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

Planting zone for 59623 — Helena, MT

ZIP code 59623 (Helena, Lewis and Clark County) is in USDA zone 5a: the coldest winter night averages −20 to −15 °F. The median last spring frost is May 9 and the first fall frost September 30.

Where ZIP 59623 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 frostMay 9
First frostSep 30
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 Helena, 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 Helena, MT

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMay 9median — 50% chance of a later frost
Safe for tender plantsMay 25only 10% chance of frost after
First fall frostSeptember 30median — 10% chance by Sep 17
Growing season142 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 29 and returns around October 7.

From the NOAA station Helena Regional Airport, MT — 4 miles away, elevation 3,829 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 59623

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 28 – Apr 11
SpinachSow outdoors · Mar 28 – Apr 18
Broccoli, cabbageStart indoors · Feb 28 – Mar 14
Broccoli, cabbageTransplant · Apr 18 – May 2
Onion setsPlant outdoors · Apr 11 – Apr 25
PotatoesPlant outdoors · Apr 18 – May 2
LettuceSow outdoors · Apr 11 – Apr 25
Carrots, beetsSow outdoors · Apr 18 – May 2
TomatoesStart indoors · Mar 14 – Mar 28
TomatoesTransplant · May 16 – May 30
Peppers, eggplantStart indoors · Feb 28 – Mar 14
Peppers, eggplantTransplant · May 23 – Jun 6
BeansSow outdoors · May 16 – May 30
Sweet cornSow outdoors · May 16 – May 30
Cucumbers, squashSow outdoors · May 16 – May 30
Fall lettuce, spinachSow outdoors · Aug 5 – Aug 19
GarlicPlant cloves · Sep 16 – Oct 21
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 9) and first fall frost (Sep 30) 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 59623 came on Mar 24 — 24 days earlier than the 1991–2020 average (Apr 17).

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.

Trees 26

American elm120 ft · blooms early spring · medium shade tolerance · to −46 °Fbalsam poplar80 ft · blooms late spring · high shade tolerance · to −79 °Fblack locust60 ft · blooms spring · high shade tolerance · to −37 °Fboxelder60 ft · blooms early spring · low shade tolerance · to −46 °FDouglas-fir200 ft · blooms mid spring · medium shade tolerance · to −33 °Feastern cottonwood190 ft · blooms spring · high shade tolerance · to −43 °F

Shrubs 96

alderleaf buckthorn3 ft · blooms late spring · low shade tolerance · to −33 °Falderleaf mountain mahogany15 ft · blooms late spring · medium shade tolerance · to −41 °Falpine laurel1 ft · blooms late spring · low shade tolerance · to −28 °FAmerican black currant3 ft · blooms early spring · to −46 °FAmerican plum24 ft · blooms mid spring · high shade tolerance · to −38 °Fantelope bitterbrush6 ft · blooms early spring · medium shade tolerance · to −36 °F

Wildflowers & perennials 253

alkali buttercup1 ft · blooms late spring · low shade tolerance · to −38 °Falpine bistort1 ft · blooms mid spring · high shade tolerance · to −38 °Falpine mountainsorrel1 ft · blooms mid summer · medium shade tolerance · to −43 °Falpine sweetvetch2 ft · blooms early summer · high shade tolerance · to −62 °FAmerican alpine speedwell1 ft · blooms summer · medium shade tolerance · to −28 °FAmerican bistort1.5 ft · blooms mid spring · high shade tolerance · to −38 °F

Grasses 161

Alaska oniongrass3 ft · blooms mid spring · low shade tolerance · to −33 °Falkali cordgrass3 ft · blooms late summer · high shade tolerance · to −38 °Falkali sacaton3 ft · blooms mid summer · high shade tolerance · to −38 °FAlpine Meadow-Foxtail0.5 ft · blooms mid spring · medium shade tolerance · to −33 °Falpine timothy1.4 ft · blooms late spring · medium shade tolerance · to −43 °Fanalogue sedge1.5 ft · blooms late spring · medium shade tolerance · to −38 °F

Vines 3

American bittersweet15 ft · blooms spring · low shade tolerance · to −38 °Friverbank grape45 ft · blooms late spring · medium shade tolerance · to −33 °Fwestern white clematis1 ft · blooms mid summer · medium shade tolerance · to −38 °F

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 Helena, MT.

Soil in Helena, MT

The soil mapped at the center of ZIP 59623 is Crago — gravelly loam, well drained.

TextureGravelly loama good balance for most vegetables
pH7.9alkaline; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter2%moderate
Hydrologic groupBwater soaks in moderately
Farmland classNot prime farmlandUSDA classification

Map unit: Crago-Musselshell gravelly loams, 4 to 35 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 59623; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Helena, MT

Summer high86 °Faverage high, July
Winter low14 °Faverage low, January
Hot days27days a year at 90 °F or more
Freezing nights170nights a year at 32 °F or less
Growing degree days2,130base 50 °F — the season’s usable heat
Longest day15 h 50 mindaylight on June 21
Shortest day8 h 34 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation14.4 inper year, rain and melted snow

Average high and low, by month

0°10°20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 32 °F, low 14 °F 32°14° JanFebruary: average high 37 °F, low 17 °F 37°17° FebMarch: average high 48 °F, low 25 °F 48°25° MarApril: average high 57 °F, low 32 °F 57°32° AprMay: average high 66 °F, low 42 °F 66°42° MayJune: average high 75 °F, low 49 °F 75°49° JunJuly: average high 86 °F, low 55 °F 86°55° JulAugust: average high 85 °F, low 53 °F 85°53° AugSeptember: average high 73 °F, low 45 °F 73°45° SepOctober: average high 58 °F, low 34 °F 58°34° OctNovember: average high 43 °F, low 23 °F 43°23° NovDecember: average high 33 °F, low 14 °F 33°14° Dec

Precipitation by month inches

0″1″2″3″January: 0.46 in precipitation 0.5 JanFebruary: 0.66 in precipitation 0.7 FebMarch: 0.69 in precipitation 0.7 MarApril: 1.25 in precipitation 1.3 AprMay: 2.47 in precipitation 2.5 MayJune: 2.71 in precipitation 2.7 JunJuly: 1.25 in precipitation 1.3 JulAugust: 1.35 in precipitation 1.4 AugSeptember: 1.33 in precipitation 1.3 SepOctober: 0.96 in precipitation 1.0 OctNovember: 0.68 in precipitation 0.7 NovDecember: 0.62 in precipitation 0.6 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Helena Regional Airport, MT (4 mi away); precipitation: Helena 2.0 Nnw, MT (2 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.

596205a
Helena, MT · <1 mi
596245a
Helena, MT · <1 mi
596264b
Helena, MT · <1 mi
596364b
Fort Harrison, MT · 4 mi
596044b
Helena, MT · 6 mi
596014b
Helena, MT · 9 mi
596344b
Clancy, MT · 10 mi
596355a
East Helena, MT · 11 mi

All 7 ZIP codes in Helena → · Montana planting zones →

Questions about 59623

What planting zone is ZIP code 59623?

ZIP code 59623 (Helena, 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 Helena, MT?

The median last spring frost (32 °F) near 59623 is May 9; after May 25 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Helena Regional Airport station.

When is the first frost in Helena, MT?

The median first fall frost near 59623 is September 30, giving a median frost-free growing season of 142 days.

When should I plant tomatoes in 59623?

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

How much rain and snow does 59623 get?

About 14.4 inches of precipitation a year (NOAA 1991–2020 normals, Helena 2.0 Nnw station).

How hot do summers get in Helena, MT?

The average high peaks at 86 °F in July, with about 27 days a year at 90 °F or more.

What kind of soil does 59623 have?

The USDA soil survey maps Crago at the center of ZIP 59623: gravelly loam, well drained, pH about 7.9 (alkaline).

Did spring come early in Helena, MT in 2026?

Spring leaf-out near 59623 came on Mar 24 in 2026, 24 days earlier than the 1991–2020 average (Apr 17), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Helena, MT?

Long-day onions: the longest day near 59623 is 15 h 50 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 59623 change?

Yes. The 2012 USDA map put 59623 in zone 4b; the 2023 map puts it in zone 5a. The newer map uses 1991–2020 data and more weather stations.