What does USDA hardiness zone 5a mean in Idaho?

Zone 5a means an average annual extreme minimum winter temperature of −20 to −15 °F — the standard USDA measure of which perennials survive winter. 21 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 5a is the growing season longest?

Yellow Pine runs the longest season on this page at about 159 days; Bennington 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 5a?

Using Moore's frost dates (this page's median-season location): start seeds indoors about Mar 29 – Apr 12, then transplant outside about May 31 – Jun 7. 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 5a 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 21 locations listed (see the table above for each location's own dates).

USDA zone 5a in Idaho

−20 to −15 °F avg. annual extreme minimum · 21 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 5a covers 21 locations across Idaho (average annual extreme minimum −20 to −15 °F): Ashton, ID, Bancroft, ID, Bennington, ID, Bloomington, ID, Donnelly, ID, Fairfield, ID, and 15 more. Average last spring frost across this group ranges May 2 (Yellow Pine) to June 17 (Bennington), and growing seasons run 77–159 days (Bennington to Yellow Pine) — 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 Moore, this group's median-season location.

USDA zone: 5a−20 to −15 °F
Last frost range: May 2–Jun 17avg, 32°F
First frost range: Sep 3–Oct 10avg, 32°F
Growing season range: 77–159days

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)
Ashton	Drummond	Ashton 1n	May 21	Sep 21	121
Bancroft	—	Mccammon	May 23	Sep 24	121
Bennington	Paris, Montpelier	Bern	Jun 17	Sep 3	77
Bloomington	St. Charles	Lifton Pumping Stn	May 19	Sep 21	123
Donnelly	—	Cascade 1 Nw	May 30	Sep 19	110
Fairfield	—	Hill City 1 W	Jun 16	Sep 6	80
Leadore	—	May 2sse	Jun 12	Sep 6	85
Lewisville	—	Idaho Falls Fanning Fld	May 23	Sep 20	114
Mackay	—	Chilly Barton Flat	Jun 14	Sep 8	86
Moore *	—	Howe	May 24	Sep 14	113
Moore	Butte	Arco	May 22	Sep 22	119
Parker	Newdale	Saint Anthony	May 17	Sep 22	126
Rexburg	—	Rexburg Byu Idaho	May 12	Sep 24	132
Roberts	Hamer	Hamer 4 Nw	Jun 3	Sep 11	98
Soda Springs	—	Grace	May 29	Sep 17	111
Sugar	Menan	Rexburg Madison Co Ap	May 15	Sep 23	129
Tetonia	—	Tetonia Exp Stn	Jun 15	Sep 7	85
Ucon	—	Idaho Falls - Kifi	May 9	Oct 1	142
Yellow Pine	—	Yellowpine Bar	May 2	Oct 10	159
Yellow Pine	—	Mccall Ap	Jun 10	Sep 10	93
Yellow Pine	—	Yellowpine	Jun 12	Sep 11	88

* Moore 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 (Moore, representative)

Computed from Moore's average frost dates — the median-season location in this Idahozone 5a 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 Moore (Idaho, zone 5a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 29 – Apr 12	May 31 – Jun 7	Jul 30 – Aug 19	—	matures comfortably
Pepper	Very tender	Mar 15 – Mar 29	Jun 7 – Jun 14	Aug 6 – Sep 5	—	matures, but with little margin
Cucumber	Tender	Apr 26 – May 3	May 31 – Jun 7	Jul 20 – Aug 9	—	matures comfortably
Summer squash / zucchini	Tender	—	May 31 – Jun 7	Jul 15 – Jul 30	—	matures comfortably
Bush bean	Tender	—	May 31 – Jun 7	Jul 20 – Jul 30	Jul 16 – Jul 26	matures comfortably
Sweet corn	Tender	—	May 24 – Jun 7	Jul 23 – Aug 22	—	matures comfortably
Basil	Very tender	Apr 12 – Apr 26	May 31 – Jun 7	Jun 30 – Jul 15	—	matures comfortably
Lettuce	Half-hardy	Apr 12 – Apr 26	Apr 26 – May 10	Jun 10 – Jun 25	Jul 2 – Jul 17	matures comfortably
Pea	Hardy	—	Apr 12 – Apr 26	Jun 6 – Jun 21	Jun 22 – Jul 7	matures comfortably
Spinach	Hardy	—	Apr 12 – Apr 26	May 22 – Jun 1	Jul 12 – Jul 22	matures comfortably
Carrot	Half-hardy	—	May 3 – May 10	Jul 2 – Jul 22	Jun 12 – Jul 2	matures comfortably
Broccoli	Half-hardy	Mar 29 – Apr 12	Apr 26 – May 10	Jun 20 – Jul 10	Jun 17 – Jul 7	matures comfortably

Which crops fit this growing season

Using Moore's own 113-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 11 of 12 crops mature here with comfortable margin, 1matures with little room to spare. The tightest fits: pepper (+9 day 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 (Moore)

From NOAA's 1991–2020 Climate Normals at station USC00104384. 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), Moore.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 11	Jul 2	Sep 3	Sep 16	82
32°F (freeze)	May 24	Jun 14	Sep 14	Sep 28	113
28°F	May 5	May 26	Sep 25	Oct 13	141
24°F	Apr 23	May 8	Oct 8	Oct 24	167

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 21 locations is 1,730; Moore's own reading is shown below.

Annual growing degree days for Moore (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,939	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,787	cool-season crops (brassicas, greens)

Hardiness zone 5a in Idaho

Zone 5a means an average annual extreme minimum winter temperature of about −20 to −15 °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, 5b, 6a, 6b, 7a, 7b.

Explore zone 5a in other states at zone 5a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 5a mean in Idaho?: Zone 5a means an average annual extreme minimum winter temperature of −20 to −15 °F — the standard USDA measure of which perennials survive winter. 21 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 5a is the growing season longest?: Yellow Pine runs the longest season on this page at about 159 days; Bennington 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 5a?: Using Moore's frost dates (this page's median-season location): start seeds indoors about Mar 29 – Apr 12, then transplant outside about May 31 – Jun 7. 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 5a 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 21 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.