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

Planting zone for 80306 — Boulder, CO

ZIP code 80306 (Boulder, Boulder County) is in USDA zone 6a: the coldest winter night averages −10 to −5 °F. The median last spring frost is June 21 and the first fall frost September 2.

Where ZIP 80306 is

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 frostJun 21
First frostSep 2
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 Boulder, 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 Boulder, CO

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostJune 21median — 50% chance of a later frost
Safe for tender plantsJuly 4only 10% chance of frost after
First fall frostSeptember 2median — 10% chance by Aug 17
Growing season71 daysfrost-free, median

A hard freeze (28 °F) usually ends around June 4 and returns around September 14.

From the NOAA station Boulder 14 W, CO — 9 miles away, elevation 9,830 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 80306

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · May 10 – May 24
SpinachSow outdoors · May 10 – May 31
Broccoli, cabbageStart indoors · Apr 12 – Apr 26
Broccoli, cabbageTransplant · May 31 – Jun 14
Onion setsPlant outdoors · May 24 – Jun 7
PotatoesPlant outdoors · May 31 – Jun 14
LettuceSow outdoors · May 24 – Jun 7
Carrots, beetsSow outdoors · May 31 – Jun 14
TomatoesStart indoors · Apr 26 – May 10
TomatoesTransplant · Jun 28 – Jul 12
Peppers, eggplantStart indoors · Apr 12 – Apr 26
Peppers, eggplantTransplant · Jul 5 – Jul 19
BeansSow outdoors · Jun 28 – Jul 12
Sweet cornSow outdoors · Jun 28 – Jul 12
Cucumbers, squashSow outdoors · Jun 28 – Jul 12
Fall lettuce, spinachSow outdoors · Jul 8 – Jul 22
GarlicPlant cloves · Aug 19 – Sep 23
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Jun 21) and first fall frost (Sep 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 80306 came on Mar 16 — 25 days earlier than the 1991–2020 average (Apr 10).

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.

Trees 28

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 104

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 251

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 167

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 4

riverbank grape45 ft · blooms late spring · medium shade tolerance · to −33 °Ftrumpet creeper2 ft · blooms summer · high shade tolerance · to −13 °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 Boulder, CO.

Soil in Boulder, CO

The soil mapped at the center of ZIP 80306 is Ratake family — very gravelly sandy loam, somewhat excessively drained.

TextureVery gravelly sandy loamdrains and warms fast; dries out and needs more feeding
pH6.8neutral; most vegetables like 6.0–7.0
DrainageSomewhat excessively drainednatural drainage class
Organic matter2%moderate
Hydrologic groupDwater soaks in very slowly
Farmland classNot prime farmlandUSDA classification

Map unit: Ratake-Cathedral families-Rock outcrop complex, 40 to 150 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 80306; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Boulder, CO

Summer high68 °Faverage high, July
Winter low12 °Faverage low, January
Hot days0days a year at 90 °F or more
Freezing nights244nights a year at 32 °F or less
Growing degree days385base 50 °F — the season’s usable heat
Longest day15 h 02 mindaylight on June 21
Shortest day9 h 19 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation21.5 inper year, rain and melted snow

Average high and low, by month

0°10°20°30°40°50°60°70°80°32 °FJanuary: average high 29 °F, low 12 °F 29°12° JanFebruary: average high 28 °F, low 12 °F 28°12° FebMarch: average high 36 °F, low 17 °F 36°17° MarApril: average high 40 °F, low 22 °F 40°22° AprMay: average high 50 °F, low 29 °F 50°29° MayJune: average high 62 °F, low 36 °F 62°36° JunJuly: average high 68 °F, low 43 °F 68°43° JulAugust: average high 66 °F, low 41 °F 66°41° AugSeptember: average high 59 °F, low 34 °F 59°34° SepOctober: average high 47 °F, low 26 °F 47°26° OctNovember: average high 35 °F, low 19 °F 35°19° NovDecember: average high 28 °F, low 12 °F 28°12° Dec

Precipitation by month inches

0″1″2″3″4″January: 0.71 in precipitation 0.7 JanFebruary: 1.02 in precipitation 1.0 FebMarch: 1.59 in precipitation 1.6 MarApril: 2.94 in precipitation 2.9 AprMay: 3.23 in precipitation 3.2 MayJune: 1.84 in precipitation 1.8 JunJuly: 2.67 in precipitation 2.7 JulAugust: 2.13 in precipitation 2.1 AugSeptember: 1.93 in precipitation 1.9 SepOctober: 1.61 in precipitation 1.6 OctNovember: 1.01 in precipitation 1.0 NovDecember: 0.81 in precipitation 0.8 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Boulder 14 W, CO (9 mi away); precipitation: Jamestown 2.5 Sse, CO (within 1 mile). 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.

804555b
Jamestown, CO · <1 mi
803076a
Boulder, CO · 1 mi
803146a
Boulder, CO · 1 mi
803086a
Boulder, CO · 5 mi
803026a
Boulder, CO · 6 mi
803046a
Boulder, CO · 6 mi
805405b
Lyons, CO · 8 mi
803106a
Boulder, CO · 9 mi

All 10 ZIP codes in Boulder → · Colorado planting zones →

Questions about 80306

What planting zone is ZIP code 80306?

ZIP code 80306 (Boulder, 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 Boulder, CO?

The median last spring frost (32 °F) near 80306 is June 21; after July 4 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Boulder 14 W station.

When is the first frost in Boulder, CO?

The median first fall frost near 80306 is September 2, giving a median frost-free growing season of 71 days.

When should I plant tomatoes in 80306?

Start tomato seeds indoors around Apr 26 – May 10 and set transplants outside around Jun 28 – Jul 12, one to three weeks after the median last frost.

How much rain and snow does 80306 get?

About 21.5 inches of precipitation a year (NOAA 1991–2020 normals, Jamestown 2.5 Sse station).

How hot do summers get in Boulder, CO?

The average high peaks at 68 °F in July, with about 0 days a year at 90 °F or more.

What kind of soil does 80306 have?

The USDA soil survey maps Ratake family at the center of ZIP 80306: very gravelly sandy loam, somewhat excessively drained, pH about 6.8 (neutral).

Did spring come early in Boulder, CO in 2026?

Spring leaf-out near 80306 came on Mar 16 in 2026, 25 days earlier than the 1991–2020 average (Apr 10), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Boulder, CO?

Long-day onions: the longest day near 80306 is 15 h 02 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).