortably
Lettuce	Half-hardy	Apr 9 – Apr 23	Apr 23 – May 7	Jun 7 – Jun 22	Jul 7 – Jul 22	matures comfortably
Pea	Hardy	—	Apr 9 – Apr 23	Jun 3 – Jun 18	Jun 27 – Jul 12	matures comfortably
Spinach	Hardy	—	Apr 9 – Apr 23	May 19 – May 29	Jul 17 – Jul 27	matures comfortably
Carrot	Half-hardy	—	Apr 30 – May 7	Jun 29 – Jul 19	Jun 17 – Jul 7	matures comfortably
Broccoli	Half-hardy	Mar 26 – Apr 9	Apr 23 – May 7	Jun 17 – Jul 7	Jun 22 – Jul 12	matures comfortably

Which crops fit this growing season

Using Rudyard'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 (Rudyard)

From NOAA's 1991–2020 Climate Normals at station USC00244512. 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), Rudyard.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 6	Jun 22	Sep 8	Sep 21	93
32°F (freeze)	May 21	Jun 6	Sep 19	Oct 1	119
28°F	May 7	May 23	Sep 29	Oct 11	143
24°F	Apr 27	May 8	Oct 7	Oct 20	162

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 13 locations is 1,839; Rudyard's own reading is shown below.

Annual growing degree days for Rudyard (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,771	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,514	cool-season crops (brassicas, greens)

Hardiness zone 3b in Montana

Zone 3b means an average annual extreme minimum winter temperature of about −35 to −30 °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 zones4a, 4b, 5a, 5b, 6a, 6b.

Explore zone 3b in other states at zone 3b, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 3b mean in Montana?: Zone 3b means an average annual extreme minimum winter temperature of −35 to −30 °F — the standard USDA measure of which perennials survive winter. 13 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 3b is the growing season longest?: Hinsdale runs the longest season on this page at about 132 days; Opheim is shortest at about 89 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 3b?: Using Rudyard's frost dates (this page's median-season location): start seeds indoors about Mar 26 – Apr 9, then transplant outside about May 28 – Jun 4. 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 3b 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 13 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.