What does USDA hardiness zone 4a mean in Idaho?

Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 1 location in Idaho fall in this zone. It does not set planting dates; see the frost calendar below for that.

When should I plant tomatoes in Idaho's zone 4a?

Using Island Park's frost dates (this page's median-season location): start seeds indoors about Apr 20 – May 4, then transplant outside about Jun 22 – Jun 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 4a 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 1 locations listed (see the table above for each location's own dates).

USDA zone 4a in Idaho

−30 to −25 °F avg. annual extreme minimum · 1 location in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 4a covers 1 location across Idaho (average annual extreme minimum −30 to −25 °F): Island Park, ID. Average last spring frost across this group ranges June 15 and growing seasons run 87 days — 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 Island Park, this group's median-season location.

USDA zone: 4a−30 to −25 °F
Last frost range: Jun 15avg, 32°F
First frost range: Sep 10avg, 32°F
Growing season range: 87days

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)
Island Park *	—	Hebgen Dam	Jun 15	Sep 10	87

* Island Park 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 (Island Park, representative)

Computed from Island Park's average frost dates — the median-season location in this Idahozone 4a 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 Island Park (Idaho, zone 4a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Apr 20 – May 4	Jun 22 – Jun 29	Aug 21 – Sep 10	—	matures, but with little margin
Pepper	Very tender	Apr 6 – Apr 20	Jun 29 – Jul 6	Aug 28 – Sep 27	—	needs season extension (cold frame, row cover, or transplants) to mature
Cucumber	Tender	May 18 – May 25	Jun 22 – Jun 29	Aug 11 – Aug 31	—	matures, but with little margin
Summer squash / zucchini	Tender	—	Jun 22 – Jun 29	Aug 6 – Aug 21	—	matures comfortably
Bush bean	Tender	—	Jun 22 – Jun 29	Aug 11 – Aug 21	Jul 12 – Jul 22	matures comfortably
Sweet corn	Tender	—	Jun 15 – Jun 29	Aug 14 – Sep 13	—	needs season extension (cold frame, row cover, or transplants) to mature
Basil	Very tender	May 4 – May 18	Jun 22 – Jun 29	Jul 22 – Aug 6	—	matures comfortably
Lettuce	Half-hardy	May 4 – May 18	May 18 – Jun 1	Jul 2 – Jul 17	Jun 28 – Jul 13	matures comfortably
Pea	Hardy	—	May 4 – May 18	Jun 28 – Jul 13	Jun 18 – Jul 3	matures comfortably
Spinach	Hardy	—	May 4 – May 18	Jun 13 – Jun 23	Jul 8 – Jul 18	matures comfortably
Carrot	Half-hardy	—	May 25 – Jun 1	Jul 24 – Aug 13	Jun 8 – Jun 28	matures comfortably
Broccoli	Half-hardy	Apr 20 – May 4	May 18 – Jun 1	Jul 12 – Aug 1	Jun 13 – Jul 3	matures comfortably

Which crops fit this growing season

Using Island Park's own 87-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 8 of 12 crops mature here with comfortable margin, 2mature with little room to spare, and 2 need season extension (starting indoors earlier, a cold frame, or row cover) to reliably reach harvest before the first fall frost. The tightest fits: tomato (+0 day margin), pepper (-17 day margin), cucumber (+10 day margin), sweet corn (-3 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 (Island Park)

From NOAA's 1991–2020 Climate Normals at station USC00244038. 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), Island Park.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	—	—	—	—	53
32°F (freeze)	Jun 15	Jul 7	Sep 10	Sep 27	87
28°F	May 24	Jun 16	Sep 26	Oct 10	122
24°F	May 7	May 27	Oct 7	Oct 24	151

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 1 locations is 1,017; Island Park's own reading is shown below.

Annual growing degree days for Island Park (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,017	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	2,425	cool-season crops (brassicas, greens)

Hardiness zone 4a in Idaho

Zone 4a means an average annual extreme minimum winter temperature of about −30 to −25 °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 zones4b, 5a, 5b, 6a, 6b, 7a, 7b.

Explore zone 4a in other states at zone 4a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 4a mean in Idaho?: Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 1 location in Idaho fall in this zone. It does not set planting dates; see the frost calendar below for that.
When should I plant tomatoes in Idaho's zone 4a?: Using Island Park's frost dates (this page's median-season location): start seeds indoors about Apr 20 – May 4, then transplant outside about Jun 22 – Jun 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 4a 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 1 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.