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

Planting zone for Longmont, CO

Longmont, CO is in USDA zones 5b to 6a across its 4 ZIP codes. The median last spring frost is May 7 and the first fall frost October 5.

Longmont 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.

Zones5b–6a−15 to −5 °F
ZIP codes4
Last frostMay 7
First frostOct 5
Growing season151 days

5b 3 ZIPs   6a 1 ZIP

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 Longmont, CO

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMay 7median — 50% chance of a later frost
Safe for tender plantsMay 22only 10% chance of frost after
First fall frostOctober 5median — 10% chance by Sep 23
Growing season151 daysfrost-free, median

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

From the NOAA station Longmont 2 Ese, CO, elevation 4,951 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 Longmont

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 26 – Apr 9
SpinachSow outdoors · Mar 26 – Apr 16
Broccoli, cabbageStart indoors · Feb 26 – Mar 12
Broccoli, cabbageTransplant · Apr 16 – Apr 30
Onion setsPlant outdoors · Apr 9 – Apr 23
PotatoesPlant outdoors · Apr 16 – Apr 30
LettuceSow outdoors · Apr 9 – Apr 23
Carrots, beetsSow outdoors · Apr 16 – Apr 30
TomatoesStart indoors · Mar 12 – Mar 26
TomatoesTransplant · May 14 – May 28
Peppers, eggplantStart indoors · Feb 26 – Mar 12
Peppers, eggplantTransplant · May 21 – Jun 4
BeansSow outdoors · May 14 – May 28
Sweet cornSow outdoors · May 14 – May 28
Cucumbers, squashSow outdoors · May 14 – May 28
Fall lettuce, spinachSow outdoors · Aug 10 – Aug 24
GarlicPlant cloves · Sep 21 – Oct 26
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 7) and first fall frost (Oct 5) 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 80502 came on Feb 26 — 25 days earlier than the 1991–2020 average (Mar 23).

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 Longmont, CO.

Soil in Longmont, CO

The soil mapped at the center of ZIP 80502 is Nunn — clay loam, well drained.

TextureClay loamholds water and nutrients; slow to drain and warm up
pH7.2neutral; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter1.5%low — add compost
Hydrologic groupCwater soaks in slowly
Farmland classPrime farmland if irrigatedUSDA classification

Map unit: Nunn clay loam, 0 to 1 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 80502; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Longmont, CO

Summer high91 °Faverage high, July
Winter low14 °Faverage low, January
Hot days49days a year at 90 °F or more
Freezing nights178nights a year at 32 °F or less
Growing degree days2,765base 50 °F — the season’s usable heat
Longest day15 h 02 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)
Precipitation16.1 inper year, rain and melted snow

Average high and low, by month

0°10°20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 45 °F, low 14 °F 45°14° JanFebruary: average high 47 °F, low 17 °F 47°17° FebMarch: average high 57 °F, low 26 °F 57°26° MarApril: average high 63 °F, low 32 °F 63°32° AprMay: average high 73 °F, low 41 °F 73°41° MayJune: average high 84 °F, low 51 °F 84°51° JunJuly: average high 91 °F, low 55 °F 91°55° JulAugust: average high 88 °F, low 54 °F 88°54° AugSeptember: average high 80 °F, low 46 °F 80°46° SepOctober: average high 66 °F, low 33 °F 66°33° OctNovember: average high 54 °F, low 24 °F 54°24° NovDecember: average high 45 °F, low 15 °F 45°15° Dec

Precipitation by month inches

0″1″2″3″January: 0.56 in precipitation 0.6 JanFebruary: 0.58 in precipitation 0.6 FebMarch: 1.30 in precipitation 1.3 MarApril: 2.05 in precipitation 2.0 AprMay: 2.53 in precipitation 2.5 MayJune: 1.75 in precipitation 1.8 JunJuly: 1.66 in precipitation 1.7 JulAugust: 1.82 in precipitation 1.8 AugSeptember: 1.29 in precipitation 1.3 SepOctober: 1.21 in precipitation 1.2 OctNovember: 0.75 in precipitation 0.8 NovDecember: 0.58 in precipitation 0.6 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Longmont 2 Ese, CO (within 1 mile); precipitation: Longmont 1.8 E, CO (within 1 mile). Daylight is calculated from latitude.

ZIP codes in Longmont

805015b
−15 to −10 °F
805025b
−15 to −10 °F
805036a
−10 to −5 °F
805045b
−15 to −10 °F

Colorado planting zones →

Questions about Longmont

What planting zone is Longmont, CO?

Longmont, CO is in USDA hardiness zones 5b to 6a on the 2023 map (5b: −15 to −10 °F; 6a: −10 to −5 °F).

When is the last frost in Longmont, CO?

The median last spring frost is May 7 and the median first fall frost is October 5 (NOAA 1991–2020 normals, Longmont 2 Ese station).

When can I plant a garden in Longmont, CO?

Cool-season crops like peas and spinach go in around Mar 26 – Apr 16; tomatoes, peppers, beans and squash after about May 14, once frost risk has passed.

What kind of onions grow in Longmont, CO?

Long-day onions: the longest day is 15 h 02 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

How much rain and snow does Longmont, CO get?

About 16.1 inches of precipitation a year (NOAA 1991–2020 normals).