What does USDA hardiness zone 6a mean in Montana?

Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °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 6a is the growing season longest?

Rollins runs the longest season on this page at about 165 days; St. Ignatius is shortest at about 120 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 6a?

Using Lakeside's frost dates (this page's median-season location): start seeds indoors about Mar 17 – Mar 31, then transplant outside about May 19 – May 26. 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 6a 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 6a in Montana

−10 to −5 °F avg. annual extreme minimum · 13 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6a covers 13 locations across Montana (average annual extreme minimum −10 to −5 °F): Dixon, MT, Frenchtown, MT, Lakeside, MT, Lonepine, MT, Noxon, MT, Pioneer Junction, MT, and 7 more. Average last spring frost across this group ranges May 1 (Rollins) to May 20 (Trout Creek), and growing seasons run 120–165 days (St. Ignatius to Rollins) — 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 Lakeside, this group's median-season location.

USDA zone: 6a−10 to −5 °F
Last frost range: May 1–May 20avg, 32°F
First frost range: Sep 21–Oct 15avg, 32°F
Growing season range: 120–165days

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)
Dixon	—	Alberton	May 18	Sep 25	126
Frenchtown	—	Missoula Intl Ap	May 16	Sep 26	130
Lakeside *	Somers	Creston	May 12	Sep 25	134
Lonepine	Pablo, Lindisfarne, Charlo, Ronan, Lake Mary Ronan, Jette, +2 more	Skq Dam	May 15	Sep 28	134
Noxon	—	Cabinet Gorge	May 9	Oct 8	153
Pioneer Junction	—	Libby Dam (base)	May 11	Oct 1	142
Ravalli	—	Saint Ignatius	May 12	Oct 2	142
Riverbend	St. Regis, Paradise	Superior	May 17	Sep 23	125
Rollins	—	Bigfork 13 S	May 1	Oct 15	165
St. Ignatius	—	Lindbergh Lake	May 20	Sep 22	120
Sylvanite	—	Troy	May 14	Sep 25	132
Thompson Falls	Plains	Thompson Falls Ph	May 7	Oct 4	150
Trout Creek	—	Trout Creek Rs	May 20	Sep 21	122

* Lakeside 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 (Lakeside, representative)

Computed from Lakeside's average frost dates — the median-season location in this Montanazone 6a 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 Lakeside (Montana, zone 6a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 17 – Mar 31	May 19 – May 26	Jul 18 – Aug 7	—	matures comfortably
Pepper	Very tender	Mar 3 – Mar 17	May 26 – Jun 2	Jul 25 – Aug 24	—	matures comfortably
Cucumber	Tender	Apr 14 – Apr 21	May 19 – May 26	Jul 8 – Jul 28	—	matures comfortably
Summer squash / zucchini	Tender	—	May 19 – May 26	Jul 3 – Jul 18	—	matures comfortably
Bush bean	Tender	—	May 19 – May 26	Jul 8 – Jul 18	Jul 27 – Aug 6	matures comfortably
Sweet corn	Tender	—	May 12 – May 26	Jul 11 – Aug 10	—	matures comfortably
Basil	Very tender	Mar 31 – Apr 14	May 19 – May 26	Jun 18 – Jul 3	—	matures comfortably
Lettuce	Half-hardy	Mar 31 – Apr 14	Apr 14 – Apr 28	May 29 – Jun 13	Jul 13 – Jul 28	matures comfortably
Pea	Hardy	—	Mar 31 – Apr 14	May 25 – Jun 9	Jul 3 – Jul 18	matures comfortably
Spinach	Hardy	—	Mar 31 – Apr 14	May 10 – May 20	Jul 23 – Aug 2	matures comfortably
Carrot	Half-hardy	—	Apr 21 – Apr 28	Jun 20 – Jul 10	Jun 23 – Jul 13	matures comfortably
Broccoli	Half-hardy	Mar 17 – Mar 31	Apr 14 – Apr 28	Jun 8 – Jun 28	Jun 28 – Jul 18	matures comfortably

Which crops fit this growing season

Using Lakeside's own 134-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 (Lakeside)

From NOAA's 1991–2020 Climate Normals at station USC00242104. 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), Lakeside.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 30	Jun 16	Sep 8	Sep 25	101
32°F (freeze)	May 12	May 26	Sep 25	Oct 8	134
28°F	Apr 28	May 11	Oct 5	Oct 22	161
24°F	Apr 8	Apr 26	Oct 18	Nov 4	191

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 1,814; Lakeside's own reading is shown below.

Annual growing degree days for Lakeside (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,586	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,347	cool-season crops (brassicas, greens)

Hardiness zone 6a in Montana

Zone 6a means an average annual extreme minimum winter temperature of about −10 to −5 °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, 6b.

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

Frequently asked questions

What does USDA hardiness zone 6a mean in Montana?: Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °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 6a is the growing season longest?: Rollins runs the longest season on this page at about 165 days; St. Ignatius is shortest at about 120 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 6a?: Using Lakeside's frost dates (this page's median-season location): start seeds indoors about Mar 17 – Mar 31, then transplant outside about May 19 – May 26. 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 6a 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.