What does USDA hardiness zone 5a mean in Colorado?

Zone 5a means an average annual extreme minimum winter temperature of −20 to −15 °F — the standard USDA measure of which perennials survive winter. 30 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 5a is the growing season longest?

Eaton runs the longest season on this page at about 159 days; Copper Mountain 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 5a?

Using Keystone's frost dates (this page's median-season location): start seeds indoors about Apr 11 – Apr 25, then transplant outside about Jun 13 – Jun 20. 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 5a 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 30 locations listed (see the table above for each location's own dates).

USDA zone 5a in Colorado

−20 to −15 °F avg. annual extreme minimum · 30 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 5a covers 30 locations across Colorado (average annual extreme minimum −20 to −15 °F): Aspen, CO, Blanca, CO, Blue River, CO, Briggsdale, CO, City of Creede, CO, Copper Mountain, CO, and 24 more. Average last spring frost across this group ranges May 2 (Eaton) to June 21 (Copper Mountain), and growing seasons run 77–159 days (Copper Mountain to Eaton) — 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 Keystone, this group's median-season location.

USDA zone: 5a−20 to −15 °F
Last frost range: May 2–Jun 21avg, 32°F
First frost range: Sep 5–Oct 8avg, 32°F
Growing season range: 77–159days

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.

Own frost dates per location.
Location	Also covers	Nearest station	Last frost	First frost	Season (days)
Aspen	Mount Crested Butte, Norrie	Aspen 1sw	Jun 2	Sep 23	113
Blanca	—	Blanca 4 Nw	May 25	Sep 22	117
Blue River	Alma	Fairplay S Park Rd	Jun 16	Sep 9	83
Briggsdale	Orchard	Briggsdale	May 12	Sep 28	135
City of Creede	Silverton	Rio Grande Rsvr	Jun 15	Sep 11	85
Copper Mountain	Vail, Red Cliff	Vail	Jun 21	Sep 7	77
Craig	—	Craig Moffat Co Ap	Jun 6	Sep 12	96
Crestone	—	Crestone 2 Se	May 26	Sep 26	120
Eaton	Kersey, La Salle, Gilcrest, Garden	Greeley Unc	May 2	Oct 8	159
Estes Park	—	Estes Park 3 Sse	May 30	Sep 17	109
Fairplay	—	Grant	Jun 15	Sep 9	84
Florissant	—	Lake George 8 Sw	Jun 2	Sep 19	108
Granby	Grand Lake	Grand Lake 6 Ssw	Jun 13	Sep 7	84
Hayden	—	Hayden	Jun 1	Sep 20	109
Hooper	—	Great Sand Dunes Np & Preserve	May 21	Oct 1	131
Keystone *	Silver Plume	Cabin Creek	Jun 6	Sep 19	103
Kremmling	—	Kremmling	Jun 14	Sep 5	80
Lake	—	Lake City	Jun 8	Sep 17	101
Monte Vista	Center	Monte Vista 2w	Jun 11	Sep 13	92
Mount Crested Butte	Pitkin	Taylor Park	Jun 20	Sep 8	80
Nunn	Ault	Nunn 7 Nne	May 13	Sep 27	136
Parshall	—	Williams Fork Dam	Jun 12	Sep 10	89
Phippsburg	McCoy, Yampa	Yampa	Jun 14	Sep 11	89
Raymer (New Raymer)	Grover	New Raymer 21 N	May 18	Sep 29	133
Red Feather Lakes	Walden	Rustic 9wsw	Jun 5	Sep 16	101
San Acacio	Capulin, Manassa, Conejos, Romeo	Manassa	May 28	Sep 21	115
Steamboat Springs	—	Steamboat Springs	Jun 9	Sep 12	93
Twin Lakes	Leadville	Twin Lakes Rsvr	Jun 16	Sep 10	85
Wiggins	Log Lane, Weldona	Ft Morgan	May 6	Oct 6	153
Winter Park	Silver Plume	Georgetown	May 30	Sep 19	112

* Keystone 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 (Keystone, representative)

Computed from Keystone's average frost dates — the median-season location in this Coloradozone 5a 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).

Crop

JanFebMarAprMayJunJulAugSepOctNovDec

Tomato

Pepper

Cucumber

Summer squash / zucchini

Start indoors
Plant out
Fall sowing
First harvest

Planting windows for Keystone (Colorado, zone 5a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Apr 11 – Apr 25	Jun 13 – Jun 20	Aug 12 – Sep 1	—	matures comfortably
Pepper	Very tender	Mar 28 – Apr 11	Jun 20 – Jun 27	Aug 19 – Sep 18	—	matures, but with little margin
Cucumber	Tender	May 9 – May 16	Jun 13 – Jun 20	Aug 2 – Aug 22	—	matures comfortably
Summer squash / zucchini	Tender	—	Jun 13 – Jun 20	Jul 28 – Aug 12	—	matures comfortably
Bush bean	Tender	—	Jun 13 – Jun 20	Aug 2 – Aug 12	Jul 21 – Jul 31	matures comfortably
Sweet corn	Tender	—	Jun 6 – Jun 20	Aug 5 – Sep 4	—	matures comfortably
Basil	Very tender	Apr 25 – May 9	Jun 13 – Jun 20	Jul 13 – Jul 28	—	matures comfortably
Lettuce	Half-hardy	Apr 25 – May 9	May 9 – May 23	Jun 23 – Jul 8	Jul 7 – Jul 22	matures comfortably
Pea	Hardy	—	Apr 25 – May 9	Jun 19 – Jul 4	Jun 27 – Jul 12	matures comfortably
Spinach	Hardy	—	Apr 25 – May 9	Jun 4 – Jun 14	Jul 17 – Jul 27	matures comfortably
Carrot	Half-hardy	—	May 16 – May 23	Jul 15 – Aug 4	Jun 17 – Jul 7	matures comfortably
Broccoli	Half-hardy	Apr 11 – Apr 25	May 9 – May 23	Jul 3 – Jul 23	Jun 22 – Jul 12	matures comfortably

Which crops fit this growing season

Using Keystone's own 103-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 11 of 12 crops mature here with comfortable margin, 1matures with little room to spare. The tightest fits: pepper (+1 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 (Keystone)

From NOAA's 1991–2020 Climate Normals at station USC00051186. 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.

Freeze probabilities by temperature threshold (MM/DD, NOAA 1991–2020), Keystone.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 19	Jul 4	Sep 5	Sep 19	77
32°F (freeze)	Jun 6	Jun 21	Sep 19	Sep 30	103
28°F	May 25	Jun 9	Sep 26	Oct 8	123
24°F	May 13	May 29	Oct 3	Oct 18	143

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 30 locations is 1,187; Keystone's own reading is shown below.

Annual growing degree days for Keystone (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	612	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	1,952	cool-season crops (brassicas, greens)

Hardiness zone 5a in Colorado

Zone 5a means an average annual extreme minimum winter temperature of about −20 to −15 °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, 5b, 6a, 6b, 7a.

Explore zone 5a in other states at zone 5a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 5a mean in Colorado?: Zone 5a means an average annual extreme minimum winter temperature of −20 to −15 °F — the standard USDA measure of which perennials survive winter. 30 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 5a is the growing season longest?: Eaton runs the longest season on this page at about 159 days; Copper Mountain 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 5a?: Using Keystone's frost dates (this page's median-season location): start seeds indoors about Apr 11 – Apr 25, then transplant outside about Jun 13 – Jun 20. 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 5a 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 30 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.