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

Planting zone for 59520 — Big Sandy, MT

ZIP code 59520 (Big Sandy, Chouteau County) is in USDA zone 4a: the coldest winter night averages −30 to −25 °F. The median last spring frost is May 17 and the first fall frost September 25.

Where ZIP 59520 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.

Zone4a−30 to −25 °F
Coldest night−34.4 to −31.7 °C
Last frostMay 17
First frostSep 25
2012 zone4a

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 4a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Big Sandy, 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 Big Sandy, MT

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMay 17median — 50% chance of a later frost
Safe for tender plantsJune 4only 10% chance of frost after
First fall frostSeptember 25median — 10% chance by Sep 12
Growing season128 daysfrost-free, median

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

From the NOAA station Big Sandy, MT — 9 miles away, elevation 2,769 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 59520

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Apr 5 – Apr 19
SpinachSow outdoors · Apr 5 – Apr 26
Broccoli, cabbageStart indoors · Mar 8 – Mar 22
Broccoli, cabbageTransplant · Apr 26 – May 10
Onion setsPlant outdoors · Apr 19 – May 3
PotatoesPlant outdoors · Apr 26 – May 10
LettuceSow outdoors · Apr 19 – May 3
Carrots, beetsSow outdoors · Apr 26 – May 10
TomatoesStart indoors · Mar 22 – Apr 5
TomatoesTransplant · May 24 – Jun 7
Peppers, eggplantStart indoors · Mar 8 – Mar 22
Peppers, eggplantTransplant · May 31 – Jun 14
BeansSow outdoors · May 24 – Jun 7
Sweet cornSow outdoors · May 24 – Jun 7
Cucumbers, squashSow outdoors · May 24 – Jun 7
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 17) 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 59520 came on Mar 21 — 23 days earlier than the 1991–2020 average (Apr 13).

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 4a in Montana

439 plants native to the lower 48 states, recorded in Montana, and rated by the USDA to survive −30 °F or colder — the bottom of zone 4a. Each links to its USDA PLANTS profile.

Trees 21

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 77

alderleaf buckthorn3 ft · blooms late spring · low shade tolerance · to −33 °Falderleaf mountain mahogany15 ft · blooms late spring · medium shade tolerance · to −41 °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 °FBarclay's willow15 ft · high shade tolerance · to −43 °F

Wildflowers & perennials 189

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 bistort1.5 ft · blooms mid spring · high shade tolerance · to −38 °FAmerican licorice4 ft · blooms late spring · medium shade tolerance · to −38 °F

Grasses 141

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 Big Sandy, MT.

Soil in Big Sandy, MT

The soil mapped at the center of ZIP 59520 is Hillon — loam, well drained.

TextureLoama good balance for most vegetables
pH7.8slightly alkaline; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter1.5%low — add compost
Hydrologic groupCwater soaks in slowly
Farmland classNot prime farmlandUSDA classification

Map unit: Hillon-Joplin loams, 8 to 25 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 59520; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Big Sandy, MT

Summer high86 °Faverage high, July
Winter low10 °Faverage low, January
Hot days28days a year at 90 °F or more
Freezing nights180nights a year at 32 °F or less
Growing degree days2,205base 50 °F — the season’s usable heat
Longest day16 h 03 mindaylight on June 21
Shortest day8 h 22 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation13.3 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 31 °F, low 10 °F 31°10° JanFebruary: average high 35 °F, low 13 °F 35°13° FebMarch: average high 46 °F, low 22 °F 46°22° MarApril: average high 58 °F, low 33 °F 58°33° AprMay: average high 68 °F, low 41 °F 68°41° MayJune: average high 76 °F, low 49 °F 76°49° JunJuly: average high 86 °F, low 55 °F 86°55° JulAugust: average high 85 °F, low 53 °F 85°53° AugSeptember: average high 74 °F, low 44 °F 74°44° SepOctober: average high 59 °F, low 33 °F 59°33° OctNovember: average high 44 °F, low 22 °F 44°22° NovDecember: average high 34 °F, low 13 °F 34°13° Dec

Precipitation by month inches

0″1″2″3″4″January: 0.45 in precipitation 0.5 JanFebruary: 0.39 in precipitation 0.4 FebMarch: 0.47 in precipitation 0.5 MarApril: 1.27 in precipitation 1.3 AprMay: 2.07 in precipitation 2.1 MayJune: 3.12 in precipitation 3.1 JunJuly: 1.40 in precipitation 1.4 JulAugust: 0.99 in precipitation 1.0 AugSeptember: 1.16 in precipitation 1.2 SepOctober: 0.93 in precipitation 0.9 OctNovember: 0.60 in precipitation 0.6 NovDecember: 0.43 in precipitation 0.4 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Big Sandy, MT (9 mi away); precipitation: Big Sandy, MT (9 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

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

595214a
Box Elder, MT · 18 mi
594604a
Loma, MT · 27 mi
594464b
Geraldine, MT · 29 mi
595354b
Lloyd, MT · 32 mi
594424a
Fort Benton, MT · 34 mi
595324a
Kremlin, MT · 39 mi
594304b
Denton, MT · 43 mi
594894b
Winifred, MT · 43 mi

Montana planting zones →

Questions about 59520

What planting zone is ZIP code 59520?

ZIP code 59520 (Big Sandy, MT) is in USDA hardiness zone 4a on the 2023 map: the coldest night of an average winter reaches −30 to −25 °F (−34.4 to −31.7 °C).

When is the last frost in Big Sandy, MT?

The median last spring frost (32 °F) near 59520 is May 17; after June 4 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Big Sandy station.

When is the first frost in Big Sandy, MT?

The median first fall frost near 59520 is September 25, giving a median frost-free growing season of 128 days.

When should I plant tomatoes in 59520?

Start tomato seeds indoors around Mar 22 – Apr 5 and set transplants outside around May 24 – Jun 7, one to three weeks after the median last frost.

How much rain and snow does 59520 get?

About 13.3 inches of precipitation a year (NOAA 1991–2020 normals, Big Sandy station).

How hot do summers get in Big Sandy, MT?

The average high peaks at 86 °F in July, with about 28 days a year at 90 °F or more.

What kind of soil does 59520 have?

The USDA soil survey maps Hillon at the center of ZIP 59520: loam, well drained, pH about 7.8 (slightly alkaline).

Did spring come early in Big Sandy, MT in 2026?

Spring leaf-out near 59520 came on Mar 21 in 2026, 23 days earlier than the 1991–2020 average (Apr 13), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Big Sandy, MT?

Long-day onions: the longest day near 59520 is 16 h 03 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).