What does USDA hardiness zone 4a mean in Colorado?

Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 1 location in Colorado fall in this zone. It does not set planting dates; see the frost calendar below for that.

When should I plant tomatoes in Colorado's zone 4a?

Using Hartsel's frost dates (this page's median-season location): start seeds indoors about Apr 7 – Apr 21, then transplant outside about Jun 9 – Jun 16. 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 4a 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 1 locations listed (see the table above for each location's own dates).

USDA zone 4a in Colorado

−30 to −25 °F avg. annual extreme minimum · 1 location in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 4a covers 1 location across Colorado (average annual extreme minimum −30 to −25 °F): Hartsel, CO. Average last spring frost across this group ranges June 2 and growing seasons run 108 days — 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 Hartsel, this group's median-season location.

USDA zone: 4a−30 to −25 °F
Last frost range: Jun 2avg, 32°F
First frost range: Sep 19avg, 32°F
Growing season range: 108days

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)
Hartsel *	—	Lake George 8 Sw	Jun 2	Sep 19	108

* Hartsel 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 (Hartsel, representative)

Computed from Hartsel's average frost dates — the median-season location in this Coloradozone 4a 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 Hartsel (Colorado, zone 4a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Apr 7 – Apr 21	Jun 9 – Jun 16	Aug 8 – Aug 28	—	matures comfortably
Pepper	Very tender	Mar 24 – Apr 7	Jun 16 – Jun 23	Aug 15 – Sep 14	—	matures, but with little margin
Cucumber	Tender	May 5 – May 12	Jun 9 – Jun 16	Jul 29 – Aug 18	—	matures comfortably
Summer squash / zucchini	Tender	—	Jun 9 – Jun 16	Jul 24 – Aug 8	—	matures comfortably
Bush bean	Tender	—	Jun 9 – Jun 16	Jul 29 – Aug 8	Jul 21 – Jul 31	matures comfortably
Sweet corn	Tender	—	Jun 2 – Jun 16	Aug 1 – Aug 31	—	matures comfortably
Basil	Very tender	Apr 21 – May 5	Jun 9 – Jun 16	Jul 9 – Jul 24	—	matures comfortably
Lettuce	Half-hardy	Apr 21 – May 5	May 5 – May 19	Jun 19 – Jul 4	Jul 7 – Jul 22	matures comfortably
Pea	Hardy	—	Apr 21 – May 5	Jun 15 – Jun 30	Jun 27 – Jul 12	matures comfortably
Spinach	Hardy	—	Apr 21 – May 5	May 31 – Jun 10	Jul 17 – Jul 27	matures comfortably
Carrot	Half-hardy	—	May 12 – May 19	Jul 11 – Jul 31	Jun 17 – Jul 7	matures comfortably
Broccoli	Half-hardy	Apr 7 – Apr 21	May 5 – May 19	Jun 29 – Jul 19	Jun 22 – Jul 12	matures comfortably

Which crops fit this growing season

Using Hartsel's own 108-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 (+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 (Hartsel)

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 — the conservative date to plant after or harvest before.

Freeze probabilities by temperature threshold (MM/DD, NOAA 1991–2020), Hartsel.
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

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 1 locations is 1,044; Hartsel's own reading is shown below.

Annual growing degree days for Hartsel (°F·days, NOAA 1991–2020).
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 in Colorado

Zone 4a means an average annual extreme minimum winter temperature of about −30 to −25 °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 zones4b, 5a, 5b, 6a, 6b, 7a.

Explore zone 4a in other states at zone 4a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 4a mean in Colorado?: Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 1 location in Colorado fall in this zone. It does not set planting dates; see the frost calendar below for that.
When should I plant tomatoes in Colorado's zone 4a?: Using Hartsel's frost dates (this page's median-season location): start seeds indoors about Apr 7 – Apr 21, then transplant outside about Jun 9 – Jun 16. 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 4a 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 1 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.