What does USDA hardiness zone 6b mean in Idaho?

Zone 6b means an average annual extreme minimum winter temperature of −5 to 0 °F — the standard USDA measure of which perennials survive winter. 37 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 6b is the growing season longest?

New Plymouth runs the longest season on this page at about 183 days; Idaho is shortest at about 80 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 6b?

Using Oakley's frost dates (this page's median-season location): start seeds indoors about Mar 20 – Apr 3, then transplant outside about May 22 – May 29. 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 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 37 locations listed (see the table above for each location's own dates).

USDA zone 6b in Idaho

−5 to 0 °F avg. annual extreme minimum · 37 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6b covers 37 locations across Idaho (average annual extreme minimum −5 to 0 °F): Blanchard, ID, Bonners Ferry, ID, Burley, ID, Castleford, ID, Chubbuck, ID, Cottonwood, ID, and 31 more. Average last spring frost across this group ranges April 19 (New Plymouth) to June 17 (Idaho), and growing seasons run 80–183 days (Idaho to New Plymouth) — 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 Oakley, this group's median-season location.

USDA zone: 6b−5 to 0 °F
Last frost range: Apr 19–Jun 17avg, 32°F
First frost range: Sep 4–Oct 20avg, 32°F
Growing season range: 80–183days

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)
Blanchard	—	Newport	May 12	Sep 21	130
Bonners Ferry	Moyie Springs	Bonners Ferry	Apr 30	Oct 4	154
Burley	Albion	Burley Muni Ap	May 7	Oct 4	150
Castleford	—	Castleford 2 N	May 17	Sep 29	132
Chubbuck	—	Pocatello 2 Ne	May 16	Oct 2	137
Cottonwood	—	Cottonwood 2 Wsw	May 6	Oct 13	156
Cottonwood	Nezperce	Nezperce	May 24	Sep 20	116
Craigmont	Winchester, Reubens	Winchester	May 23	Sep 25	121
Dover	Ponderay, Kootenai	Sandpoint Exp Stn	May 8	Sep 28	141
Downey	—	Downey	May 31	Sep 18	110
Elk River	—	Elk Rvr	May 16	Sep 23	128
Fernwood	St. Maries	Saint Maries	May 3	Oct 4	152
Garden Valley	Placerville	Garden Valley	Jun 1	Sep 10	102
Gooding	—	Shoshone 1 Wnw	Apr 24	Oct 16	173
Grangeville	—	Grangeville	May 9	Oct 3	146
Hazelton	Eden, Murtaugh	Hazelton	May 7	Oct 5	149
Idaho	—	Idaho City	Jun 17	Sep 5	80
Jerome	—	Jerome Co Ap	May 1	Oct 14	165
Kellogg	Pinehurst, Wardner, Smelterville	Kellogg	May 14	Sep 26	131
Kimberly	Hollister, Hansen	Twin Falls Sun Vly Rgnl Ap	May 9	Oct 4	147
Laclede	—	Priest Rvr Exp Stn	May 20	Sep 13	114
Lava Hot Springs	Arimo	Mccammon	May 23	Sep 24	121
Moscow	—	Pullman Moscow Rgnl Ap	Apr 29	Oct 2	159
Mountain Home	—	Mtn Home	May 2	Oct 8	157
New Plymouth	—	Payette	Apr 19	Oct 20	183
Oakley *	—	Oakley	May 15	Oct 2	137
Osburn	Wallace, Silverton	Wallace	May 5	Oct 2	149
Oxford	—	Preston	May 16	Sep 28	133
Plummer	—	Plummer 3 Wsw	May 11	Oct 5	143
Post Falls	Coeur d'Alene, Rathdrum, Harrison, Hayden Lake, Worley, Fernan Lake	Coeur D'alene	Apr 22	Oct 18	179
Riggins	—	Riggins	May 6	Oct 9	155
Spirit Lake	Laclede, Athol, Clark Fork	Bayview Model Basin	May 22	Sep 22	122
Stanley	—	Lowman	Jun 6	Sep 4	87
Viola	De Smet, Garfield, Troy, Deary, Princeton, Farmington, +1 more	Potlatch 3 Nne	May 15	Sep 21	126
Weippe	—	Pierce	Jun 3	Sep 7	93
Wendell	—	Jerome	May 2	Oct 14	166
White Bird	—	Enterprise 20 Nne	Jun 8	Sep 5	84

* Oakley 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 (Oakley, representative)

Computed from Oakley's average frost dates — the median-season location in this Idahozone 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 Oakley (Idaho, zone 6b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 20 – Apr 3	May 22 – May 29	Jul 21 – Aug 10	—	matures comfortably
Pepper	Very tender	Mar 6 – Mar 20	May 29 – Jun 5	Jul 28 – Aug 27	—	matures comfortably
Cucumber	Tender	Apr 17 – Apr 24	May 22 – May 29	Jul 11 – Jul 31	—	matures comfortably
Summer squash / zucchini	Tender	—	May 22 – May 29	Jul 6 – Jul 21	—	matures comfortably
Bush bean	Tender	—	May 22 – May 29	Jul 11 – Jul 21	Aug 3 – Aug 13	matures comfortably
Sweet corn	Tender	—	May 15 – May 29	Jul 14 – Aug 13	—	matures comfortably
Basil	Very tender	Apr 3 – Apr 17	May 22 – May 29	Jun 21 – Jul 6	—	matures comfortably
Lettuce	Half-hardy	Apr 3 – Apr 17	Apr 17 – May 1	Jun 1 – Jun 16	Jul 20 – Aug 4	matures comfortably
Pea	Hardy	—	Apr 3 – Apr 17	May 28 – Jun 12	Jul 10 – Jul 25	matures comfortably
Spinach	Hardy	—	Apr 3 – Apr 17	May 13 – May 23	Jul 30 – Aug 9	matures comfortably
Carrot	Half-hardy	—	Apr 24 – May 1	Jun 23 – Jul 13	Jun 30 – Jul 20	matures comfortably
Broccoli	Half-hardy	Mar 20 – Apr 3	Apr 17 – May 1	Jun 11 – Jul 1	Jul 5 – Jul 25	matures comfortably

Which crops fit this growing season

Using Oakley's own 137-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 (Oakley)

From NOAA's 1991–2020 Climate Normals at station USC00106542. 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), Oakley.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 2	Jun 19	Sep 22	Oct 6	110
32°F (freeze)	May 15	Jun 6	Oct 2	Oct 17	137
28°F	Apr 30	May 15	Oct 16	Nov 1	170
24°F	Apr 14	May 4	Oct 28	Nov 14	197

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 37 locations is 1,995; Oakley's own reading is shown below.

Annual growing degree days for Oakley (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	2,445	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	4,586	cool-season crops (brassicas, greens)

Hardiness zone 6b in Idaho

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

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 Idaho?: Zone 6b means an average annual extreme minimum winter temperature of −5 to 0 °F — the standard USDA measure of which perennials survive winter. 37 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 6b is the growing season longest?: New Plymouth runs the longest season on this page at about 183 days; Idaho is shortest at about 80 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 6b?: Using Oakley's frost dates (this page's median-season location): start seeds indoors about Mar 20 – Apr 3, then transplant outside about May 22 – May 29. 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 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 37 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.