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

Planting zone for 99176 — Thornton, WA

ZIP code 99176 (Thornton, Whitman County) is in USDA zone 6b: the coldest winter night averages −5 to 0 °F. The median last spring frost is April 29 and the first fall frost October 4.

Where ZIP 99176 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 frostApr 29

First frostOct 4

2012 zone6b

Same zone as on the 2012 map. 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 Thornton, WA…

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 Thornton, WA

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostApril 29median — 50% chance of a later frost

Safe for tender plantsMay 15only 10% chance of frost after

First fall frostOctober 4median — 10% chance by Sep 19

Growing season161 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 5 and returns around October 20.

From the NOAA station St John, WA — 8 miles away, elevation 1,952 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 99176

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Mar 18 – Apr 1

SpinachSow outdoors · Mar 18 – Apr 8

Broccoli, cabbageStart indoors · Feb 18 – Mar 4

Broccoli, cabbageTransplant · Apr 8 – Apr 22

Onion setsPlant outdoors · Apr 1 – Apr 15

PotatoesPlant outdoors · Apr 8 – Apr 22

LettuceSow outdoors · Apr 1 – Apr 15

Carrots, beetsSow outdoors · Apr 8 – Apr 22

TomatoesStart indoors · Mar 4 – Mar 18

TomatoesTransplant · May 6 – May 20

Peppers, eggplantStart indoors · Feb 18 – Mar 4

Peppers, eggplantTransplant · May 13 – May 27

BeansSow outdoors · May 6 – May 20

Sweet cornSow outdoors · May 6 – May 20

Cucumbers, squashSow outdoors · May 6 – May 20

Fall lettuce, spinachSow outdoors · Aug 9 – Aug 23

GarlicPlant cloves · Sep 20 – Oct 25

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 29) and first fall frost (Oct 4) 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 99176 came on Mar 16 — 18 days earlier than the 1991–2020 average (Apr 3).

average
Apr 32026
Mar 16

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 Washington

581 plants native to the lower 48 states, recorded in Washington, 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 Thornton, WA.

Soil in Thornton, WA

The soil mapped at the center of ZIP 99176 is Palouse — silt loam, well drained.

TextureSilt loama good balance for most vegetables

pH6.5neutral; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter3%moderate

Hydrologic groupBwater soaks in moderately

Farmland classFarmland of statewide importanceUSDA classification

Map unit: Palouse-Thatuna silt loams, 7 to 25 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 99176; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Thornton, WA

Summer high84 °Faverage high, July

Winter low28 °Faverage low, December

Hot days28days a year at 90 °F or more

Freezing nights123nights a year at 32 °F or less

Growing degree days2,154base 50 °F — the season’s usable heat

Longest day15 h 55 mindaylight on June 21

Shortest day8 h 30 mindaylight on December 21

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

Precipitation17.1 inper year, rain and melted snow

Snowfall27 inper year

Average high and low, by month

Precipitation by month inches · ❄ = snowfall

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: St John, WA (8 mi away); precipitation: St John, WA (8 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 6b.

Saint John, WA · 11 mi

991306b

Garfield, WA · 12 mi

991286b

Farmington, WA · 16 mi

Washington planting zones →

Questions about 99176

What planting zone is ZIP code 99176?

ZIP code 99176 (Thornton, WA) 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 Thornton, WA?

The median last spring frost (32 °F) near 99176 is April 29; after May 15 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, St John station.

When is the first frost in Thornton, WA?

The median first fall frost near 99176 is October 4, giving a median frost-free growing season of 161 days.

When should I plant tomatoes in 99176?

Start tomato seeds indoors around Mar 4 – Mar 18 and set transplants outside around May 6 – May 20, one to three weeks after the median last frost.

How much rain and snow does 99176 get?

About 17.1 inches of precipitation a year and 27 inches of snowfall (NOAA 1991–2020 normals, St John station).

How hot do summers get in Thornton, WA?

The average high peaks at 84 °F in July, with about 28 days a year at 90 °F or more.

What kind of soil does 99176 have?

The USDA soil survey maps Palouse at the center of ZIP 99176: silt loam, well drained, pH about 6.5 (neutral).

Did spring come early in Thornton, WA in 2026?

Spring leaf-out near 99176 came on Mar 16 in 2026, 18 days earlier than the 1991–2020 average (Apr 3), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Thornton, WA?

Long-day onions: the longest day near 99176 is 15 h 55 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).