USDA zone 4b in Idaho
USDA plant hardiness zone 4b covers 3 locations across Idaho (average annual extreme minimum −25 to −20 °F): Mud Lake, ID, Mud Lake, ID, Rockford, ID. Average last spring frost across this group ranges June 3 (Mud Lake) to June 12 (Mud Lake), and growing seasons run 87–98 days (Mud Lake to Mud Lake) — 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 Rockford, this group's median-season location.
- USDA zone
- 4b−25 to −20 °F
- Last frost range
- Jun 3–Jun 12avg, 32°F
- First frost range
- Sep 6–Sep 11avg, 32°F
- Growing season range
- 87–98days
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) |
|---|---|---|---|---|---|
| Mud Lake | — | Monteview | Jun 12 | Sep 7 | 87 |
| Mud Lake | — | Hamer 4 Nw | Jun 3 | Sep 11 | 98 |
| Rockford * | — | Idaho Falls 46 W | Jun 11 | Sep 6 | 89 |
* Rockford 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 (Rockford, representative)
Computed from Rockford's average frost dates — the median-season location in this Idahozone 4b 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 | Apr 16 – Apr 30 | Jun 18 – Jun 25 | Aug 17 – Sep 6 | — | matures, but with little margin |
| Pepper | Very tender | Apr 2 – Apr 16 | Jun 25 – Jul 2 | Aug 24 – Sep 23 | — | needs season extension (cold frame, row cover, or transplants) to mature |
| Cucumber | Tender | May 14 – May 21 | Jun 18 – Jun 25 | Aug 7 – Aug 27 | — | matures, but with little margin |
| Summer squash / zucchini | Tender | — | Jun 18 – Jun 25 | Aug 2 – Aug 17 | — | matures comfortably |
| Bush bean | Tender | — | Jun 18 – Jun 25 | Aug 7 – Aug 17 | Jul 8 – Jul 18 | matures comfortably |
| Sweet corn | Tender | — | Jun 11 – Jun 25 | Aug 10 – Sep 9 | — | needs season extension (cold frame, row cover, or transplants) to mature |
| Basil | Very tender | Apr 30 – May 14 | Jun 18 – Jun 25 | Jul 18 – Aug 2 | — | matures comfortably |
| Lettuce | Half-hardy | Apr 30 – May 14 | May 14 – May 28 | Jun 28 – Jul 13 | Jun 24 – Jul 9 | matures comfortably |
| Pea | Hardy | — | Apr 30 – May 14 | Jun 24 – Jul 9 | Jun 14 – Jun 29 | matures comfortably |
| Spinach | Hardy | — | Apr 30 – May 14 | Jun 9 – Jun 19 | Jul 4 – Jul 14 | matures comfortably |
| Carrot | Half-hardy | — | May 21 – May 28 | Jul 20 – Aug 9 | Jun 4 – Jun 24 | matures comfortably |
| Broccoli | Half-hardy | Apr 16 – Apr 30 | May 14 – May 28 | Jul 8 – Jul 28 | Jun 9 – Jun 29 | matures comfortably |
Which crops fit this growing season
Using Rockford's own 89-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 (Rockford)
From NOAA's 1991–2020 Climate Normals at station USW00094143. 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 | — | — | — | — | 63 |
| 32°F (freeze) | Jun 11 | Jul 1 | Sep 6 | Sep 21 | 89 |
| 28°F | May 23 | Jun 16 | Sep 18 | Oct 3 | 118 |
| 24°F | May 6 | May 27 | Sep 30 | Oct 15 | 144 |
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 3 locations is 1,861; Rockford's own reading is shown below.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 1,861 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 3,591 | cool-season crops (brassicas, greens) |
Hardiness zone 4b in Idaho
Zone 4b means an average annual extreme minimum winter temperature of about −25 to −20 °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, 5a, 5b, 6a, 6b, 7a, 7b.
Explore zone 4b in other states at zone 4b, or see all USDA hardiness zones.
Frequently asked questions
- What does USDA hardiness zone 4b mean in Idaho?
- Zone 4b means an average annual extreme minimum winter temperature of −25 to −20 °F — the standard USDA measure of which perennials survive winter. 3 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 4b is the growing season longest?
- Mud Lake runs the longest season on this page at about 98 days; Mud Lake is shortest at about 87 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 4b?
- Using Rockford's frost dates (this page's median-season location): start seeds indoors about Apr 16 – Apr 30, then transplant outside about Jun 18 – Jun 25. 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 4b 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 3 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.