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

Planting zone for 59484 — Sweet Grass, MT

ZIP code 59484 (Sweet Grass, Toole 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 59484 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 Sweet Grass, 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 Sweet Grass, 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 plantsMay 31only 10% chance of frost after
First fall frostSeptember 25median — 10% chance by Sep 9
Growing season129 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 5 and returns around October 4.

From the NOAA station Sunburst 8 E, MT — 10 miles away, elevation 3,701 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 59484

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 59484 came on Apr 28 — 5 days earlier than the 1991–2020 average (May 3).

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).

Soil in Sweet Grass, MT

The soil mapped at the center of ZIP 59484 is Stemple — slightly decomposed plant material, well drained.

TextureSlightly decomposed plant materialtop layer
pH5.0strongly acidic; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter75%high
Hydrologic groupBwater soaks in moderately
Farmland classNot prime farmlandUSDA classification

Map unit: Stemple, low elevation-Rubble land complex, 25 to 70 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 59484; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Sweet Grass, MT

Summer high80 °Faverage high, July
Winter low10 °Faverage low, January
Hot days11days a year at 90 °F or more
Freezing nights190nights a year at 32 °F or less
Growing degree days1,418base 50 °F — the season’s usable heat
Longest day16 h 12 mindaylight on June 21
Shortest day8 h 14 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation14.8 inper year, rain and melted snow
Snowfall86 inper year

Average high and low, by month

0°10°20°30°40°50°60°70°80°90°32 °FJanuary: average high 33 °F, low 10 °F 33°10° JanFebruary: average high 35 °F, low 11 °F 35°11° FebMarch: average high 44 °F, low 19 °F 44°19° MarApril: average high 55 °F, low 27 °F 55°27° AprMay: average high 64 °F, low 36 °F 64°36° MayJune: average high 71 °F, low 43 °F 71°43° JunJuly: average high 80 °F, low 47 °F 80°47° JulAugust: average high 80 °F, low 46 °F 80°46° AugSeptember: average high 70 °F, low 39 °F 70°39° SepOctober: average high 55 °F, low 29 °F 55°29° OctNovember: average high 42 °F, low 19 °F 42°19° NovDecember: average high 34 °F, low 11 °F 34°11° Dec

Precipitation by month inches · ❄ = snowfall

0″1″2″3″4″January: 0.45 in precipitation, 12.4 in snowfall 0.5❄12 JanFebruary: 0.47 in precipitation, 12.9 in snowfall 0.5❄13 FebMarch: 0.63 in precipitation, 15.1 in snowfall 0.6❄15 MarApril: 1.31 in precipitation, 10.8 in snowfall 1.3❄11 AprMay: 1.89 in precipitation, 3.7 in snowfall 1.9❄4 MayJune: 3.53 in precipitation, 0.3 in snowfall 3.5 JunJuly: 1.25 in precipitation 1.3 JulAugust: 1.54 in precipitation, 0.4 in snowfall 1.5 AugSeptember: 1.41 in precipitation, 1.4 in snowfall 1.4❄1 SepOctober: 1.19 in precipitation, 5.2 in snowfall 1.2❄5 OctNovember: 0.64 in precipitation, 10.6 in snowfall 0.6❄11 NovDecember: 0.50 in precipitation, 13.1 in snowfall 0.5❄13 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Goldbutte 7 N, MT (6 mi away); precipitation: Goldbutte 7 N, MT (6 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 4a.

595454a
Whitlash, MT · 15 mi
594824a
Sunburst, MT · 16 mi
594664a
Oilmont, MT · 20 mi
594444a
Galata, MT · 23 mi
594544a
Kevin, MT · 24 mi
594744a
Shelby, MT · 31 mi
594354a
Ethridge, MT · 35 mi
594614a
Lothair, MT · 37 mi

Montana planting zones →

Questions about 59484

What planting zone is ZIP code 59484?

ZIP code 59484 (Sweet Grass, 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 Sweet Grass, MT?

The median last spring frost (32 °F) near 59484 is May 17; after May 31 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Sunburst 8 E station.

When is the first frost in Sweet Grass, MT?

The median first fall frost near 59484 is September 25, giving a median frost-free growing season of 129 days.

When should I plant tomatoes in 59484?

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 59484 get?

About 14.8 inches of precipitation a year and 86 inches of snowfall (NOAA 1991–2020 normals, Goldbutte 7 N station).

How hot do summers get in Sweet Grass, MT?

The average high peaks at 80 °F in July, with about 11 days a year at 90 °F or more.

What kind of soil does 59484 have?

The USDA soil survey maps Stemple at the center of ZIP 59484: slightly decomposed plant material, well drained, pH about 5.0 (strongly acidic).

Did spring come early in Sweet Grass, MT in 2026?

Spring leaf-out near 59484 came on Apr 28 in 2026, 5 days earlier than the 1991–2020 average (May 3), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Sweet Grass, MT?

Long-day onions: the longest day near 59484 is 16 h 12 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).