USDA zone 6a in Colorado
USDA plant hardiness zone 6a covers 72 locations across Colorado (average annual extreme minimum −10 to −5 °F): Arvada, CO, Arvada, CO, Aurora, CO, Aurora, CO, Aurora, CO, Avon, CO, and 66 more. Average last spring frost across this group ranges April 23 (Orchard Mesa) to June 21 (Avon), and growing seasons run 77–179 days (Avon to Orchard Mesa) — 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 Commerce, this group's median-season location.
- USDA zone
- 6a−10 to −5 °F
- Last frost range
- Apr 23–Jun 21avg, 32°F
- First frost range
- Sep 7–Oct 20avg, 32°F
- Growing season range
- 77–179days
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) |
|---|---|---|---|---|---|
| Arvada | — | Ralston Rsvr | May 3 | Oct 14 | 161 |
| Arvada | Wheat Ridge, Berkley, Mountain View | Wheat Ridge 2 | May 5 | Oct 5 | 151 |
| Aurora | Denver, Centennial, Highlands Ranch, Lone Tree, Greenwood | Denver Centennial Ap | May 5 | Oct 8 | 154 |
| Aurora | Denver | Denver-stapleton | May 4 | Oct 8 | 156 |
| Aurora | Commerce, Bennett | Denver Intl Ap | May 5 | Oct 9 | 156 |
| Avon | Wolcott | Vail | Jun 21 | Sep 7 | 77 |
| Bayfield | — | Vallecito Dam | May 31 | Sep 26 | 119 |
| Bethune | — | Burlington Carson Ap | May 2 | Oct 10 | 160 |
| Boulder | Lafayette, Louisville, Eldorado Springs, Paragon Estates, Tall Timber, Valmont | Boulder | May 6 | Oct 5 | 151 |
| Branson | — | Kim 10sse | Apr 24 | Oct 18 | 175 |
| Branson | — | Lake Maloya | May 21 | Sep 29 | 128 |
| Burlington | Kanorado | Burlington | May 6 | Oct 9 | 155 |
| Cheraw | — | Cheraw 1 N | Apr 30 | Oct 10 | 163 |
| Cheyenne Wells | Arapahoe | Cheyenne Wells 2ne | May 1 | Oct 14 | 165 |
| Collbran | — | Collbran 1wsw | May 17 | Sep 30 | 136 |
| Colona | — | Montrose #2 | Apr 28 | Oct 14 | 167 |
| Colorado | Beulah Valley, Rye | Rye 1sw | May 5 | Oct 10 | 156 |
| Colorado Springs | Rock Creek Park | Ruxton Park | Jun 3 | Sep 17 | 102 |
| Colorado Springs | Fort Carson, Fountain, Security-Widefield, Ellicott, Cimarron Hills, Rock Creek Park | Colorado Springs Muni Ap | May 3 | Oct 8 | 158 |
| Colorado Springs | — | Cumbre Vista - Colorado Spgs 1 | May 9 | Oct 6 | 150 |
| Columbine | — | Kassler | May 8 | Oct 3 | 147 |
| Commerce * | Todd Creek | Brighton 3 Se | May 3 | Oct 7 | 155 |
| Crawford | — | Montrose 11 Ene | May 20 | Oct 1 | 132 |
| Denver | Sheridan, Edgewater, Holly Hills | Denver Water Dept | Apr 30 | Oct 12 | 165 |
| Dinosaur | — | Dinosaur 2 E | May 2 | Oct 14 | 161 |
| Eads | — | Eads | Apr 30 | Oct 13 | 165 |
| Ellicott | — | Ellicott 7s | May 6 | Oct 3 | 149 |
| Fowler | — | La Junta 17 Wsw | May 8 | Oct 4 | 148 |
| Foxfield | — | Parker | May 9 | Oct 3 | 145 |
| Gardner | — | Sheep Mtn | May 27 | Sep 25 | 119 |
| Glenwood Springs | — | Glenwood Spgs #2 | May 13 | Oct 3 | 142 |
| Hasty | McClave | John Martin Dam | Apr 24 | Oct 15 | 172 |
| Highlands Ranch | Littleton, Englewood, Cherry Hills, Sheridan, Bow Mar | Marston Fltr Plt | May 2 | Oct 11 | 160 |
| Hoehne | — | Trinidad Perry Stokes Ap | May 4 | Oct 9 | 156 |
| Holly | Granada, Coolidge, Hartman | Holly | May 1 | Oct 12 | 163 |
| Idalia | — | Idalia | May 4 | Oct 9 | 155 |
| Kim | — | Kim 15 Nne | May 5 | Oct 9 | 156 |
| Kit Carson | — | Wild Horse 6n | May 4 | Oct 8 | 155 |
| La Junta | — | La Junta 20 S | Apr 27 | Oct 14 | 168 |
| La Veta | — | San Luis 1 S | May 31 | Sep 23 | 115 |
| Lakewood | Edgewater | Lakewood | May 3 | Oct 7 | 155 |
| Lamar | — | Lamar | Apr 30 | Oct 10 | 163 |
| Las Animas | — | Las Animas | Apr 29 | Oct 12 | 166 |
| Lyons | — | Longmont 2 Ese | May 7 | Oct 5 | 151 |
| Mancos | — | Mancos 1sw | Jun 4 | Sep 24 | 111 |
| Manzanola | — | Ordway 1 Ene | May 2 | Oct 8 | 159 |
| Marvel | — | Ft Lewis | Jun 1 | Sep 27 | 116 |
| Maybell | — | Maybell | Jun 16 | Sep 9 | 83 |
| New Castle | Silt | Rifle Garfield Co Ap | May 11 | Oct 4 | 143 |
| Norwood | — | Norwood | May 25 | Oct 1 | 128 |
| Nucla | — | Uravan | Apr 26 | Oct 18 | 174 |
| Olney Springs | — | Tacony 13 Se | May 5 | Oct 6 | 153 |
| Orchard Mesa | — | Grand Junction 6ese | Apr 23 | Oct 20 | 179 |
| Parachute | — | Altenbern | May 25 | Sep 25 | 120 |
| Penrose | Park Center, Florence, Rockvale, Coal Creek | Canon City | May 4 | Oct 7 | 153 |
| Poncha Springs | — | Salida | May 29 | Sep 20 | 112 |
| Pueblo | Blende, Vineland, Boone | Pueblo Mem Ap | Apr 30 | Oct 9 | 161 |
| Pueblo | Pueblo West, Colorado | Pueblo Rsvr | Apr 26 | Oct 13 | 166 |
| Rangely | Bonanza | Rangely 1e | May 15 | Oct 1 | 136 |
| Rico | — | Lemon Dam | May 28 | Sep 30 | 122 |
| Rico | — | Placerville | May 26 | Sep 29 | 125 |
| Ridgway | — | Ridgway | Jun 11 | Sep 13 | 92 |
| Rocky Ford | Swink | Rocky Ford 2 Se | May 1 | Oct 8 | 159 |
| Sheridan Lake | — | Sheridan Lake | Apr 27 | Oct 15 | 170 |
| Snowmass | Woody Creek, Redstone | Aspen Pitkin Co Ap | May 31 | Sep 23 | 114 |
| Somerset | — | Paonia 1sw | May 3 | Oct 12 | 159 |
| Southern Ute | — | Ignacio 6ese | May 23 | Sep 29 | 128 |
| Stonewall Gap | Valdez | Trinidad Lake | May 11 | Oct 4 | 145 |
| Stratton | Vona | Stratton | May 6 | Oct 9 | 155 |
| Thornton | Broomfield, Westminster, Welby, Berkley, Federal Heights, Twin Lakes | Northglenn | May 2 | Oct 11 | 160 |
| Trinidad | — | Trinidad | Apr 30 | Oct 15 | 167 |
| Walsenburg | Lynn | Walsenburg 1 Nw | May 6 | Oct 4 | 149 |
* Commerce 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 (Commerce, representative)
Computed from Commerce's average frost dates — the median-season location in this Coloradozone 6a 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 | Mar 8 – Mar 22 | May 10 – May 17 | Jul 9 – Jul 29 | — | matures comfortably |
| Pepper | Very tender | Feb 22 – Mar 8 | May 17 – May 24 | Jul 16 – Aug 15 | — | matures comfortably |
| Cucumber | Tender | Apr 5 – Apr 12 | May 10 – May 17 | Jun 29 – Jul 19 | — | matures comfortably |
| Summer squash / zucchini | Tender | — | May 10 – May 17 | Jun 24 – Jul 9 | — | matures comfortably |
| Bush bean | Tender | — | May 10 – May 17 | Jun 29 – Jul 9 | Aug 8 – Aug 18 | matures comfortably |
| Sweet corn | Tender | — | May 3 – May 17 | Jul 2 – Aug 1 | — | matures comfortably |
| Basil | Very tender | Mar 22 – Apr 5 | May 10 – May 17 | Jun 9 – Jun 24 | — | matures comfortably |
| Lettuce | Half-hardy | Mar 22 – Apr 5 | Apr 5 – Apr 19 | May 20 – Jun 4 | Jul 25 – Aug 9 | matures comfortably |
| Pea | Hardy | — | Mar 22 – Apr 5 | May 16 – May 31 | Jul 15 – Jul 30 | matures comfortably |
| Spinach | Hardy | — | Mar 22 – Apr 5 | May 1 – May 11 | Aug 4 – Aug 14 | matures comfortably |
| Carrot | Half-hardy | — | Apr 12 – Apr 19 | Jun 11 – Jul 1 | Jul 5 – Jul 25 | matures comfortably |
| Broccoli | Half-hardy | Mar 8 – Mar 22 | Apr 5 – Apr 19 | May 30 – Jun 19 | Jul 10 – Jul 30 | matures comfortably |
Which crops fit this growing season
Using Commerce's own 155-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 12 of 12 crops mature here with comfortable 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 (Commerce)
From NOAA's 1991–2020 Climate Normals at station USC00050950. 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 | May 13 | May 27 | Sep 30 | Oct 10 | 137 |
| 32°F (freeze) | May 3 | May 16 | Oct 7 | Oct 19 | 155 |
| 28°F | Apr 23 | May 7 | Oct 16 | Oct 29 | 174 |
| 24°F | Apr 9 | Apr 28 | Oct 26 | Nov 10 | 197 |
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 72 locations is 2,860; Commerce's own reading is shown below.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 2,923 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 5,133 | cool-season crops (brassicas, greens) |
Hardiness zone 6a in Colorado
Zone 6a means an average annual extreme minimum winter temperature of about −10 to −5 °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, 4b, 5a, 5b, 6b, 7a.
Explore zone 6a in other states at zone 6a, or see all USDA hardiness zones.
Frequently asked questions
- What does USDA hardiness zone 6a mean in Colorado?
- Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 72 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 6a is the growing season longest?
- Orchard Mesa runs the longest season on this page at about 179 days; Avon is shortest at about 77 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 6a?
- Using Commerce's frost dates (this page's median-season location): start seeds indoors about Mar 8 – Mar 22, then transplant outside about May 10 – May 17. 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 6a 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 72 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.