USDA zone 4b in Colorado
USDA plant hardiness zone 4b covers 5 locations across Colorado (average annual extreme minimum −25 to −20 °F): Alamosa, CO, Pitkin, CO, Walden, CO, Walden, CO, Walden, CO. Average last spring frost across this group ranges June 2 (Walden) to June 21 (Pitkin), and growing seasons run 74–106 days (Pitkin to Walden) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Colorado location's own frost dates; the printable planting calendar uses Walden, this group's median-season location.
- USDA zone
- 4b−25 to −20 °F
- Last frost range
- Jun 2–Jun 21avg, 32°F
- First frost range
- Sep 5–Sep 19avg, 32°F
- Growing season range
- 74–106days
Locations in this group
Every location below shares this zone and a nearby weather station, but keeps its own frost dates — find your town for its own numbers, or use the ZIP planner on the home page for an instant personalized answer.
| Location | Also covers | Nearest station | Last frost | First frost | Season (days) |
|---|---|---|---|---|---|
| Alamosa | — | Alamosa San Luis Ap | Jun 7 | Sep 15 | 98 |
| Pitkin | — | Cochetopa Creek | Jun 21 | Sep 5 | 74 |
| Walden * | — | Steamboat Springs | Jun 9 | Sep 12 | 93 |
| Walden | — | Grand Lake 6 Ssw | Jun 13 | Sep 7 | 84 |
| Walden | — | Centennial 1ne | Jun 2 | Sep 19 | 106 |
* Walden is this group's median-season location — the calendar below is computed from its data.
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.
Planting calendar (Walden, representative)
Computed from Walden's average frost dates — the median-season location in this Coloradozone 4b group. 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 — your own location's dates may run a week or two earlier or later (see the table above).
- Start indoors
- Plant out
- Fall sowing
- First harvest
| Crop | Frost tolerance | Start indoors | Plant out | First harvest | Fall planting | Season fit |
|---|---|---|---|---|---|---|
| Tomato | Tender | Apr 14 – Apr 28 | Jun 16 – Jun 23 | Aug 15 – Sep 4 | — | matures, but with little margin |
| Pepper | Very tender | Mar 31 – Apr 14 | Jun 23 – Jun 30 | Aug 22 – Sep 21 | — | needs season extension (cold frame, row cover, or transplants) to mature |
| Cucumber | Tender | May 12 – May 19 | Jun 16 – Jun 23 | Aug 5 – Aug 25 | — | matures comfortably |
| Summer squash / zucchini | Tender | — | Jun 16 – Jun 23 | Jul 31 – Aug 15 | — | matures comfortably |
| Bush bean | Tender | — | Jun 16 – Jun 23 | Aug 5 – Aug 15 | Jul 14 – Jul 24 | matures comfortably |
| Sweet corn | Tender | — | Jun 9 – Jun 23 | Aug 8 – Sep 7 | — | matures, but with little margin |
| Basil | Very tender | Apr 28 – May 12 | Jun 16 – Jun 23 | Jul 16 – Jul 31 | — | matures comfortably |
| Lettuce | Half-hardy | Apr 28 – May 12 | May 12 – May 26 | Jun 26 – Jul 11 | Jun 30 – Jul 15 | matures comfortably |
| Pea | Hardy | — | Apr 28 – May 12 | Jun 22 – Jul 7 | Jun 20 – Jul 5 | matures comfortably |
| Spinach | Hardy | — | Apr 28 – May 12 | Jun 7 – Jun 17 | Jul 10 – Jul 20 | matures comfortably |
| Carrot | Half-hardy | — | May 19 – May 26 | Jul 18 – Aug 7 | Jun 10 – Jun 30 | matures comfortably |
| Broccoli | Half-hardy | Apr 14 – Apr 28 | May 12 – May 26 | Jul 6 – Jul 26 | Jun 15 – Jul 5 | matures comfortably |
Which crops fit this growing season
Using Walden's own 93-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 9 of 12 crops mature here with comfortable margin, 2mature with little room to spare, and 1 needs season extension (starting indoors earlier, a cold frame, or row cover) to reliably reach harvest before the first fall frost. The tightest fits: tomato (+8 day margin), pepper (-9 day margin), sweet corn (+5 day margin). A different location in the table above with a longer or shorter season will get a different mix — the ZIP planner on the home page gives your own.
Frost & freeze dates (Walden)
From NOAA's 1991–2020 Climate Normals at station USC00057936. 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 — 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 | — | — | — | — | 64 |
| 32°F (freeze) | Jun 9 | Jun 24 | Sep 12 | Sep 24 | 93 |
| 28°F | May 20 | Jun 6 | Sep 23 | Oct 6 | 126 |
| 24°F | May 2 | May 21 | Oct 4 | Oct 19 | 155 |
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. This group's median GDD (base 50°F) across all 5 locations is 1,108; Walden's own reading is shown below.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 1,359 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 3,028 | cool-season crops (brassicas, greens) |
Hardiness zone 4b in Colorado
Zone 4b means an average annual extreme minimum winter temperature of about −25 to −20 °F on the USDA's 2023 map — the number that 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. Colorado spans 7 hardiness zones in this dataset — see all Colorado locations for the full range, including zones4a, 5a, 5b, 6a, 6b, 7a.
Explore zone 4b in other states at zone 4b, or see all USDA hardiness zones.
Frequently asked questions
- What does USDA hardiness zone 4b mean in Colorado?
- Zone 4b means an average annual extreme minimum winter temperature of −25 to −20 °F — the standard USDA measure of which perennials survive winter. 5 locations in Colorado fall in this zone. It does not set planting dates; see the frost calendar below for that.
- Where in Colorado's zone 4b is the growing season longest?
- Walden runs the longest season on this page at about 106 days; Pitkin is shortest at about 74 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.
- When should I plant tomatoes in Colorado's zone 4b?
- Using Walden's frost dates (this page's median-season location): start seeds indoors about Apr 14 – Apr 28, then transplant outside about Jun 16 – Jun 23. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.
- Does every location in Colorado's zone 4b have the same frost dates?
- No — they do not all share a weather station, so frost dates vary by a week or more across the 5 locations listed (see the table above for each location's own dates).
Sources & method
Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020, one reading per location's own nearest station (see the table above). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to each 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.