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

Planting zone for 59632 — Boulder, MT

ZIP code 59632 (Boulder, Jefferson County) is in USDA zone 5a: the coldest winter night averages −20 to −15 °F. The median last spring frost is June 7 and the first fall frost September 8.

Where ZIP 59632 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.

Zone5a−20 to −15 °F
Coldest night−28.9 to −26.1 °C
Last frostJun 7
First frostSep 8
2012 zone4a

Was 4a on the 2012 map — a full zone warmer now. The zone is the one at the center of the ZIP code; hilly ZIPs can span two zones. What zone 5a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Boulder, MT…

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, MT

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostJune 7median — 50% chance of a later frost
Safe for tender plantsJune 25only 10% chance of frost after
First fall frostSeptember 8median — 10% chance by Aug 25
Growing season90 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 21 and returns around September 21.

From the NOAA station Boulder, MT — 7 miles away, elevation 4,905 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 59632

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Apr 26 – May 10
SpinachSow outdoors · Apr 26 – May 17
Broccoli, cabbageStart indoors · Mar 29 – Apr 12
Broccoli, cabbageTransplant · May 17 – May 31
Onion setsPlant outdoors · May 10 – May 24
PotatoesPlant outdoors · May 17 – May 31
LettuceSow outdoors · May 10 – May 24
Carrots, beetsSow outdoors · May 17 – May 31
TomatoesStart indoors · Apr 12 – Apr 26
TomatoesTransplant · Jun 14 – Jun 28
Peppers, eggplantStart indoors · Mar 29 – Apr 12
Peppers, eggplantTransplant · Jun 21 – Jul 5
BeansSow outdoors · Jun 14 – Jun 28
Sweet cornSow outdoors · Jun 14 – Jun 28
Cucumbers, squashSow outdoors · Jun 14 – Jun 28
Fall lettuce, spinachSow outdoors · Jul 14 – Jul 28
GarlicPlant cloves · Aug 25 – Sep 29
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Jun 7) and first fall frost (Sep 8) 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 59632 came on Mar 23 — 26 days earlier than the 1991–2020 average (Apr 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 5a in Montana

548 plants native to the lower 48 states, recorded in Montana, and rated by the USDA to survive −20 °F or colder — the bottom of zone 5a. Each links to its USDA PLANTS profile.

Trees 26

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 °Fboxelder60 ft · blooms early spring · low shade tolerance · to −46 °FDouglas-fir200 ft · blooms mid spring · medium shade tolerance · to −33 °Feastern cottonwood190 ft · blooms spring · high shade tolerance · to −43 °F

Shrubs 96

alderleaf buckthorn3 ft · blooms late spring · low shade tolerance · to −33 °Falderleaf 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 °F

Wildflowers & perennials 253

alkali buttercup1 ft · blooms late spring · low shade tolerance · to −38 °Falpine bistort1 ft · blooms mid spring · high shade tolerance · to −38 °Falpine mountainsorrel1 ft · blooms mid summer · medium shade tolerance · to −43 °Falpine sweetvetch2 ft · blooms early summer · high shade tolerance · to −62 °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 161

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

American bittersweet15 ft · blooms spring · low shade tolerance · to −38 °Friverbank grape45 ft · blooms late spring · medium shade tolerance · to −33 °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, MT.

Soil in Boulder, MT

The soil mapped at the center of ZIP 59632 is Bonebasin — slightly decomposed plant material, very poorly drained.

TextureSlightly decomposed plant materialtop layer
pH6.5neutral; most vegetables like 6.0–7.0
DrainageVery poorly drainedconsider raised beds
Organic matter50%high
Hydrologic groupB/Dmoderate when drained; wet otherwise
Farmland classNot prime farmlandUSDA classification

Map unit: Bonebasin-Wetsand complex, 0 to 2 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 59632; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Boulder, MT

Summer high83 °Faverage high, July
Winter low12 °Faverage low, January
Hot days12days a year at 90 °F or more
Freezing nights205nights a year at 32 °F or less
Growing degree days1,477base 50 °F — the season’s usable heat
Longest day15 h 47 mindaylight on June 21
Shortest day8 h 37 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation10.8 inper year, rain and melted snow

Average high and low, by month

0°10°20°30°40°50°60°70°80°90°32 °FJanuary: average high 35 °F, low 12 °F 35°12° JanFebruary: average high 38 °F, low 14 °F 38°14° FebMarch: average high 47 °F, low 21 °F 47°21° MarApril: average high 55 °F, low 27 °F 55°27° AprMay: average high 64 °F, low 35 °F 64°35° MayJune: average high 72 °F, low 43 °F 72°43° JunJuly: average high 83 °F, low 48 °F 83°48° JulAugust: average high 82 °F, low 46 °F 82°46° AugSeptember: average high 72 °F, low 38 °F 72°38° SepOctober: average high 57 °F, low 28 °F 57°28° OctNovember: average high 42 °F, low 19 °F 42°19° NovDecember: average high 34 °F, low 12 °F 34°12° Dec

Precipitation by month inches

0″1″2″3″January: 0.28 in precipitation 0.3 JanFebruary: 0.19 in precipitation 0.2 FebMarch: 0.40 in precipitation 0.4 MarApril: 0.78 in precipitation 0.8 AprMay: 1.86 in precipitation 1.9 MayJune: 2.30 in precipitation 2.3 JunJuly: 1.38 in precipitation 1.4 JulAugust: 1.12 in precipitation 1.1 AugSeptember: 1.10 in precipitation 1.1 SepOctober: 0.64 in precipitation 0.6 OctNovember: 0.48 in precipitation 0.5 NovDecember: 0.27 in precipitation 0.3 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Boulder, MT (7 mi away); precipitation: Boulder, MT (7 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.

596384b
Jefferson City, MT · 13 mi
596344b
Clancy, MT · 19 mi
596315a
Basin, MT · 20 mi
596435a
Toston, MT · 20 mi
597215a
Cardwell, MT · 23 mi
597595a
Whitehall, MT · 24 mi
597015a
Butte, MT · 24 mi
596475a
Winston, MT · 26 mi

Montana planting zones →

Questions about 59632

What planting zone is ZIP code 59632?

ZIP code 59632 (Boulder, MT) is in USDA hardiness zone 5a on the 2023 map: the coldest night of an average winter reaches −20 to −15 °F (−28.9 to −26.1 °C).

When is the last frost in Boulder, MT?

The median last spring frost (32 °F) near 59632 is June 7; after June 25 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Boulder station.

When is the first frost in Boulder, MT?

The median first fall frost near 59632 is September 8, giving a median frost-free growing season of 90 days.

When should I plant tomatoes in 59632?

Start tomato seeds indoors around Apr 12 – Apr 26 and set transplants outside around Jun 14 – Jun 28, one to three weeks after the median last frost.

How much rain and snow does 59632 get?

About 10.8 inches of precipitation a year (NOAA 1991–2020 normals, Boulder station).

How hot do summers get in Boulder, MT?

The average high peaks at 83 °F in July, with about 12 days a year at 90 °F or more.

What kind of soil does 59632 have?

The USDA soil survey maps Bonebasin at the center of ZIP 59632: slightly decomposed plant material, very poorly drained, pH about 6.5 (neutral).

Did spring come early in Boulder, MT in 2026?

Spring leaf-out near 59632 came on Mar 23 in 2026, 26 days earlier than the 1991–2020 average (Apr 18), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Boulder, MT?

Long-day onions: the longest day near 59632 is 15 h 47 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 59632 change?

Yes. The 2012 USDA map put 59632 in zone 4a; the 2023 map puts it in zone 5a. The newer map uses 1991–2020 data and more weather stations.