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

Planting zone for 59843 — Helmville, MT

ZIP code 59843 (Helmville, Powell County) is in USDA zone 5a: the coldest winter night averages −20 to −15 °F. The median last spring frost is June 9 and the first fall frost September 8.

Where ZIP 59843 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 9

First frostSep 8

2012 zone4a

Was 4a on the 2012 map — a full 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 Helmville, 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 Helmville, MT

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostJune 9median — 50% chance of a later frost

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

First fall frostSeptember 8median — 10% chance by Aug 26

Growing season91 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 16 and returns around September 20.

From the NOAA station Elliston, MT — 34 miles away, elevation 5,079 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 59843

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Apr 28 – May 12

SpinachSow outdoors · Apr 28 – May 19

Broccoli, cabbageStart indoors · Mar 31 – Apr 14

Broccoli, cabbageTransplant · May 19 – Jun 2

Onion setsPlant outdoors · May 12 – May 26

PotatoesPlant outdoors · May 19 – Jun 2

LettuceSow outdoors · May 12 – May 26

Carrots, beetsSow outdoors · May 19 – Jun 2

TomatoesStart indoors · Apr 14 – Apr 28

TomatoesTransplant · Jun 16 – Jun 30

Peppers, eggplantStart indoors · Mar 31 – Apr 14

Peppers, eggplantTransplant · Jun 23 – Jul 7

BeansSow outdoors · Jun 16 – Jun 30

Sweet cornSow outdoors · Jun 16 – Jun 30

Cucumbers, squashSow outdoors · Jun 16 – Jun 30

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 9) 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 59843 came on Apr 18 — 7 days earlier than the 1991–2020 average (Apr 25).

average
Apr 252026
Apr 18

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 Helmville, MT.

Soil in Helmville, MT

The soil mapped at the center of ZIP 59843 is Kleinschmidt — gravelly loam, somewhat poorly drained.

TextureGravelly loama good balance for most vegetables

pH7.2neutral; most vegetables like 6.0–7.0

DrainageSomewhat poorly drainedconsider raised beds

Organic matter4%moderate

Hydrologic groupCwater soaks in slowly

Farmland classPrime farmland if irrigatedUSDA classification

Map unit: Kleinschmidt gravelly loam, cool, 0 to 4 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 59843; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Helmville, MT

Summer high80 °Faverage high, July

Winter low8 °Faverage low, January

Hot days10days a year at 90 °F or more

Freezing nights247nights a year at 32 °F or less

Growing degree days998base 50 °F — the season’s usable heat

Longest day15 h 52 mindaylight on June 21

Shortest day8 h 31 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: Ovando 9 Sse, MT (3 mi away); precipitation: Ovando 9 Sse, MT (3 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.

598325b

Drummond, MT · 16 mi

597334b

Gold Creek, MT · 21 mi

Montana planting zones →

Questions about 59843

What planting zone is ZIP code 59843?

ZIP code 59843 (Helmville, 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 Helmville, MT?

The median last spring frost (32 °F) near 59843 is June 9; after June 26 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Elliston station.

When is the first frost in Helmville, MT?

The median first fall frost near 59843 is September 8, giving a median frost-free growing season of 91 days.

When should I plant tomatoes in 59843?

Start tomato seeds indoors around Apr 14 – Apr 28 and set transplants outside around Jun 16 – Jun 30, one to three weeks after the median last frost.

How much rain and snow does 59843 get?

About 11.8 inches of precipitation a year (NOAA 1991–2020 normals, Ovando 9 Sse station).

How hot do summers get in Helmville, MT?

The average high peaks at 80 °F in July, with about 10 days a year at 90 °F or more.

What kind of soil does 59843 have?

The USDA soil survey maps Kleinschmidt at the center of ZIP 59843: gravelly loam, somewhat poorly drained, pH about 7.2 (neutral).

Did spring come early in Helmville, MT in 2026?

Spring leaf-out near 59843 came on Apr 18 in 2026, 7 days earlier than the 1991–2020 average (Apr 25), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Helmville, MT?

Long-day onions: the longest day near 59843 is 15 h 52 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 59843 change?

Yes. The 2012 USDA map put 59843 in zone 4a; the 2023 map puts it in zone 5a. The newer map uses 1991–2020 data and more weather stations.