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

Planting zone for 59631 — Basin, MT

ZIP code 59631 (Basin, 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 59631 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 zone4b

Was 4b on the 2012 map — half a 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 Basin, 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 Basin, 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 — 14 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 59631

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 59631 came on Apr 28 — 12 days earlier than the 1991–2020 average (May 10).

average
May 102026
Apr 28

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.

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 Basin, MT.

Soil in Basin, MT

The soil mapped at the center of ZIP 59631 is Elve — slightly decomposed plant material, somewhat excessively drained.

TextureSlightly decomposed plant materialtop layer

pH6.5neutral; most vegetables like 6.0–7.0

DrainageSomewhat excessively drainednatural drainage class

Organic matter50%high

Hydrologic groupAwater soaks in fast

Farmland classNot prime farmlandUSDA classification

Map unit: Elve-Vision families-Rock outcrop complex, low relief mountain slopes and ridges.

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 59631; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Basin, 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 48 mindaylight on June 21

Shortest day8 h 36 mindaylight on December 21

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

Precipitation29.5 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: Boulder, MT (14 mi away); precipitation: Rocker Peak, MT (8 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

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

597284b

Elliston, MT · 15 mi

596384b

Jefferson City, MT · 18 mi

Montana planting zones →

Questions about 59631

What planting zone is ZIP code 59631?

ZIP code 59631 (Basin, 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 Basin, MT?

The median last spring frost (32 °F) near 59631 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 Basin, MT?

The median first fall frost near 59631 is September 8, giving a median frost-free growing season of 90 days.

When should I plant tomatoes in 59631?

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 59631 get?

About 29.5 inches of precipitation a year (NOAA 1991–2020 normals, Rocker Peak station).

How hot do summers get in Basin, 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 59631 have?

The USDA soil survey maps Elve at the center of ZIP 59631: slightly decomposed plant material, somewhat excessively drained, pH about 6.5 (neutral).

Did spring come early in Basin, MT in 2026?

Spring leaf-out near 59631 came on Apr 28 in 2026, 12 days earlier than the 1991–2020 average (May 10), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Basin, MT?

Long-day onions: the longest day near 59631 is 15 h 48 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 59631 change?

Yes. The 2012 USDA map put 59631 in zone 4b; the 2023 map puts it in zone 5a. The newer map uses 1991–2020 data and more weather stations.