St. Marie, MT planting calendar
- USDA zone
- 4a−30 to −25 °F
- Last frost
- May 13avg, 32°F
- First frost
- Sep 27avg, 32°F
- Growing season
- 134days
St. Marie, Montana is in USDA plant hardiness zone 4a. Its average last spring frost is around May 13 and the first fall frost around September 27, giving a growing season of about 134 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.
St. Marie planting calendar
Each crop's windows are counted from St. Marie'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 18 – Apr 1 | May 20 – May 27 | Jul 19 – Aug 8 | — |
| Pepper | Very tender | Mar 4 – Mar 18 | May 27 – Jun 3 | Jul 26 – Aug 25 | — |
| Cucumber | Tender | Apr 15 – Apr 22 | May 20 – May 27 | Jul 9 – Jul 29 | — |
| Summer squash / zucchini | Tender | — | May 20 – May 27 | Jul 4 – Jul 19 | — |
| Bush bean | Tender | — | May 20 – May 27 | Jul 9 – Jul 19 | Jul 29 – Aug 8 |
| Sweet corn | Tender | — | May 13 – May 27 | Jul 12 – Aug 11 | — |
| Basil | Very tender | Apr 1 – Apr 15 | May 20 – May 27 | Jun 19 – Jul 4 | — |
| Lettuce | Half-hardy | Apr 1 – Apr 15 | Apr 15 – Apr 29 | May 30 – Jun 14 | Jul 15 – Jul 30 |
| Pea | Hardy | — | Apr 1 – Apr 15 | May 26 – Jun 10 | Jul 5 – Jul 20 |
| Spinach | Hardy | — | Apr 1 – Apr 15 | May 11 – May 21 | Jul 25 – Aug 4 |
| Carrot | Half-hardy | — | Apr 22 – Apr 29 | Jun 21 – Jul 11 | Jun 25 – Jul 15 |
| Broccoli | Half-hardy | Mar 18 – Apr 1 | Apr 15 – Apr 29 | Jun 9 – Jun 29 | Jun 30 – Jul 20 |
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 USW00094010. 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 24 | Jun 4 | Sep 19 | Sep 30 | 117 |
| 32°F (freeze) | May 13 | May 26 | Sep 27 | Oct 9 | 134 |
| 28°F | May 3 | May 16 | Oct 5 | Oct 21 | 154 |
| 24°F | Apr 22 | May 6 | Oct 15 | Oct 30 | 174 |
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) | 2,220 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 4,055 | cool-season crops (brassicas, greens) |
Hardiness zone 4a
St. Marie 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 St. Marie?
- St. Marie, 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 St. Marie?
- The average (median) last spring frost at 32°F is around May 13, 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 26, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in St. Marie?
- The average first fall frost at 32°F is around September 27. That leaves a growing season of about 134 days between the average last spring and first fall frosts.
- When should I start tomatoes in St. Marie?
- Start tomato seeds indoors about Mar 18 – Apr 1 and transplant them outside about May 20 – May 27, once the danger of frost has passed. Estimated first harvest is around Jul 19 – Aug 8.
- How long is the growing season in St. Marie?
- About 134 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~May 13) and the average first fall frost (~September 27). 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 USW00094010 (St Marie, 2.3 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.