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

Planting zone for 99733 — Circle, AK

ZIP code 99733 (Circle, Yukon-Koyukuk (CA) County) is in USDA zone 1a: the coldest winter night averages −60 to −55 °F. The median last spring frost is May 25 and the first fall frost August 26.

Where ZIP 99733 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.

Zone1a−60 to −55 °F

Coldest night−51.1 to −48.3 °C

Last frostMay 25

First frostAug 26

2012 zone1b

Was 1b on the 2012 map — half a zone colder now. The zone is the one at the center of the ZIP code; hilly ZIPs can span two zones. What zone 1a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Circle, 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 Circle, AK

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostMay 25median — 50% chance of a later frost

Safe for tender plantsJune 6only 10% chance of frost after

First fall frostAugust 26median — 10% chance by Aug 12

Growing season92 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 14 and returns around September 5.

From the NOAA station Central #2, AK — 22 miles away, elevation 919 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 99733

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Apr 13 – Apr 27

SpinachSow outdoors · Apr 13 – May 4

Broccoli, cabbageStart indoors · Mar 16 – Mar 30

Broccoli, cabbageTransplant · May 4 – May 18

Onion setsPlant outdoors · Apr 27 – May 11

PotatoesPlant outdoors · May 4 – May 18

LettuceSow outdoors · Apr 27 – May 11

Carrots, beetsSow outdoors · May 4 – May 18

TomatoesStart indoors · Mar 30 – Apr 13

TomatoesTransplant · Jun 1 – Jun 15

Peppers, eggplantStart indoors · Mar 16 – Mar 30

Peppers, eggplantTransplant · Jun 8 – Jun 22

BeansSow outdoors · Jun 1 – Jun 15

Sweet cornSow outdoors · Jun 1 – Jun 15

Cucumbers, squashSow outdoors · Jun 1 – Jun 15

Fall lettuce, spinachSow outdoors · Jul 1 – Jul 15

GarlicPlant cloves · Aug 12 – Sep 16

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 25) and first fall frost (Aug 26) 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 1a in Alaska

37 plants native to Alaska, recorded in Alaska, and rated by the USDA to survive −60 °F or colder — the bottom of zone 1a. 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 Circle, AK.

Soil in Circle, AK

The soil mapped at the center of ZIP 99733 is Albert — slightly decomposed plant material, well drained.

TextureSlightly decomposed plant materialtop layer

DrainageWell drainednatural drainage class

Organic matter75%high

Hydrologic groupCwater soaks in slowly

Farmland classNot prime farmlandUSDA classification

Map unit: Albert-Eastcrazy-Westcrazy complex, 1 to 8 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 99733; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Circle, AK

Summer high73 °Faverage high, July

Winter low−24 °Faverage low, January

Hot days0days a year at 90 °F or more

Freezing nights254nights a year at 32 °F or less

Growing degree days859base 50 °F — the season’s usable heat

Longest day24 h 00 mindaylight on June 21

Shortest day2 h 58 mindaylight on December 21

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

Precipitation12.1 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: Central #2, AK (22 mi away); precipitation: Circle Hot Springs, AK (24 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

2 of the 3 nearest ZIP codes are in a different zone — elevation and distance from water change winter lows quickly.

997303b

Central, AK · 41 mi

997881a

Chalkyitsik, AK · 60 mi

997382b

Eagle, AK · 63 mi

Alaska planting zones →

Questions about 99733

What planting zone is ZIP code 99733?

ZIP code 99733 (Circle, AK) is in USDA hardiness zone 1a on the 2023 map: the coldest night of an average winter reaches −60 to −55 °F (−51.1 to −48.3 °C).

When is the last frost in Circle, AK?

The median last spring frost (32 °F) near 99733 is May 25; after June 6 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Central #2 station.

When is the first frost in Circle, AK?

The median first fall frost near 99733 is August 26, giving a median frost-free growing season of 92 days.

When should I plant tomatoes in 99733?

Start tomato seeds indoors around Mar 30 – Apr 13 and set transplants outside around Jun 1 – Jun 15, one to three weeks after the median last frost.

How much rain and snow does 99733 get?

About 12.1 inches of precipitation a year (NOAA 1991–2020 normals, Circle Hot Springs station).

How hot do summers get in Circle, AK?

The average high peaks at 73 °F in July, with about 0 days a year at 90 °F or more.

What kind of soil does 99733 have?

The USDA soil survey maps Albert at the center of ZIP 99733: slightly decomposed plant material, well drained.

What kind of onions grow in Circle, AK?

Long-day onions: the longest day near 99733 is 24 h 00 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 99733 change?

Yes. The 2012 USDA map put 99733 in zone 1b; the 2023 map puts it in zone 1a. The newer map uses 1991–2020 data and more weather stations.