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

Planting zone for 83631 — Idaho City, ID

ZIP code 83631 (Idaho City, Boise County) is in USDA zone 6b: the coldest winter night averages −5 to 0 °F. The median last spring frost is June 17 and the first fall frost September 5.

Where ZIP 83631 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.

Zone6b−5 to 0 °F

Coldest night−20.6 to −17.8 °C

Last frostJun 17

First frostSep 5

2012 zone6a

Was 6a 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 6b means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Idaho City, ID…

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 Idaho City, ID

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostJune 17median — 50% chance of a later frost

Safe for tender plantsJuly 4only 10% chance of frost after

First fall frostSeptember 5median — 10% chance by Aug 23

Growing season80 daysfrost-free, median

A hard freeze (28 °F) usually ends around May 22 and returns around September 19.

From the NOAA station Idaho City, ID — 4 miles away, elevation 3,967 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 83631

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · May 6 – May 20

SpinachSow outdoors · May 6 – May 27

Broccoli, cabbageStart indoors · Apr 8 – Apr 22

Broccoli, cabbageTransplant · May 27 – Jun 10

Onion setsPlant outdoors · May 20 – Jun 3

PotatoesPlant outdoors · May 27 – Jun 10

LettuceSow outdoors · May 20 – Jun 3

Carrots, beetsSow outdoors · May 27 – Jun 10

TomatoesStart indoors · Apr 22 – May 6

TomatoesTransplant · Jun 24 – Jul 8

Peppers, eggplantStart indoors · Apr 8 – Apr 22

Peppers, eggplantTransplant · Jul 1 – Jul 15

BeansSow outdoors · Jun 24 – Jul 8

Sweet cornSow outdoors · Jun 24 – Jul 8

Cucumbers, squashSow outdoors · Jun 24 – Jul 8

Fall lettuce, spinachSow outdoors · Jul 11 – Jul 25

GarlicPlant cloves · Aug 22 – Sep 26

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Jun 17) and first fall frost (Sep 5) 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 83631 came on Apr 4 — 14 days earlier than the 1991–2020 average (Apr 18).

average
Apr 182026
Apr 4

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 6b in Idaho

570 plants native to the lower 48 states, recorded in Idaho, and rated by the USDA to survive −5 °F or colder — the bottom of zone 6b. 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 Idaho City, ID.

Soil in Idaho City, ID

The soil mapped at the center of ZIP 83631 is Shirts — slightly decomposed plant material, somewhat excessively drained.

TextureSlightly decomposed plant materialtop layer

pH5.0strongly acidic; most vegetables like 6.0–7.0

DrainageSomewhat excessively drainednatural drainage class

Organic matter75%high

Hydrologic groupBwater soaks in moderately

Farmland classNot prime farmlandUSDA classification

Map unit: Shirts-Kosh-Charters complex, 15 to 50 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 83631; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Idaho City, ID

Summer high87 °Faverage high, July

Winter low14 °Faverage low, January

Hot days31days a year at 90 °F or more

Freezing nights224nights a year at 32 °F or less

Growing degree days1,586base 50 °F — the season’s usable heat

Longest day15 h 28 mindaylight on June 21

Shortest day8 h 54 mindaylight on December 21

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

Precipitation21.1 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: Idaho City, ID (4 mi away); precipitation: Idaho City, ID (4 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.

836666b

Placerville, ID · 11 mi

836226b

Garden Valley, ID · 16 mi

836297a

Horseshoe Bend, ID · 18 mi

Idaho planting zones →

Questions about 83631

What planting zone is ZIP code 83631?

ZIP code 83631 (Idaho City, ID) is in USDA hardiness zone 6b on the 2023 map: the coldest night of an average winter reaches −5 to 0 °F (−20.6 to −17.8 °C).

When is the last frost in Idaho City, ID?

The median last spring frost (32 °F) near 83631 is June 17; after July 4 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Idaho City station.

When is the first frost in Idaho City, ID?

The median first fall frost near 83631 is September 5, giving a median frost-free growing season of 80 days.

When should I plant tomatoes in 83631?

Start tomato seeds indoors around Apr 22 – May 6 and set transplants outside around Jun 24 – Jul 8, one to three weeks after the median last frost.

How much rain and snow does 83631 get?

About 21.1 inches of precipitation a year (NOAA 1991–2020 normals, Idaho City station).

How hot do summers get in Idaho City, ID?

The average high peaks at 87 °F in July, with about 31 days a year at 90 °F or more.

What kind of soil does 83631 have?

The USDA soil survey maps Shirts at the center of ZIP 83631: slightly decomposed plant material, somewhat excessively drained, pH about 5.0 (strongly acidic).

Did spring come early in Idaho City, ID in 2026?

Spring leaf-out near 83631 came on Apr 4 in 2026, 14 days earlier than the 1991–2020 average (Apr 18), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Idaho City, ID?

Long-day onions: the longest day near 83631 is 15 h 28 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 83631 change?

Yes. The 2012 USDA map put 83631 in zone 6a; the 2023 map puts it in zone 6b. The newer map uses 1991–2020 data and more weather stations.