What does USDA hardiness zone 6a mean in Idaho?

Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 32 locations in Idaho fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in Idaho's zone 6a is the growing season longest?

Shoshone runs the longest season on this page at about 173 days; Grace is shortest at about 77 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 Idaho's zone 6a?

Using Salmon's frost dates (this page's median-season location): start seeds indoors about Mar 22 – Apr 5, then transplant outside about May 24 – May 31. 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 Idaho'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 32 locations listed (see the table above for each location's own dates).

USDA zone 6a in Idaho

−10 to −5 °F avg. annual extreme minimum · 32 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6a covers 32 locations across Idaho (average annual extreme minimum −10 to −5 °F): Acequia, ID, Bonners Ferry, ID, Carey, ID, Coolin, ID, Council, ID, Fairfield, ID, and 26 more. Average last spring frost across this group ranges April 24 (Shoshone) to June 17 (Grace), and growing seasons run 77–173 days (Grace to Shoshone) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Idaho location's own frost dates; the printable planting calendar uses Salmon, this group's median-season location.

USDA zone: 6a−10 to −5 °F
Last frost range: Apr 24–Jun 17avg, 32°F
First frost range: Sep 3–Oct 16avg, 32°F
Growing season range: 77–173days

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)
Acequia	Albion	Minidoka Dam	May 16	Oct 1	136
Bonners Ferry	Moyie Springs	Porthill 1 Sw	May 4	Sep 30	145
Carey	Gannett	Picabo	Jun 16	Sep 9	85
Coolin	Cusick	Priest Rvr Exp Stn	May 20	Sep 13	114
Council	—	Council	May 10	Oct 2	145
Fairfield	—	Fairfield Rs	Jun 8	Sep 14	97
Ferdinand	—	Cottonwood 2 Wsw	May 6	Oct 13	156
Fernwood	Bovill	Elk Rvr	May 16	Sep 23	128
Grace	—	Bern	Jun 17	Sep 3	77
Hailey	—	Bellevue	May 13	Oct 2	139
Irwin	—	Alta 1 Nnw	Jun 14	Sep 11	85
Kootenai	—	Sandpoint Exp Stn	May 8	Sep 28	141
Malad	Snowville	Malad City	May 22	Sep 20	116
Malta	—	Malta 4 Ese	Jun 1	Sep 19	111
Midvale	Council, Cambridge	Cambridge	May 15	Sep 27	134
Mullan	Haugan, Saltese	Mullan Pass Vor/dme	Jun 10	Sep 23	104
Mullan	—	Wallace	May 5	Oct 2	149
Oakley	—	Oakley	May 15	Oct 2	137
Paul	—	Paul 1ene	May 6	Oct 6	151
Pierce	—	Pierce	Jun 3	Sep 7	93
Pocatello	—	Pocatello Rgnl Ap	May 23	Sep 18	117
Pocatello	Inkom	Pocatello 2 Ne	May 16	Oct 2	137
Preston	Dayton, Weston, Franklin	Preston	May 16	Sep 28	133
Richfield	—	Richfield	May 24	Sep 26	123
Riggins	—	Riggins	May 6	Oct 9	155
Rockland	—	Massacre Rocks Sp	May 20	Sep 26	125
Rockland	—	Mccammon	May 23	Sep 24	121
Salmon *	—	Salmon-ksra	May 17	Sep 24	127
Shoshone	Dietrich	Shoshone 1 Wnw	Apr 24	Oct 16	173
Stanley	—	Lowman	Jun 6	Sep 4	87
Swan Valley	—	Idaho Falls 16 Se	Jun 9	Sep 11	93
Tyhee	—	Ft Hall 1 Nne	May 20	Sep 20	119

* Salmon 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 (Salmon, representative)

Computed from Salmon's average frost dates — the median-season location in this Idahozone 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 Salmon (Idaho, zone 6a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 22 – Apr 5	May 24 – May 31	Jul 23 – Aug 12	—	matures comfortably
Pepper	Very tender	Mar 8 – Mar 22	May 31 – Jun 7	Jul 30 – Aug 29	—	matures comfortably
Cucumber	Tender	Apr 19 – Apr 26	May 24 – May 31	Jul 13 – Aug 2	—	matures comfortably
Summer squash / zucchini	Tender	—	May 24 – May 31	Jul 8 – Jul 23	—	matures comfortably
Bush bean	Tender	—	May 24 – May 31	Jul 13 – Jul 23	Jul 26 – Aug 5	matures comfortably
Sweet corn	Tender	—	May 17 – May 31	Jul 16 – Aug 15	—	matures comfortably
Basil	Very tender	Apr 5 – Apr 19	May 24 – May 31	Jun 23 – Jul 8	—	matures comfortably
Lettuce	Half-hardy	Apr 5 – Apr 19	Apr 19 – May 3	Jun 3 – Jun 18	Jul 12 – Jul 27	matures comfortably
Pea	Hardy	—	Apr 5 – Apr 19	May 30 – Jun 14	Jul 2 – Jul 17	matures comfortably
Spinach	Hardy	—	Apr 5 – Apr 19	May 15 – May 25	Jul 22 – Aug 1	matures comfortably
Carrot	Half-hardy	—	Apr 26 – May 3	Jun 25 – Jul 15	Jun 22 – Jul 12	matures comfortably
Broccoli	Half-hardy	Mar 22 – Apr 5	Apr 19 – May 3	Jun 13 – Jul 3	Jun 27 – Jul 17	matures comfortably

Which crops fit this growing season

Using Salmon's own 127-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 (Salmon)

From NOAA's 1991–2020 Climate Normals at station USC00108080. 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), Salmon.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 7	Jun 23	Sep 13	Sep 24	95
32°F (freeze)	May 17	Jun 9	Sep 24	Oct 5	127
28°F	May 2	May 19	Oct 5	Oct 18	153
24°F	Apr 17	May 1	Oct 18	Nov 1	182

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 32 locations is 2,037; Salmon's own reading is shown below.

Annual growing degree days for Salmon (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,911	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,749	cool-season crops (brassicas, greens)

Hardiness zone 6a in Idaho

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. Idaho spans 8 hardiness zones in this dataset — see all Idaho locations for the full range, including zones4a, 4b, 5a, 5b, 6b, 7a, 7b.

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 Idaho?: Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 32 locations in Idaho fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in Idaho's zone 6a is the growing season longest?: Shoshone runs the longest season on this page at about 173 days; Grace is shortest at about 77 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 Idaho's zone 6a?: Using Salmon's frost dates (this page's median-season location): start seeds indoors about Mar 22 – Apr 5, then transplant outside about May 24 – May 31. 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 Idaho'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 32 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.