Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated Oct 29, 2026

Planting zone for 81647 — New Castle, CO

ZIP code 81647 (New Castle, Garfield County) is in USDA zone 6a: the coldest winter night averages −10 to −5 °F. The median last spring frost is May 23 and the first fall frost September 25.

Where ZIP 81647 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.

Zone6a−10 to −5 °F

Coldest night−23.3 to −20.6 °C

Last frostMay 23

First frostSep 25

2012 zone6a

Same zone as on the 2012 map. The zone is the one at the center of the ZIP code; hilly ZIPs can span two zones. What zone 6a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for New Castle, 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 New Castle, CO

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostMay 23median — 50% chance of a later frost

Safe for tender plantsJune 6only 10% chance of frost after

First fall frostSeptember 25median — 10% chance by Sep 13

Growing season124 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 9 and returns around October 4.

From the NOAA station Rifle 3ene, CO — 11 miles away, elevation 5,469 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 81647

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Apr 11 – Apr 25

SpinachSow outdoors · Apr 11 – May 2

Broccoli, cabbageStart indoors · Mar 14 – Mar 28

Broccoli, cabbageTransplant · May 2 – May 16

Onion setsPlant outdoors · Apr 25 – May 9

PotatoesPlant outdoors · May 2 – May 16

LettuceSow outdoors · Apr 25 – May 9

Carrots, beetsSow outdoors · May 2 – May 16

TomatoesStart indoors · Mar 28 – Apr 11

TomatoesTransplant · May 30 – Jun 13

Peppers, eggplantStart indoors · Mar 14 – Mar 28

Peppers, eggplantTransplant · Jun 6 – Jun 20

BeansSow outdoors · May 30 – Jun 13

Sweet cornSow outdoors · May 30 – Jun 13

Cucumbers, squashSow outdoors · May 30 – Jun 13

Fall lettuce, spinachSow outdoors · Jul 31 – Aug 14

GarlicPlant cloves · Sep 11 – Oct 16

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 23) and first fall frost (Sep 25) 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 81647 came on Mar 18 — 18 days earlier than the 1991–2020 average (Apr 5).

average
Apr 52026
Mar 18

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 6a in Colorado

563 plants native to the lower 48 states, recorded in Colorado, and rated by the USDA to survive −10 °F or colder — the bottom of zone 6a. 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 New Castle, CO.

Soil in New Castle, CO

The soil mapped at the center of ZIP 81647 is Ascalon — fine sandy loam, well drained.

TextureFine sandy loamdrains and warms fast; dries out and needs more feeding

pH7.2neutral; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter1.5%low — add compost

Hydrologic groupBwater soaks in moderately

Farmland classPrime farmland if irrigatedUSDA classification

Map unit: Ascalon fine sandy loam, 1 to 6 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 81647; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in New Castle, CO

Summer high88 °Faverage high, July

Winter low10 °Faverage low, January

Hot days37days a year at 90 °F or more

Freezing nights202nights a year at 32 °F or less

Growing degree days2,130base 50 °F — the season’s usable heat

Longest day14 h 58 mindaylight on June 21

Shortest day9 h 22 mindaylight on December 21

Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully

Precipitation20.1 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: Rifle 3ene, CO (11 mi away); precipitation: Glenwood Springs 1.7 Nw, CO (10 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

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

816016a

Glenwood Springs, CO · 10 mi

816526a

Silt, CO · 11 mi

816026a

Glenwood Springs, CO · 11 mi

816375b

Gypsum, CO · 27 mi

816366b

Battlement Mesa, CO · 28 mi

816356a

Parachute, CO · 29 mi

816236a

Carbondale, CO · 30 mi

816246a

Collbran, CO · 31 mi

Colorado planting zones →

Questions about 81647

What planting zone is ZIP code 81647?

ZIP code 81647 (New Castle, CO) is in USDA hardiness zone 6a on the 2023 map: the coldest night of an average winter reaches −10 to −5 °F (−23.3 to −20.6 °C).

When is the last frost in New Castle, CO?

The median last spring frost (32 °F) near 81647 is May 23; after June 6 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Rifle 3ene station.

When is the first frost in New Castle, CO?

The median first fall frost near 81647 is September 25, giving a median frost-free growing season of 124 days.

When should I plant tomatoes in 81647?

Start tomato seeds indoors around Mar 28 – Apr 11 and set transplants outside around May 30 – Jun 13, one to three weeks after the median last frost.

How much rain and snow does 81647 get?

About 20.1 inches of precipitation a year (NOAA 1991–2020 normals, Glenwood Springs 1.7 Nw station).

How hot do summers get in New Castle, CO?

The average high peaks at 88 °F in July, with about 37 days a year at 90 °F or more.

What kind of soil does 81647 have?

The USDA soil survey maps Ascalon at the center of ZIP 81647: fine sandy loam, well drained, pH about 7.2 (neutral).

Did spring come early in New Castle, CO in 2026?

Spring leaf-out near 81647 came on Mar 18 in 2026, 18 days earlier than the 1991–2020 average (Apr 5), according to the USA National Phenology Network Spring Index.

What kind of onions grow in New Castle, CO?

Intermediate onions: the longest day near 81647 is 14 h 58 min, so day-neutral types bulb best; long-day varieties may not bulb fully.