Corvallis, MT planting calendar
- USDA zone
- 5b−15 to −10 °F
- Last frost
- May 23avg, 32°F
- First frost
- Sep 21avg, 32°F
- Growing season
- 118days
Corvallis, Montana is in USDA plant hardiness zone 5b. Its average last spring frost is around May 23 and the first fall frost around September 21, giving a growing season of about 118 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.
Corvallis planting calendar
Each crop's windows are counted from Corvallis'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 28 – Apr 11 | May 30 – Jun 6 | Jul 29 – Aug 18 | — |
| Pepper | Very tender | Mar 14 – Mar 28 | Jun 6 – Jun 13 | Aug 5 – Sep 4 | — |
| Cucumber | Tender | Apr 25 – May 2 | May 30 – Jun 6 | Jul 19 – Aug 8 | — |
| Summer squash / zucchini | Tender | — | May 30 – Jun 6 | Jul 14 – Jul 29 | — |
| Bush bean | Tender | — | May 30 – Jun 6 | Jul 19 – Jul 29 | Jul 23 – Aug 2 |
| Sweet corn | Tender | — | May 23 – Jun 6 | Jul 22 – Aug 21 | — |
| Basil | Very tender | Apr 11 – Apr 25 | May 30 – Jun 6 | Jun 29 – Jul 14 | — |
| Lettuce | Half-hardy | Apr 11 – Apr 25 | Apr 25 – May 9 | Jun 9 – Jun 24 | Jul 9 – Jul 24 |
| Pea | Hardy | — | Apr 11 – Apr 25 | Jun 5 – Jun 20 | Jun 29 – Jul 14 |
| Spinach | Hardy | — | Apr 11 – Apr 25 | May 21 – May 31 | Jul 19 – Jul 29 |
| Carrot | Half-hardy | — | May 2 – May 9 | Jul 1 – Jul 21 | Jun 19 – Jul 9 |
| Broccoli | Half-hardy | Mar 28 – Apr 11 | Apr 25 – May 9 | Jun 19 – Jul 9 | Jun 24 – Jul 14 |
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 USC00248783. 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 1 | Sep 8 | Sep 22 | 84 |
| 32°F (freeze) | May 23 | Jun 10 | Sep 21 | Oct 4 | 118 |
| 28°F | May 5 | May 19 | Oct 2 | Oct 16 | 148 |
| 24°F | Apr 23 | May 8 | Oct 14 | Oct 29 | 173 |
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) | 1,789 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 3,644 | cool-season crops (brassicas, greens) |
Hardiness zone 5b
Corvallis sits in USDA plant hardiness zone 5b on the 2023 map — meaning its average annual extreme minimum winter temperature is about −15 to −10 °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 5b, or see all USDA hardiness zones.
Frequently asked questions
- What USDA hardiness zone is Corvallis?
- Corvallis, Montana is in USDA plant hardiness zone 5b on the 2023 map (average annual extreme minimum temperature −15 to −10 °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 Corvallis?
- The average (median) last spring frost at 32°F is around May 23, 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 10, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in Corvallis?
- The average first fall frost at 32°F is around September 21. That leaves a growing season of about 118 days between the average last spring and first fall frosts.
- When should I start tomatoes in Corvallis?
- Start tomato seeds indoors about Mar 28 – Apr 11 and transplant them outside about May 30 – Jun 6, once the danger of frost has passed. Estimated first harvest is around Jul 29 – Aug 18.
- How long is the growing season in Corvallis?
- About 118 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~May 23) and the average first fall frost (~September 21). 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 USC00248783 (Western Ag Rsch Ctr, 9.7 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.