Columbus, MT planting calendar
- USDA zone
- 5a−20 to −15 °F
- Last frost
- May 27avg, 32°F
- First frost
- Sep 14avg, 32°F
- Growing season
- 108days
Columbus, Montana is in USDA plant hardiness zone 5a. Its average last spring frost is around May 27 and the first fall frost around September 14, giving a growing season of about 108 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.
Columbus planting calendar
Each crop's windows are counted from Columbus'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 1 – Apr 15 | Jun 3 – Jun 10 | Aug 2 – Aug 22 | — |
| Pepper | Very tender | Mar 18 – Apr 1 | Jun 10 – Jun 17 | Aug 9 – Sep 8 | — |
| Cucumber | Tender | Apr 29 – May 6 | Jun 3 – Jun 10 | Jul 23 – Aug 12 | — |
| Summer squash / zucchini | Tender | — | Jun 3 – Jun 10 | Jul 18 – Aug 2 | — |
| Bush bean | Tender | — | Jun 3 – Jun 10 | Jul 23 – Aug 2 | Jul 16 – Jul 26 |
| Sweet corn | Tender | — | May 27 – Jun 10 | Jul 26 – Aug 25 | — |
| Basil | Very tender | Apr 15 – Apr 29 | Jun 3 – Jun 10 | Jul 3 – Jul 18 | — |
| Lettuce | Half-hardy | Apr 15 – Apr 29 | Apr 29 – May 13 | Jun 13 – Jun 28 | Jul 2 – Jul 17 |
| Pea | Hardy | — | Apr 15 – Apr 29 | Jun 9 – Jun 24 | Jun 22 – Jul 7 |
| Spinach | Hardy | — | Apr 15 – Apr 29 | May 25 – Jun 4 | Jul 12 – Jul 22 |
| Carrot | Half-hardy | — | May 6 – May 13 | Jul 5 – Jul 25 | Jun 12 – Jul 2 |
| Broccoli | Half-hardy | Apr 1 – Apr 15 | Apr 29 – May 13 | Jun 23 – Jul 13 | Jun 17 – Jul 7 |
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 USC00241938. 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 12 | Jul 3 | Sep 2 | Sep 15 | 79 |
| 32°F (freeze) | May 27 | Jun 12 | Sep 14 | Sep 28 | 108 |
| 28°F | May 13 | May 27 | Sep 26 | Oct 9 | 134 |
| 24°F | May 1 | May 14 | Oct 5 | Oct 20 | 156 |
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) | 1,788 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 3,632 | cool-season crops (brassicas, greens) |
Hardiness zone 5a
Columbus sits in USDA plant hardiness zone 5a on the 2023 map — meaning its average annual extreme minimum winter temperature is about −20 to −15 °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 5a, or see all USDA hardiness zones.
Frequently asked questions
- What USDA hardiness zone is Columbus?
- Columbus, Montana is in USDA plant hardiness zone 5a on the 2023 map (average annual extreme minimum temperature −20 to −15 °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 Columbus?
- The average (median) last spring frost at 32°F is around May 27, 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 12, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in Columbus?
- The average first fall frost at 32°F is around September 14. That leaves a growing season of about 108 days between the average last spring and first fall frosts.
- When should I start tomatoes in Columbus?
- Start tomato seeds indoors about Apr 1 – Apr 15 and transplant them outside about Jun 3 – Jun 10, once the danger of frost has passed. Estimated first harvest is around Aug 2 – Aug 22.
- How long is the growing season in Columbus?
- About 108 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~May 27) and the average first fall frost (~September 14). 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 USC00241938 (Columbus, 2.7 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.