Hartsel, CO planting calendar
- USDA zone
- 4a−30 to −25 °F
- Last frost
- Jun 2avg, 32°F
- First frost
- Sep 19avg, 32°F
- Growing season
- 108days
Hartsel, Colorado is in USDA plant hardiness zone 4a. Its average last spring frost is around June 2 and the first fall frost around September 19, 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.
Hartsel planting calendar
Each crop's windows are counted from Hartsel'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 7 – Apr 21 | Jun 9 – Jun 16 | Aug 8 – Aug 28 | — |
| Pepper | Very tender | Mar 24 – Apr 7 | Jun 16 – Jun 23 | Aug 15 – Sep 14 | — |
| Cucumber | Tender | May 5 – May 12 | Jun 9 – Jun 16 | Jul 29 – Aug 18 | — |
| Summer squash / zucchini | Tender | — | Jun 9 – Jun 16 | Jul 24 – Aug 8 | — |
| Bush bean | Tender | — | Jun 9 – Jun 16 | Jul 29 – Aug 8 | Jul 21 – Jul 31 |
| Sweet corn | Tender | — | Jun 2 – Jun 16 | Aug 1 – Aug 31 | — |
| Basil | Very tender | Apr 21 – May 5 | Jun 9 – Jun 16 | Jul 9 – Jul 24 | — |
| Lettuce | Half-hardy | Apr 21 – May 5 | May 5 – May 19 | Jun 19 – Jul 4 | Jul 7 – Jul 22 |
| Pea | Hardy | — | Apr 21 – May 5 | Jun 15 – Jun 30 | Jun 27 – Jul 12 |
| Spinach | Hardy | — | Apr 21 – May 5 | May 31 – Jun 10 | Jul 17 – Jul 27 |
| Carrot | Half-hardy | — | May 12 – May 19 | Jul 11 – Jul 31 | Jun 17 – Jul 7 |
| Broccoli | Half-hardy | Apr 7 – Apr 21 | May 5 – May 19 | Jun 29 – Jul 19 | Jun 22 – Jul 12 |
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 USC00054742. 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 16 | Jul 3 | Sep 10 | Sep 21 | 83 |
| 32°F (freeze) | Jun 2 | Jun 17 | Sep 19 | Sep 30 | 108 |
| 28°F | May 19 | Jun 2 | Sep 29 | Oct 12 | 132 |
| 24°F | May 7 | May 21 | Oct 8 | Oct 20 | 151 |
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) | 1,044 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 2,580 | cool-season crops (brassicas, greens) |
Hardiness zone 4a
Hartsel 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 Hartsel?
- Hartsel, Colorado 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 Hartsel?
- The average (median) last spring frost at 32°F is around June 2, 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 17, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in Hartsel?
- The average first fall frost at 32°F is around September 19. That leaves a growing season of about 108 days between the average last spring and first fall frosts.
- When should I start tomatoes in Hartsel?
- Start tomato seeds indoors about Apr 7 – Apr 21 and transplant them outside about Jun 9 – Jun 16, once the danger of frost has passed. Estimated first harvest is around Aug 8 – Aug 28.
- How long is the growing season in Hartsel?
- About 108 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~June 2) and the average first fall frost (~September 19). 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 USC00054742 (Lake George 8 Sw, 27.8 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.