Jefferson, MT planting calendar
- USDA zone
- 4b−25 to −20 °F
- Last frost
- Jun 7avg, 32°F
- First frost
- Sep 8avg, 32°F
- Growing season
- 90days
Jefferson, Montana is in USDA plant hardiness zone 4b. Its average last spring frost is around June 7 and the first fall frost around September 8, giving a growing season of about 90 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.
Jefferson planting calendar
Each crop's windows are counted from Jefferson'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 | Apr 12 – Apr 26 | Jun 14 – Jun 21 | Aug 13 – Sep 2 | — |
| Pepper | Very tender | Mar 29 – Apr 12 | Jun 21 – Jun 28 | Aug 20 – Sep 19 | — |
| Cucumber | Tender | May 10 – May 17 | Jun 14 – Jun 21 | Aug 3 – Aug 23 | — |
| Summer squash / zucchini | Tender | — | Jun 14 – Jun 21 | Jul 29 – Aug 13 | — |
| Bush bean | Tender | — | Jun 14 – Jun 21 | Aug 3 – Aug 13 | Jul 10 – Jul 20 |
| Sweet corn | Tender | — | Jun 7 – Jun 21 | Aug 6 – Sep 5 | — |
| Basil | Very tender | Apr 26 – May 10 | Jun 14 – Jun 21 | Jul 14 – Jul 29 | — |
| Lettuce | Half-hardy | Apr 26 – May 10 | May 10 – May 24 | Jun 24 – Jul 9 | Jun 26 – Jul 11 |
| Pea | Hardy | — | Apr 26 – May 10 | Jun 20 – Jul 5 | Jun 16 – Jul 1 |
| Spinach | Hardy | — | Apr 26 – May 10 | Jun 5 – Jun 15 | Jul 6 – Jul 16 |
| Carrot | Half-hardy | — | May 17 – May 24 | Jul 16 – Aug 5 | Jun 6 – Jun 26 |
| Broccoli | Half-hardy | Apr 12 – Apr 26 | May 10 – May 24 | Jul 4 – Jul 24 | Jun 11 – Jul 1 |
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 USC00241008. 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 | — | — | — | — | 63 |
| 32°F (freeze) | Jun 7 | Jun 25 | Sep 8 | Sep 21 | 90 |
| 28°F | May 21 | Jun 5 | Sep 21 | Oct 3 | 121 |
| 24°F | May 8 | May 22 | Sep 30 | Oct 12 | 144 |
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) | 1,477 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 3,143 | cool-season crops (brassicas, greens) |
Hardiness zone 4b
Jefferson 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 Jefferson?
- Jefferson, 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 Jefferson?
- The average (median) last spring frost at 32°F is around June 7, 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 25, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in Jefferson?
- The average first fall frost at 32°F is around September 8. That leaves a growing season of about 90 days between the average last spring and first fall frosts.
- When should I start tomatoes in Jefferson?
- Start tomato seeds indoors about Apr 12 – Apr 26 and transplant them outside about Jun 14 – Jun 21, once the danger of frost has passed. Estimated first harvest is around Aug 13 – Sep 2.
- How long is the growing season in Jefferson?
- About 90 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~June 7) and the average first fall frost (~September 8). 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 USC00241008 (Boulder, 15.6 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.