Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated Oct 30, 2026

Planting zone for 81638 — Hamilton, CO

ZIP code 81638 (Hamilton, Moffat County) is in USDA zone 5b: the coldest winter night averages −15 to −10 °F. The median last spring frost is May 29 and the first fall frost September 20.

Where ZIP 81638 is

Outlined: the ZIP code area · tap the map for the zone at any spot · scroll or pinch to zoom

Not the official USDA Plant Hardiness Zone Map. Zones come from the 2023 USDA hardiness-zone data by the PRISM Group, Oregon State University; we re-mapped and recolored them. The official map is at planthardiness.ars.usda.gov.

Zone5b−15 to −10 °F

Coldest night−26.1 to −23.3 °C

Last frostMay 29

First frostSep 20

2012 zone5a

Was 5a on the 2012 map — half a zone warmer now. The zone is the one at the center of the ZIP code; hilly ZIPs can span two zones. What zone 5b means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Hamilton, CO…

Forecast data: National Weather Service (api.weather.gov), public domain. This is not a warning service — for official frost and freeze warnings see weather.gov.

Frost dates in Hamilton, CO

JanFebMarAprMayJunJulAugSepOctNovDec

Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free

Last spring frostMay 29median — 50% chance of a later frost

Safe for tender plantsJune 13only 10% chance of frost after

First fall frostSeptember 20median — 10% chance by Sep 5

Growing season113 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 13 and returns around September 30.

From the NOAA station Craig 4sw, CO — 11 miles away, elevation 6,496 ft. NOAA NCEI U.S. Climate Normals 1991–2020, 32 °F threshold. Low spots and valleys can freeze later in spring than the station.

When to plant vegetables in 81638

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Apr 17 – May 1

SpinachSow outdoors · Apr 17 – May 8

Broccoli, cabbageStart indoors · Mar 20 – Apr 3

Broccoli, cabbageTransplant · May 8 – May 22

Onion setsPlant outdoors · May 1 – May 15

PotatoesPlant outdoors · May 8 – May 22

LettuceSow outdoors · May 1 – May 15

Carrots, beetsSow outdoors · May 8 – May 22

TomatoesStart indoors · Apr 3 – Apr 17

TomatoesTransplant · Jun 5 – Jun 19

Peppers, eggplantStart indoors · Mar 20 – Apr 3

Peppers, eggplantTransplant · Jun 12 – Jun 26

BeansSow outdoors · Jun 5 – Jun 19

Sweet cornSow outdoors · Jun 5 – Jun 19

Cucumbers, squashSow outdoors · Jun 5 – Jun 19

Fall lettuce, spinachSow outdoors · Jul 26 – Aug 9

GarlicPlant cloves · Sep 6 – Oct 11

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 29) and first fall frost (Sep 20) with the rules of thumb U.S. extension services publish. Cool-season crops take a light frost; warm-season crops do best once the soil is above about 60 °F.

When spring arrived in 2026

In 2026, spring leaf-out near 81638 came on Mar 25 — 28 days earlier than the 1991–2020 average (Apr 22).

average
Apr 222026
Mar 25

JanFebMarAprMayJun

The Spring Index marks the day when accumulated warmth is enough for lilac and honeysuckle to leaf out — a widely used signal that the growing season has started. Source: USA National Phenology Network, www.usanpn.org — Spring Indices, first leaf, 2026 (data as of 2026-09-08) and 30-year average 1991–2020 (CC BY 4.0).

Native plants for zone 5b in Colorado

546 plants native to the lower 48 states, recorded in Colorado, and rated by the USDA to survive −15 °F or colder — the bottom of zone 5b. Each links to its USDA PLANTS profile.

Source: USDA, NRCS. 2026. The PLANTS Database (plants.usda.gov), National Plant Data Team — state distribution, native status and the “Temperature, Minimum” characteristic. Check light, moisture and soil needs before buying; local nurseries and your state native plant society can confirm what thrives in Hamilton, CO.

Soil in Hamilton, CO

The soil mapped at the center of ZIP 81638 is Ustorthents — very channery sandy loam, well drained.

TextureVery channery sandy loamdrains and warms fast; dries out and needs more feeding

pH7.5slightly alkaline; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter1.25%low — add compost

Hydrologic groupDwater soaks in very slowly

Farmland classNot prime farmlandUSDA classification

Map unit: Ustorthents, frigid-Borolls complex, 25 to 75 percent slopes.

Soil Survey Staff, Natural Resources Conservation Service, USDA. Soil Survey Geographic (SSURGO) Database, via Soil Data Access, accessed 2026-10-29. The soil at the center point of ZIP 81638; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Hamilton, CO

Summer high85 °Faverage high, July

Winter low7 °Faverage low, January

Hot days15days a year at 90 °F or more

Freezing nights202nights a year at 32 °F or less

Growing degree days1,727base 50 °F — the season’s usable heat

Longest day15 h 03 mindaylight on June 21

Shortest day9 h 18 mindaylight on December 21

Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)

Precipitation17.6 inper year, rain and melted snow

Average high and low, by month

Precipitation by month inches

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Craig 4sw, CO (11 mi away); precipitation: Hamilton, CO (5 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

6 of the 8 nearest ZIP codes are in a different zone — elevation and distance from water change winter lows quickly.

Phippsburg, CO · 36 mi

804835a

Yampa, CO · 40 mi

804675a

Oak Creek, CO · 44 mi

816335b

Dinosaur, CO · 44 mi

Colorado planting zones →

Questions about 81638

What planting zone is ZIP code 81638?

ZIP code 81638 (Hamilton, CO) is in USDA hardiness zone 5b on the 2023 map: the coldest night of an average winter reaches −15 to −10 °F (−26.1 to −23.3 °C).

When is the last frost in Hamilton, CO?

The median last spring frost (32 °F) near 81638 is May 29; after June 13 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Craig 4sw station.

When is the first frost in Hamilton, CO?

The median first fall frost near 81638 is September 20, giving a median frost-free growing season of 113 days.

When should I plant tomatoes in 81638?

Start tomato seeds indoors around Apr 3 – Apr 17 and set transplants outside around Jun 5 – Jun 19, one to three weeks after the median last frost.

How much rain and snow does 81638 get?

About 17.6 inches of precipitation a year (NOAA 1991–2020 normals, Hamilton station).

How hot do summers get in Hamilton, CO?

The average high peaks at 85 °F in July, with about 15 days a year at 90 °F or more.

What kind of soil does 81638 have?

The USDA soil survey maps Ustorthents at the center of ZIP 81638: very channery sandy loam, well drained, pH about 7.5 (slightly alkaline).

Did spring come early in Hamilton, CO in 2026?

Spring leaf-out near 81638 came on Mar 25 in 2026, 28 days earlier than the 1991–2020 average (Apr 22), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Hamilton, CO?

Long-day onions: the longest day near 81638 is 15 h 03 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 81638 change?

Yes. The 2012 USDA map put 81638 in zone 5a; the 2023 map puts it in zone 5b. The newer map uses 1991–2020 data and more weather stations.