Knife River, MN planting calendar
- USDA zone
- 4b−25 to −20 °F
- Last frost
- May 12avg, 32°F
- First frost
- Oct 6avg, 32°F
- Growing season
- 145days
Knife River, Minnesota is in USDA plant hardiness zone 4b. Its average last spring frost is around May 12 and the first fall frost around October 6, giving a growing season of about 145 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.
Knife River planting calendar
Each crop's windows are counted from Knife River'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 17 – Mar 31 | May 19 – May 26 | Jul 18 – Aug 7 | — |
| Pepper | Very tender | Mar 3 – Mar 17 | May 26 – Jun 2 | Jul 25 – Aug 24 | — |
| Cucumber | Tender | Apr 14 – Apr 21 | May 19 – May 26 | Jul 8 – Jul 28 | — |
| Summer squash / zucchini | Tender | — | May 19 – May 26 | Jul 3 – Jul 18 | — |
| Bush bean | Tender | — | May 19 – May 26 | Jul 8 – Jul 18 | Aug 7 – Aug 17 |
| Sweet corn | Tender | — | May 12 – May 26 | Jul 11 – Aug 10 | — |
| Basil | Very tender | Mar 31 – Apr 14 | May 19 – May 26 | Jun 18 – Jul 3 | — |
| Lettuce | Half-hardy | Mar 31 – Apr 14 | Apr 14 – Apr 28 | May 29 – Jun 13 | Jul 24 – Aug 8 |
| Pea | Hardy | — | Mar 31 – Apr 14 | May 25 – Jun 9 | Jul 14 – Jul 29 |
| Spinach | Hardy | — | Mar 31 – Apr 14 | May 10 – May 20 | Aug 3 – Aug 13 |
| Carrot | Half-hardy | — | Apr 21 – Apr 28 | Jun 20 – Jul 10 | Jul 4 – Jul 24 |
| Broccoli | Half-hardy | Mar 17 – Mar 31 | Apr 14 – Apr 28 | Jun 8 – Jun 28 | Jul 9 – Jul 29 |
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 USC00218419. 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 28 | Jun 11 | Sep 27 | Oct 10 | 120 |
| 32°F (freeze) | May 12 | May 30 | Oct 6 | Oct 21 | 145 |
| 28°F | Apr 29 | May 11 | Oct 20 | Nov 4 | 174 |
| 24°F | Apr 16 | Apr 28 | Nov 2 | Nov 15 | 200 |
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) | 1,492 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 3,169 | cool-season crops (brassicas, greens) |
Hardiness zone 4b
Knife River 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 Knife River?
- Knife River, Minnesota 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 Knife River?
- The average (median) last spring frost at 32°F is around May 12, 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 30, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in Knife River?
- The average first fall frost at 32°F is around October 6. That leaves a growing season of about 145 days between the average last spring and first fall frosts.
- When should I start tomatoes in Knife River?
- Start tomato seeds indoors about Mar 17 – Mar 31 and transplant them outside about May 19 – May 26, once the danger of frost has passed. Estimated first harvest is around Jul 18 – Aug 7.
- How long is the growing season in Knife River?
- About 145 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~May 12) and the average first fall frost (~October 6). 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 USC00218419 (Two Harbors, 11.5 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.