Last frost date in Helena, MT

Average last frost

May 9

Expect the last frost in Helena around May 9. Frost after May 25 happens in only one year out of ten. The first fall frost usually arrives about September 30, which leaves 142 frost-free days. An early freeze before September 17 happens one year in ten.

Safe after (10% risk): May 25
Average first frost: Sep 30
Frost-free season: 142 days
Hard freeze (28°F) ends: Apr 29

Next 7 nights in Helena

Hard freeze likely Fri night, low 24°F

National Weather Service forecast, updated Oct 22, 12:57 UTC

ThuOct 2242°
FriOct 2324°hard freeze
SatOct 2423°hard freeze
SunOct 2524°hard freeze
MonOct 2625°hard freeze
TueOct 2726°hard freeze
WedOct 2824°hard freeze

Frost risk through the year at Helena Regional Airport, 2.7 miles from Helena.

Planting calendar for Helena

Windows count forward or back from the average frost dates above. Click a crop for dates across Montana.

Start indoorsPlant outSow outdoorsFall plantingAverage last and first frost

Warm-season crops

JanFebMarAprMayJunJulAugSepOctNovDec

TomatoesMay 16–May 30

PumpkinsMay 23–Jun 6

SunflowersMay 9–May 30

CucumbersMay 23–May 30

CornMay 16–May 30

Sweet potatoesMay 30–Jun 13

WatermelonMay 23–Jun 6

Green beansMay 16–Jun 6

ZucchiniMay 16–May 30

OkraMay 30–Jun 13

PeppersMay 23–Jun 6

BasilMay 16–May 30

Butternut squashMay 16–May 30

CantaloupeMay 23–Jun 6

EggplantMay 23–Jun 6

Cool-season crops

JanFebMarAprMayJunJulAugSepOctNovDec

PotatoesApr 11–Apr 25

OnionsMar 28–Apr 18

CarrotsApr 18–May 23

LettuceApr 25–May 16

BroccoliApr 11–Apr 25

BeetsApr 11–May 23

PeasMar 28–Apr 11

CabbageApr 11–Apr 25

RadishesApr 4–May 16

Brussels sproutsApr 25–May 9

SpinachMar 28–Apr 18

CilantroApr 25–May 16

KaleApr 11–Apr 25

Collard greensApr 11–Apr 25

TurnipsApr 11–Apr 25

DillMay 9–May 23

CeleryMay 16–May 23

CauliflowerApr 25–May 9

LeeksApr 25–May 9

Swiss chardApr 25–May 23

ParsleyApr 25–May 16

Fall bulbs and garlic

JanFebMarAprMayJunJulAugSepOctNovDec

Tulip bulbsSep 16–Oct 21

GarlicSep 9–Oct 14

Daffodil bulbsSep 2–Oct 14

Perennials

JanFebMarAprMayJunJulAugSepOctNovDec

StrawberriesApr 11–Apr 25

AsparagusApr 11–Apr 25

Frost probabilities

Threshold	Spring, 90% chance later	Spring, 50%	Spring, 10%	Fall, 10% chance earlier	Fall, 50%	Fall, 90%
Light frost, 32°F	Apr 29	May 9	May 25	Sep 17	Sep 30	Oct 11
Hard freeze, 28°F	Apr 15	Apr 29	May 11	Sep 25	Oct 7	Oct 22

Read the spring 10% column as the date after which only one year in ten still gets frost. That is the safer date for tomatoes, peppers and basil.

Where these numbers come from

NOAA station Helena Regional Airport (USW00024144) sits 2.7 miles from the center of Helena at 3,828 feet. Its averages cover 1991 to 2020. Low spots, valleys and open fields can keep getting frost a week or two after the station stops; city centers and lakeshores usually get less of it.

Questions

When is the last frost in Helena, MT?

The average last frost date in Helena is May 9, based on NOAA 1991–2020 climate normals. There is a 10% chance of frost after May 25.

When is the first frost in Helena?

The first frost in fall usually comes around September 30. One year in ten it arrives before September 17.

How long is the growing season in Helena?

About 142 days between the average last and first frost.

When should I plant tomatoes in Helena?

Plant tomato seedlings outside from May 16 to May 30. Start seeds indoors six to eight weeks earlier.

Nearby in Montana

All 7 cities in Montana