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

Planting zone for 99631 — Moose Pass, AK

ZIP code 99631 (Moose Pass, Kenai Peninsula County) is in USDA zone 5a: the coldest winter night averages −20 to −15 °F. The median last spring frost is May 31 and the first fall frost September 8.

Where ZIP 99631 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 31
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 Moose Pass, AK…

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 Moose Pass, AK

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMay 31median — 50% chance of a later frost
Safe for tender plantsJune 15only 10% chance of frost after
First fall frostSeptember 8median — 10% chance by Aug 24
Growing season99 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 14 and returns around September 22.

From the NOAA station Moose Pass 3nw, AK — 10 miles away, elevation 492 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 99631

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Apr 19 – May 3
SpinachSow outdoors · Apr 19 – May 10
Broccoli, cabbageStart indoors · Mar 22 – Apr 5
Broccoli, cabbageTransplant · May 10 – May 24
Onion setsPlant outdoors · May 3 – May 17
PotatoesPlant outdoors · May 10 – May 24
LettuceSow outdoors · May 3 – May 17
Carrots, beetsSow outdoors · May 10 – May 24
TomatoesStart indoors · Apr 5 – Apr 19
TomatoesTransplant · Jun 7 – Jun 21
Peppers, eggplantStart indoors · Mar 22 – Apr 5
Peppers, eggplantTransplant · Jun 14 – Jun 28
BeansSow outdoors · Jun 7 – Jun 21
Sweet cornSow outdoors · Jun 7 – Jun 21
Cucumbers, squashSow outdoors · Jun 7 – Jun 21
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 (May 31) 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.

Native plants for zone 5a in Alaska

275 plants native to Alaska, recorded in Alaska, and rated by the USDA to survive −20 °F or colder — the bottom of zone 5a. Each links to its USDA PLANTS profile.

Trees 14

Alaska cedar75 ft · blooms mid spring · low shade tolerance · to −22 °Fbalsam poplar80 ft · blooms late spring · high shade tolerance · to −79 °Fblack spruce65 ft · blooms late spring · low shade tolerance · to −79 °Flodgepole pine99 ft · blooms mid summer · high shade tolerance · to −70 °FPacific silver fir165 ft · blooms late spring · low shade tolerance · to −28 °Fpaper birch70 ft · blooms mid spring · high shade tolerance · to −62 °F

Shrubs 46

alpine bearberry0.5 ft · blooms early spring · medium shade tolerance · to −62 °Falpine laurel1 ft · blooms late spring · low shade tolerance · to −28 °FBarclay's willow15 ft · high shade tolerance · to −43 °Fblack crowberry1 ft · blooms mid summer · high shade tolerance · to −43 °Fblack hawthorn15 ft · blooms mid spring · medium shade tolerance · to −47 °Fbog blueberry2 ft · blooms early summer · medium shade tolerance · to −38 °F

Wildflowers & perennials 119

alkali buttercup1 ft · blooms late spring · low shade tolerance · to −38 °Falpine aster0.3 ft · blooms early summer · medium shade tolerance · to −28 °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 °F

Grasses 87

Alaska oniongrass3 ft · blooms mid spring · low shade tolerance · to −33 °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 °Farctic bluegrass2 ft · high shade tolerance · to −62 °Fawlfruit sedge4 ft · blooms mid spring · low shade tolerance · to −38 °Fbeaked sedge4 ft · blooms late spring · low 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 Moose Pass, AK.

Soil in Moose Pass, AK

The soil mapped at the center of ZIP 99631 is E23-boreal alpine dwarf scrub-gravelly colluvial slopes — slightly decomposed plant material, well drained.

TextureSlightly decomposed plant materialtop layer
pH4.0strongly acidic; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter75%high
Hydrologic groupBwater soaks in moderately
Farmland classNot prime farmlandUSDA classification

Map unit: Cook Inlet Mountains-Boreal Alpine-Barren Mountains.

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 99631; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Moose Pass, AK

Summer high66 °Faverage high, July
Winter low9 °Faverage low, January
Hot days0days a year at 90 °F or more
Freezing nights197nights a year at 32 °F or less
Growing degree days471base 50 °F — the season’s usable heat
Longest day19 h 06 mindaylight on June 21
Shortest day5 h 40 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation30.7 inper year, rain and melted snow

Average high and low, by month

0°10°20°30°40°50°60°70°80°32 °FJanuary: average high 24 °F, low 9 °F 24°9° JanFebruary: average high 29 °F, low 13 °F 29°13° FebMarch: average high 35 °F, low 14 °F 35°14° MarApril: average high 45 °F, low 27 °F 45°27° AprMay: average high 55 °F, low 34 °F 55°34° MayJune: average high 63 °F, low 42 °F 63°42° JunJuly: average high 66 °F, low 47 °F 66°47° JulAugust: average high 64 °F, low 45 °F 64°45° AugSeptember: average high 55 °F, low 38 °F 55°38° SepOctober: average high 42 °F, low 27 °F 42°27° OctNovember: average high 30 °F, low 17 °F 30°17° NovDecember: average high 27 °F, low 12 °F 27°12° Dec

Precipitation by month inches

0″1″2″3″4″5″January: 2.31 in precipitation 2.3 JanFebruary: 2.16 in precipitation 2.2 FebMarch: 1.71 in precipitation 1.7 MarApril: 1.26 in precipitation 1.3 AprMay: 1.75 in precipitation 1.8 MayJune: 1.31 in precipitation 1.3 JunJuly: 2.07 in precipitation 2.1 JulAugust: 3.17 in precipitation 3.2 AugSeptember: 4.48 in precipitation 4.5 SepOctober: 4.12 in precipitation 4.1 OctNovember: 2.95 in precipitation 3.0 NovDecember: 3.38 in precipitation 3.4 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Moose Pass 3nw, AK (10 mi away); precipitation: Summit Creek, AK (4 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

4 of the 8 nearest ZIP codes are in a different zone — elevation and distance from water change winter lows quickly.

995725a
Cooper Landing, AK · 12 mi
996055b
Hope, AK · 20 mi
995405b
Indian, AK · 31 mi
996645b
Seward, AK · 32 mi
995115a
Anchorage, AK · 33 mi
995165b
Anchorage, AK · 33 mi
995075a
Anchorage, AK · 36 mi
995155a
Anchorage, AK · 37 mi

Alaska planting zones →

Questions about 99631

What planting zone is ZIP code 99631?

ZIP code 99631 (Moose Pass, AK) 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 Moose Pass, AK?

The median last spring frost (32 °F) near 99631 is May 31; after June 15 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Moose Pass 3nw station.

When is the first frost in Moose Pass, AK?

The median first fall frost near 99631 is September 8, giving a median frost-free growing season of 99 days.

When should I plant tomatoes in 99631?

Start tomato seeds indoors around Apr 5 – Apr 19 and set transplants outside around Jun 7 – Jun 21, one to three weeks after the median last frost.

How much rain and snow does 99631 get?

About 30.7 inches of precipitation a year (NOAA 1991–2020 normals, Summit Creek station).

How hot do summers get in Moose Pass, AK?

The average high peaks at 66 °F in July, with about 0 days a year at 90 °F or more.

What kind of soil does 99631 have?

The USDA soil survey maps E23-boreal alpine dwarf scrub-gravelly colluvial slopes at the center of ZIP 99631: slightly decomposed plant material, well drained, pH about 4.0 (strongly acidic).

What kind of onions grow in Moose Pass, AK?

Long-day onions: the longest day near 99631 is 19 h 06 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 99631 change?

Yes. The 2012 USDA map put 99631 in zone 4a; the 2023 map puts it in zone 5a. The newer map uses 1991–2020 data and more weather stations.