Shawmut, MT planting calendar
- USDA zone
- 4b−25 to −20 °F
- Last frost
- May 26avg, 32°F
- First frost
- Sep 18avg, 32°F
- Growing season
- 112days
Shawmut, Montana is in USDA plant hardiness zone 4b. Its average last spring frost is around May 26 and the first fall frost around September 18, giving a growing season of about 112 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.
Shawmut planting calendar
Each crop's windows are counted from Shawmut'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 31 – Apr 14 | Jun 2 – Jun 9 | Aug 1 – Aug 21 | — |
| Pepper | Very tender | Mar 17 – Mar 31 | Jun 9 – Jun 16 | Aug 8 – Sep 7 | — |
| Cucumber | Tender | Apr 28 – May 5 | Jun 2 – Jun 9 | Jul 22 – Aug 11 | — |
| Summer squash / zucchini | Tender | — | Jun 2 – Jun 9 | Jul 17 – Aug 1 | — |
| Bush bean | Tender | — | Jun 2 – Jun 9 | Jul 22 – Aug 1 | Jul 20 – Jul 30 |
| Sweet corn | Tender | — | May 26 – Jun 9 | Jul 25 – Aug 24 | — |
| Basil | Very tender | Apr 14 – Apr 28 | Jun 2 – Jun 9 | Jul 2 – Jul 17 | — |
| Lettuce | Half-hardy | Apr 14 – Apr 28 | Apr 28 – May 12 | Jun 12 – Jun 27 | Jul 6 – Jul 21 |
| Pea | Hardy | — | Apr 14 – Apr 28 | Jun 8 – Jun 23 | Jun 26 – Jul 11 |
| Spinach | Hardy | — | Apr 14 – Apr 28 | May 24 – Jun 3 | Jul 16 – Jul 26 |
| Carrot | Half-hardy | — | May 5 – May 12 | Jul 4 – Jul 24 | Jun 16 – Jul 6 |
| Broccoli | Half-hardy | Mar 31 – Apr 14 | Apr 28 – May 12 | Jun 22 – Jul 12 | Jun 21 – Jul 11 |
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 USC00247263. 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 | Jun 13 | Jul 3 | Sep 6 | Sep 21 | 83 |
| 32°F (freeze) | May 26 | Jun 13 | Sep 18 | Oct 1 | 112 |
| 28°F | May 11 | May 28 | Sep 29 | Oct 12 | 139 |
| 24°F | Apr 30 | May 11 | Oct 7 | Oct 23 | 159 |
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) | 1,711 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 3,473 | cool-season crops (brassicas, greens) |
Hardiness zone 4b
Shawmut sits in USDA plant hardiness zone 4b on the 2023 map — meaning its average annual extreme minimum winter temperature is about −25 to −20 °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 4b, or see all USDA hardiness zones.
Frequently asked questions
- What USDA hardiness zone is Shawmut?
- Shawmut, Montana is in USDA plant hardiness zone 4b on the 2023 map (average annual extreme minimum temperature −25 to −20 °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 Shawmut?
- The average (median) last spring frost at 32°F is around May 26, from NOAA's 1991–2020 climate normals at the nearest reporting station. Roughly one year in ten the last frost is as late as June 13, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in Shawmut?
- The average first fall frost at 32°F is around September 18. That leaves a growing season of about 112 days between the average last spring and first fall frosts.
- When should I start tomatoes in Shawmut?
- Start tomato seeds indoors about Mar 31 – Apr 14 and transplant them outside about Jun 2 – Jun 9, once the danger of frost has passed. Estimated first harvest is around Aug 1 – Aug 21.
- How long is the growing season in Shawmut?
- About 112 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~May 26) and the average first fall frost (~September 18). 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 USC00247263 (Ryegate 18 Nnw, 16.6 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.