Fort Benton, MT planting calendar
- USDA zone
- 4a−30 to −25 °F
- Last frost
- May 15avg, 32°F
- First frost
- Sep 22avg, 32°F
- Growing season
- 129days
Fort Benton, Montana is in USDA plant hardiness zone 4a. Its average last spring frost is around May 15 and the first fall frost around September 22, giving a growing season of about 129 days (NOAA 1991–2020 normals, 32°F, median). Start tender crops like tomatoes and peppers indoors weeks before the last frost and set them out afterward; sow hardy crops such as peas, spinach, and lettuce before it. The planner below turns those frost dates into a printable per-crop planting calendar.
Fort Benton planting calendar
Each crop's windows are counted from Fort Benton's average frost dates. 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.
- Start indoors
- Plant out
- Fall sowing
- First harvest
| Crop | Frost tolerance | Start indoors | Plant out | First harvest | Fall planting |
|---|---|---|---|---|---|
| Tomato | Tender | Mar 20 – Apr 3 | May 22 – May 29 | Jul 21 – Aug 10 | — |
| Pepper | Very tender | Mar 6 – Mar 20 | May 29 – Jun 5 | Jul 28 – Aug 27 | — |
| Cucumber | Tender | Apr 17 – Apr 24 | May 22 – May 29 | Jul 11 – Jul 31 | — |
| Summer squash / zucchini | Tender | — | May 22 – May 29 | Jul 6 – Jul 21 | — |
| Bush bean | Tender | — | May 22 – May 29 | Jul 11 – Jul 21 | Jul 24 – Aug 3 |
| Sweet corn | Tender | — | May 15 – May 29 | Jul 14 – Aug 13 | — |
| Basil | Very tender | Apr 3 – Apr 17 | May 22 – May 29 | Jun 21 – Jul 6 | — |
| Lettuce | Half-hardy | Apr 3 – Apr 17 | Apr 17 – May 1 | Jun 1 – Jun 16 | Jul 10 – Jul 25 |
| Pea | Hardy | — | Apr 3 – Apr 17 | May 28 – Jun 12 | Jun 30 – Jul 15 |
| Spinach | Hardy | — | Apr 3 – Apr 17 | May 13 – May 23 | Jul 20 – Jul 30 |
| Carrot | Half-hardy | — | Apr 24 – May 1 | Jun 23 – Jul 13 | Jun 20 – Jul 10 |
| Broccoli | Half-hardy | Mar 20 – Apr 3 | Apr 17 – May 1 | Jun 11 – Jul 1 | Jun 25 – Jul 15 |
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.
Frost & freeze dates
From NOAA's 1991–2020 Climate Normals at station USC00243113. 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 (p10 for spring, p90 for fall) — 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 | May 28 | Jun 13 | Sep 12 | Sep 25 | 106 |
| 32°F (freeze) | May 15 | May 30 | Sep 22 | Oct 4 | 129 |
| 28°F | May 4 | May 14 | Oct 1 | Oct 14 | 149 |
| 24°F | Apr 23 | May 6 | Oct 10 | Oct 24 | 170 |
32°F is the standard "freeze" line that damages tender crops; lighter 36°F frost can nip the most cold-sensitive plants, while hardy crops shrug off light frost down toward 28°F. Use the threshold that matches what you are protecting.
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. Warm-season crops need a long, warm GDD total; a short, cool GDD total favors greens and brassicas.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 2,164 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 4,105 | cool-season crops (brassicas, greens) |
Hardiness zone 4a
Fort Benton sits in USDA plant hardiness zone 4a on the 2023 map — meaning its average annual extreme minimum winter temperature is about −30 to −25 °F. That number 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.
Explore more places in zone 4a, or see all USDA hardiness zones.
Frequently asked questions
- What USDA hardiness zone is Fort Benton?
- Fort Benton, Montana is in USDA plant hardiness zone 4a on the 2023 map (average annual extreme minimum temperature −30 to −25 °F) — from the USDA Plant Hardiness Zone Map 2023, matched to this location's ZIP. See the methodology page for sources.
- When is the last frost in Fort Benton?
- The average (median) last spring frost at 32°F is around May 15, from NOAA's 1991–2020 climate normals at the nearest reporting station. Roughly one year in ten the last frost is as late as May 30, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in Fort Benton?
- The average first fall frost at 32°F is around September 22. That leaves a growing season of about 129 days between the average last spring and first fall frosts.
- When should I start tomatoes in Fort Benton?
- Start tomato seeds indoors about Mar 20 – Apr 3 and transplant them outside about May 22 – May 29, once the danger of frost has passed. Estimated first harvest is around Jul 21 – Aug 10.
- How long is the growing season in Fort Benton?
- About 129 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~May 15) and the average first fall frost (~September 22). Cold-hardy crops extend usable time at both ends; frost-tender crops fit inside it.
Sources & method
Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020 for station USC00243113 (Ft Benton, 11 km away). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to this 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.