Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated Oct 29, 2026

Planting zone for 59231 — Saint Marie, MT

ZIP code 59231 (Saint Marie, Valley County) is in USDA zone 4a: the coldest winter night averages −30 to −25 °F. The median last spring frost is May 13 and the first fall frost September 27.

Where ZIP 59231 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 13

First frostSep 27

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 Saint Marie, 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 Saint Marie, MT

JanFebMarAprMayJunJulAugSepOctNovDec

Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free

Last spring frostMay 13median — 50% chance of a later frost

Safe for tender plantsMay 26only 10% chance of frost after

First fall frostSeptember 27median — 10% chance by Sep 13

Growing season134 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 3 and returns around October 5.

From the NOAA station St Marie, MT — within 1 mile, elevation 2,756 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 59231

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Apr 1 – Apr 15

SpinachSow outdoors · Apr 1 – Apr 22

Broccoli, cabbageStart indoors · Mar 4 – Mar 18

Broccoli, cabbageTransplant · Apr 22 – May 6

Onion setsPlant outdoors · Apr 15 – Apr 29

PotatoesPlant outdoors · Apr 22 – May 6

LettuceSow outdoors · Apr 15 – Apr 29

Carrots, beetsSow outdoors · Apr 22 – May 6

TomatoesStart indoors · Mar 18 – Apr 1

TomatoesTransplant · May 20 – Jun 3

Peppers, eggplantStart indoors · Mar 4 – Mar 18

Peppers, eggplantTransplant · May 27 – Jun 10

BeansSow outdoors · May 20 – Jun 3

Sweet cornSow outdoors · May 20 – Jun 3

Cucumbers, squashSow outdoors · May 20 – Jun 3

Fall lettuce, spinachSow outdoors · Aug 2 – Aug 16

GarlicPlant cloves · Sep 13 – Oct 18

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 13) and first fall frost (Sep 27) 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 59231 came on Apr 21 — about the same time as the 1991–2020 average (Apr 19).

average
Apr 192026
Apr 21

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.

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 Saint Marie, MT.

Soil in Saint Marie, MT

The soil mapped at the center of ZIP 59231 is Thoeny — loam, well drained.

TextureLoama good balance for most vegetables

pH6.7neutral; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter1.5%low — add compost

Hydrologic groupDwater soaks in very slowly

Farmland classNot prime farmlandUSDA classification

Map unit: Thoeny-Phillips complex, 1 to 5 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 59231; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Saint Marie, MT

Summer high85 °Faverage high, July

Winter low6 °Faverage low, January

Hot days21days a year at 90 °F or more

Freezing nights179nights a year at 32 °F or less

Growing degree days2,220base 50 °F — the season’s usable heat

Longest day16 h 07 mindaylight on June 21

Shortest day8 h 19 mindaylight on December 21

Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)

Precipitation11.8 inper year, rain and melted snow

Average high and low, by month

Precipitation by month inches

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: St Marie, MT (within 1 mile); precipitation: St Marie, MT (within 1 mile). Daylight is calculated from latitude.

Nearby ZIP codes

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

Montana planting zones →

Questions about 59231

What planting zone is ZIP code 59231?

ZIP code 59231 (Saint Marie, 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 Saint Marie, MT?

The median last spring frost (32 °F) near 59231 is May 13; after May 26 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, St Marie station.

When is the first frost in Saint Marie, MT?

The median first fall frost near 59231 is September 27, giving a median frost-free growing season of 134 days.

When should I plant tomatoes in 59231?

Start tomato seeds indoors around Mar 18 – Apr 1 and set transplants outside around May 20 – Jun 3, one to three weeks after the median last frost.

How much rain and snow does 59231 get?

About 11.8 inches of precipitation a year (NOAA 1991–2020 normals, St Marie station).

How hot do summers get in Saint Marie, MT?

The average high peaks at 85 °F in July, with about 21 days a year at 90 °F or more.

What kind of soil does 59231 have?

The USDA soil survey maps Thoeny at the center of ZIP 59231: loam, well drained, pH about 6.7 (neutral).

Did spring come early in Saint Marie, MT in 2026?

Spring leaf-out near 59231 came on Apr 21 in 2026, about the same as the 1991–2020 average (Apr 19), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Saint Marie, MT?

Long-day onions: the longest day near 59231 is 16 h 07 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).