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

Planting zone for Castle Rock, CO

Castle Rock, CO is in USDA zone 5b across its 3 ZIP codes. The median last spring frost is May 9 and the first fall frost October 2.

Castle Rock on the zone map

Outlined: its 3 ZIP code areas · 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
ZIP codes3
Last frostMay 9
First frostOct 2
Growing season143 days

5b 3 ZIPs

Frost forecast — next 7 nights

live from the National Weather Service

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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 Castle Rock, CO

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMay 9median — 50% chance of a later frost
Safe for tender plantsMay 24only 10% chance of frost after
First fall frostOctober 2median — 10% chance by Sep 17
Growing season143 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 1 and returns around October 12.

From the NOAA station Castle Rock, CO, elevation 6,185 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 Castle Rock

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 28 – Apr 11
SpinachSow outdoors · Mar 28 – Apr 18
Broccoli, cabbageStart indoors · Feb 28 – Mar 14
Broccoli, cabbageTransplant · Apr 18 – May 2
Onion setsPlant outdoors · Apr 11 – Apr 25
PotatoesPlant outdoors · Apr 18 – May 2
LettuceSow outdoors · Apr 11 – Apr 25
Carrots, beetsSow outdoors · Apr 18 – May 2
TomatoesStart indoors · Mar 14 – Mar 28
TomatoesTransplant · May 16 – May 30
Peppers, eggplantStart indoors · Feb 28 – Mar 14
Peppers, eggplantTransplant · May 23 – Jun 6
BeansSow outdoors · May 16 – May 30
Sweet cornSow outdoors · May 16 – May 30
Cucumbers, squashSow outdoors · May 16 – May 30
Fall lettuce, spinachSow outdoors · Aug 7 – Aug 21
GarlicPlant cloves · Sep 18 – Oct 23
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 9) and first fall frost (Oct 2) 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 80109 came on Feb 21 — 31 days earlier than the 1991–2020 average (Mar 24).

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.

Trees 27

American elm120 ft · blooms early spring · medium shade tolerance · to −46 °Fbalsam poplar80 ft · blooms late spring · high shade tolerance · to −79 °Fblack locust60 ft · blooms spring · high shade tolerance · to −37 °Fblack walnut100 ft · blooms late spring · high shade tolerance · to −28 °Fblack willow100 ft · blooms early spring · high shade tolerance · to −58 °Fblue spruce100 ft · blooms late spring · medium shade tolerance · to −38 °F

Shrubs 98

alderleaf mountain mahogany15 ft · blooms late spring · medium shade tolerance · to −41 °Falpine laurel1 ft · blooms late spring · low shade tolerance · to −28 °FAmerican black currant3 ft · blooms early spring · to −46 °FAmerican plum24 ft · blooms mid spring · high shade tolerance · to −38 °Fantelope bitterbrush6 ft · blooms early spring · medium shade tolerance · to −36 °FApache plume6 ft · blooms late spring · high shade tolerance · to −23 °F

Wildflowers & perennials 246

alkali buttercup1 ft · blooms late spring · low shade tolerance · to −38 °Falpine aster0.3 ft · blooms early summer · medium shade tolerance · to −28 °Falpine bistort1 ft · blooms mid spring · high shade tolerance · to −38 °Falpine mountainsorrel1 ft · blooms mid summer · medium shade tolerance · to −43 °FAmerican alpine speedwell1 ft · blooms summer · medium shade tolerance · to −28 °FAmerican bistort1.5 ft · blooms mid spring · high shade tolerance · to −38 °F

Grasses 163

Alaska oniongrass3 ft · blooms mid spring · low shade tolerance · to −33 °Falkali cordgrass3 ft · blooms late summer · high shade tolerance · to −38 °Falkali sacaton3 ft · blooms mid summer · high shade tolerance · to −38 °FAlpine Meadow-Foxtail0.5 ft · blooms mid spring · medium shade tolerance · to −33 °Falpine timothy1.4 ft · blooms late spring · medium shade tolerance · to −43 °Fanalogue sedge1.5 ft · blooms late spring · medium shade tolerance · to −38 °F

Vines 3

riverbank grape45 ft · blooms late spring · medium shade tolerance · to −33 °FVirginia creeper1 ft · blooms late spring · medium shade tolerance · to −28 °Fwestern white clematis1 ft · blooms mid summer · medium shade tolerance · to −38 °F

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 Castle Rock, CO.

Soil in Castle Rock, CO

The center of ZIP 80109 is mapped as Stony rough land: built-over ground without natural soil data. Garden soil here is usually imported or disturbed — test before you plant.

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 80109; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Castle Rock, CO

Summer high86 °Faverage high, July
Winter low18 °Faverage low, January
Hot days23days a year at 90 °F or more
Freezing nights167nights a year at 32 °F or less
Growing degree days2,491base 50 °F — the season’s usable heat
Longest day14 h 57 mindaylight on June 21
Shortest day9 h 23 mindaylight on December 21
Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully
Precipitation17.7 inper year, rain and melted snow

Average high and low, by month

10°20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 46 °F, low 18 °F 46°18° JanFebruary: average high 47 °F, low 20 °F 47°20° FebMarch: average high 54 °F, low 27 °F 54°27° MarApril: average high 59 °F, low 33 °F 59°33° AprMay: average high 68 °F, low 42 °F 68°42° MayJune: average high 80 °F, low 51 °F 80°51° JunJuly: average high 86 °F, low 56 °F 86°56° JulAugust: average high 84 °F, low 55 °F 84°55° AugSeptember: average high 77 °F, low 47 °F 77°47° SepOctober: average high 65 °F, low 35 °F 65°35° OctNovember: average high 54 °F, low 26 °F 54°26° NovDecember: average high 46 °F, low 18 °F 46°18° Dec

Precipitation by month inches

0″1″2″3″January: 0.56 in precipitation 0.6 JanFebruary: 0.62 in precipitation 0.6 FebMarch: 1.36 in precipitation 1.4 MarApril: 2.27 in precipitation 2.3 AprMay: 2.26 in precipitation 2.3 MayJune: 1.83 in precipitation 1.8 JunJuly: 2.36 in precipitation 2.4 JulAugust: 2.37 in precipitation 2.4 AugSeptember: 1.40 in precipitation 1.4 SepOctober: 1.24 in precipitation 1.2 OctNovember: 0.79 in precipitation 0.8 NovDecember: 0.62 in precipitation 0.6 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Castle Rock, CO (2 mi away); precipitation: Castle Rock 3.7 W, CO (within 1 mile). Daylight is calculated from latitude.

ZIP codes in Castle Rock

801045b
−15 to −10 °F
801085b
−15 to −10 °F
801095b
−15 to −10 °F

Colorado planting zones →

Questions about Castle Rock

What planting zone is Castle Rock, CO?

Castle Rock, CO is in USDA hardiness zone 5b on the 2023 map (5b: −15 to −10 °F).

When is the last frost in Castle Rock, CO?

The median last spring frost is May 9 and the median first fall frost is October 2 (NOAA 1991–2020 normals, Castle Rock station).

When can I plant a garden in Castle Rock, CO?

Cool-season crops like peas and spinach go in around Mar 28 – Apr 18; tomatoes, peppers, beans and squash after about May 16, once frost risk has passed.

What kind of onions grow in Castle Rock, CO?

Intermediate onions: the longest day is 14 h 57 min, so day-neutral types bulb best; long-day varieties may not bulb fully.

How much rain and snow does Castle Rock, CO get?

About 17.7 inches of precipitation a year (NOAA 1991–2020 normals).