USDA zone 5b in Idaho
USDA plant hardiness zone 5b covers 16 locations across Idaho (average annual extreme minimum −15 to −10 °F): Ammon, ID, Challis, ID, Elk, ID, Fort Hall, ID, Gannett, ID, Grace, ID, and 10 more. Average last spring frost across this group ranges May 2 (Elk) to June 17 (Montpelier), and growing seasons run 77–159 days (Montpelier to Elk) — 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
- 5b−15 to −10 °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.
| Location | Also covers | Nearest station | Last frost | First frost | Season (days) |
|---|---|---|---|---|---|
| Ammon | Iona, Ririe | Idaho Falls - Kifi | May 9 | Oct 1 | 142 |
| Challis | Stanley, Clayton | Challis Ap | May 14 | Sep 25 | 131 |
| Elk | — | Yellowpine Bar | May 2 | Oct 10 | 159 |
| Fort Hall | — | Ft Hall 1 Nne | May 20 | Sep 20 | 119 |
| Gannett | — | Picabo | Jun 16 | Sep 9 | 85 |
| Grace | — | Grace | May 29 | Sep 17 | 111 |
| Idaho Falls | Shelley | Idaho Falls Fanning Fld | May 23 | Sep 20 | 114 |
| McCammon | — | Mccammon | May 23 | Sep 24 | 121 |
| Montpelier | Georgetown | Bern | Jun 17 | Sep 3 | 77 |
| Moreland | Basalt | Blackfoot Fire Dept | May 5 | Oct 3 | 149 |
| Oxford | — | Preston | May 16 | Sep 28 | 133 |
| Rockford | Aberdeen | Aberdeen Exp Stn | May 21 | Sep 20 | 118 |
| Salmon * | Leadore | Salmon-ksra | May 17 | Sep 24 | 127 |
| Spencer | — | Dubois Exp Stn | May 18 | Sep 30 | 132 |
| Sun Valley | Ketchum | Bellevue | May 13 | Oct 2 | 139 |
| Swan Valley | Ammon, Etna, Basalt | Idaho Falls 16 Se | Jun 9 | Sep 11 | 93 |
* 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 5b 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).
- Start indoors
- Plant out
- Fall sowing
- First harvest
| 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.
| 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 16 locations is 1,983; Salmon's own reading is shown below.
| 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 5b in Idaho
Zone 5b means an average annual extreme minimum winter temperature of about −15 to −10 °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, 6a, 6b, 7a, 7b.
Explore zone 5b in other states at zone 5b, or see all USDA hardiness zones.
Frequently asked questions
- What does USDA hardiness zone 5b mean in Idaho?
- Zone 5b means an average annual extreme minimum winter temperature of −15 to −10 °F — the standard USDA measure of which perennials survive winter. 16 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 5b is the growing season longest?
- Elk runs the longest season on this page at about 159 days; Montpelier 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 5b?
- 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 5b 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 16 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.