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

Planting zone for 59923 — Libby, MT

ZIP code 59923 (Libby, Lincoln County) is in USDA zone 6a: the coldest winter night averages −10 to −5 °F. The median last spring frost is May 11 and the first fall frost October 1.

Where ZIP 59923 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.

Zone6a−10 to −5 °F

Coldest night−23.3 to −20.6 °C

Last frostMay 11

First frostOct 1

2012 zone5b

Was 5b 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 6a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Libby, 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 Libby, MT

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostMay 11median — 50% chance of a later frost

Safe for tender plantsMay 27only 10% chance of frost after

First fall frostOctober 1median — 10% chance by Sep 14

Growing season142 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 28 and returns around October 16.

From the NOAA station Libby Dam (Base), MT — 10 miles away, elevation 2,110 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 59923

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Mar 30 – Apr 13

SpinachSow outdoors · Mar 30 – Apr 20

Broccoli, cabbageStart indoors · Mar 2 – Mar 16

Broccoli, cabbageTransplant · Apr 20 – May 4

Onion setsPlant outdoors · Apr 13 – Apr 27

PotatoesPlant outdoors · Apr 20 – May 4

LettuceSow outdoors · Apr 13 – Apr 27

Carrots, beetsSow outdoors · Apr 20 – May 4

TomatoesStart indoors · Mar 16 – Mar 30

TomatoesTransplant · May 18 – Jun 1

Peppers, eggplantStart indoors · Mar 2 – Mar 16

Peppers, eggplantTransplant · May 25 – Jun 8

BeansSow outdoors · May 18 – Jun 1

Sweet cornSow outdoors · May 18 – Jun 1

Cucumbers, squashSow outdoors · May 18 – Jun 1

Fall lettuce, spinachSow outdoors · Aug 6 – Aug 20

GarlicPlant cloves · Sep 17 – Oct 22

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 11) and first fall frost (Oct 1) 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 59923 came on Apr 9 — 12 days earlier than the 1991–2020 average (Apr 21).

average
Apr 212026
Apr 9

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 6a in Montana

572 plants native to the lower 48 states, recorded in Montana, and rated by the USDA to survive −10 °F or colder — the bottom of zone 6a. 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 Libby, MT.

Soil in Libby, MT

The soil mapped at the center of ZIP 59923 is Andic Dystric Eutrochrept — slightly decomposed plant material, moderately well drained.

TextureSlightly decomposed plant materialtop layer

pH6.5neutral; most vegetables like 6.0–7.0

DrainageModerately well drainednatural drainage class

Organic matter50%high

Hydrologic groupBwater soaks in moderately

Farmland classNot prime farmlandUSDA classification

Map unit: Andic Dystric Eutrochrepts, lacustrine terraces.

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 59923; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Libby, MT

Summer high85 °Faverage high, July

Winter low22 °Faverage low, January

Hot days24days a year at 90 °F or more

Freezing nights178nights a year at 32 °F or less

Growing degree days1,864base 50 °F — the season’s usable heat

Longest day16 h 05 mindaylight on June 21

Shortest day8 h 20 mindaylight on December 21

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

Precipitation17.1 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: Libby Dam (Base), MT (10 mi away); precipitation: Libby Dam (Base), MT (10 mi away). 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.

Trout Creek, MT · 35 mi

Montana planting zones →

Questions about 59923

What planting zone is ZIP code 59923?

ZIP code 59923 (Libby, MT) is in USDA hardiness zone 6a on the 2023 map: the coldest night of an average winter reaches −10 to −5 °F (−23.3 to −20.6 °C).

When is the last frost in Libby, MT?

The median last spring frost (32 °F) near 59923 is May 11; after May 27 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Libby Dam (Base) station.

When is the first frost in Libby, MT?

The median first fall frost near 59923 is October 1, giving a median frost-free growing season of 142 days.

When should I plant tomatoes in 59923?

Start tomato seeds indoors around Mar 16 – Mar 30 and set transplants outside around May 18 – Jun 1, one to three weeks after the median last frost.

How much rain and snow does 59923 get?

About 17.1 inches of precipitation a year (NOAA 1991–2020 normals, Libby Dam (Base) station).

How hot do summers get in Libby, MT?

The average high peaks at 85 °F in July, with about 24 days a year at 90 °F or more.

What kind of soil does 59923 have?

The USDA soil survey maps Andic Dystric Eutrochrept at the center of ZIP 59923: slightly decomposed plant material, moderately well drained, pH about 6.5 (neutral).

Did spring come early in Libby, MT in 2026?

Spring leaf-out near 59923 came on Apr 9 in 2026, 12 days earlier than the 1991–2020 average (Apr 21), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Libby, MT?

Long-day onions: the longest day near 59923 is 16 h 05 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 59923 change?

Yes. The 2012 USDA map put 59923 in zone 5b; the 2023 map puts it in zone 6a. The newer map uses 1991–2020 data and more weather stations.