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

Planting zone for 59218 — Culbertson, MT

ZIP code 59218 (Culbertson, Roosevelt County) is in USDA zone 4a: the coldest winter night averages −30 to −25 °F. The median last spring frost is May 21 and the first fall frost September 20.

Where ZIP 59218 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 21

First frostSep 20

2012 zone3b

Was 3b 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 4a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Culbertson, 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 Culbertson, MT

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostMay 21median — 50% chance of a later frost

Safe for tender plantsJune 3only 10% chance of frost after

First fall frostSeptember 20median — 10% chance by Sep 6

Growing season120 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 10 and returns around September 27.

From the NOAA station Culbertson, MT — 5 miles away, elevation 1,939 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 59218

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Apr 9 – Apr 23

SpinachSow outdoors · Apr 9 – Apr 30

Broccoli, cabbageStart indoors · Mar 12 – Mar 26

Broccoli, cabbageTransplant · Apr 30 – May 14

Onion setsPlant outdoors · Apr 23 – May 7

PotatoesPlant outdoors · Apr 30 – May 14

LettuceSow outdoors · Apr 23 – May 7

Carrots, beetsSow outdoors · Apr 30 – May 14

TomatoesStart indoors · Mar 26 – Apr 9

TomatoesTransplant · May 28 – Jun 11

Peppers, eggplantStart indoors · Mar 12 – Mar 26

Peppers, eggplantTransplant · Jun 4 – Jun 18

BeansSow outdoors · May 28 – Jun 11

Sweet cornSow outdoors · May 28 – Jun 11

Cucumbers, squashSow outdoors · May 28 – Jun 11

Fall lettuce, spinachSow outdoors · Jul 26 – Aug 9

GarlicPlant cloves · Sep 6 – Oct 11

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 21) and first fall frost (Sep 20) 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 59218 came on Apr 20 — 3 days later than the 1991–2020 average (Apr 17).

average
Apr 172026
Apr 20

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 Culbertson, MT.

Soil in Culbertson, MT

The soil mapped at the center of ZIP 59218 is Lambert — silt loam, well drained.

TextureSilt loama good balance for most vegetables

pH8.2alkaline; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter2%moderate

Hydrologic groupDwater soaks in very slowly

Farmland classNot prime farmlandUSDA classification

Map unit: Lambert-Dimyaw complex, 15 to 65 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 59218; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Culbertson, MT

Summer high87 °Faverage high, July

Winter low2 °Faverage low, January

Hot days31days a year at 90 °F or more

Freezing nights191nights a year at 32 °F or less

Growing degree days2,350base 50 °F — the season’s usable heat

Longest day16 h 03 mindaylight on June 21

Shortest day8 h 21 mindaylight on December 21

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

Precipitation14.5 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: Culbertson, MT (5 mi away); precipitation: Culbertson, MT (5 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.

592264a

Froid, MT · 19 mi

592124a

Bainville, MT · 19 mi

Homestead, MT · 23 mi

592434a

Lambert, MT · 24 mi

592473b

Medicine Lake, MT · 30 mi

588384a

Cartwright, ND · 35 mi

Montana planting zones →

Questions about 59218

What planting zone is ZIP code 59218?

ZIP code 59218 (Culbertson, 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 Culbertson, MT?

The median last spring frost (32 °F) near 59218 is May 21; after June 3 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Culbertson station.

When is the first frost in Culbertson, MT?

The median first fall frost near 59218 is September 20, giving a median frost-free growing season of 120 days.

When should I plant tomatoes in 59218?

Start tomato seeds indoors around Mar 26 – Apr 9 and set transplants outside around May 28 – Jun 11, one to three weeks after the median last frost.

How much rain and snow does 59218 get?

About 14.5 inches of precipitation a year (NOAA 1991–2020 normals, Culbertson station).

How hot do summers get in Culbertson, MT?

The average high peaks at 87 °F in July, with about 31 days a year at 90 °F or more.

What kind of soil does 59218 have?

The USDA soil survey maps Lambert at the center of ZIP 59218: silt loam, well drained, pH about 8.2 (alkaline).

Did spring come early in Culbertson, MT in 2026?

Spring leaf-out near 59218 came on Apr 20 in 2026, 3 days later than the 1991–2020 average (Apr 17), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Culbertson, MT?

Long-day onions: the longest day near 59218 is 16 h 03 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 59218 change?

Yes. The 2012 USDA map put 59218 in zone 3b; the 2023 map puts it in zone 4a. The newer map uses 1991–2020 data and more weather stations.