What does USDA hardiness zone 6b mean in Montana?

Zone 6b means an average annual extreme minimum winter temperature of −5 to 0 °F — the standard USDA measure of which perennials survive winter. 2 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 6b is the growing season longest?

Heron runs the longest season on this page at about 153 days; Saltese is shortest at about 104 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 6b?

Using Heron's frost dates (this page's median-season location): start seeds indoors about Mar 14 – Mar 28, then transplant outside about May 16 – May 23. 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 6b 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 2 locations listed (see the table above for each location's own dates).

USDA zone 6b in Montana

−5 to 0 °F avg. annual extreme minimum · 2 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6b covers 2 locations across Montana (average annual extreme minimum −5 to 0 °F): Heron, MT, Saltese, MT. Average last spring frost across this group ranges May 9 (Heron) to June 10 (Saltese), and growing seasons run 104–153 days (Saltese to Heron) — 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 Heron, this group's median-season location.

USDA zone: 6b−5 to 0 °F
Last frost range: May 9–Jun 10avg, 32°F
First frost range: Sep 23–Oct 8avg, 32°F
Growing season range: 104–153days

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)
Heron *	Clark Fork, East Hope	Cabinet Gorge	May 9	Oct 8	153
Saltese	—	Mullan Pass Vor/dme	Jun 10	Sep 23	104

* Heron 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 (Heron, representative)

Computed from Heron's average frost dates — the median-season location in this Montanazone 6b 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 Heron (Montana, zone 6b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 14 – Mar 28	May 16 – May 23	Jul 15 – Aug 4	—	matures comfortably
Pepper	Very tender	Feb 28 – Mar 14	May 23 – May 30	Jul 22 – Aug 21	—	matures comfortably
Cucumber	Tender	Apr 11 – Apr 18	May 16 – May 23	Jul 5 – Jul 25	—	matures comfortably
Summer squash / zucchini	Tender	—	May 16 – May 23	Jun 30 – Jul 15	—	matures comfortably
Bush bean	Tender	—	May 16 – May 23	Jul 5 – Jul 15	Aug 9 – Aug 19	matures comfortably
Sweet corn	Tender	—	May 9 – May 23	Jul 8 – Aug 7	—	matures comfortably
Basil	Very tender	Mar 28 – Apr 11	May 16 – May 23	Jun 15 – Jun 30	—	matures comfortably
Lettuce	Half-hardy	Mar 28 – Apr 11	Apr 11 – Apr 25	May 26 – Jun 10	Jul 26 – Aug 10	matures comfortably
Pea	Hardy	—	Mar 28 – Apr 11	May 22 – Jun 6	Jul 16 – Jul 31	matures comfortably
Spinach	Hardy	—	Mar 28 – Apr 11	May 7 – May 17	Aug 5 – Aug 15	matures comfortably
Carrot	Half-hardy	—	Apr 18 – Apr 25	Jun 17 – Jul 7	Jul 6 – Jul 26	matures comfortably
Broccoli	Half-hardy	Mar 14 – Mar 28	Apr 11 – Apr 25	Jun 5 – Jun 25	Jul 11 – Jul 31	matures comfortably

Which crops fit this growing season

Using Heron's own 153-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 (Heron)

From NOAA's 1991–2020 Climate Normals at station USC00101363. 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), Heron.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 28	Jun 14	Sep 23	Oct 6	116
32°F (freeze)	May 9	May 25	Oct 8	Oct 26	153
28°F	Apr 22	May 9	Oct 25	Nov 13	187
24°F	Mar 30	Apr 21	Nov 9	Dec 1	226

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 2 locations is 1,332; Heron's own reading is shown below.

Annual growing degree days for Heron (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,665	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,504	cool-season crops (brassicas, greens)

Hardiness zone 6b in Montana

Zone 6b means an average annual extreme minimum winter temperature of about −5 to 0 °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, 4b, 5a, 5b, 6a.

Explore zone 6b in other states at zone 6b, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 6b mean in Montana?: Zone 6b means an average annual extreme minimum winter temperature of −5 to 0 °F — the standard USDA measure of which perennials survive winter. 2 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 6b is the growing season longest?: Heron runs the longest season on this page at about 153 days; Saltese is shortest at about 104 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 6b?: Using Heron's frost dates (this page's median-season location): start seeds indoors about Mar 14 – Mar 28, then transplant outside about May 16 – May 23. 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 6b 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 2 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.