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

Planting zone for 99128 — Farmington, WA

ZIP code 99128 (Farmington, Whitman County) is in USDA zone 6b: the coldest winter night averages −5 to 0 °F. The median last spring frost is May 15 and the first fall frost September 21.

Where ZIP 99128 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 frostMay 15

First frostSep 21

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 Farmington, 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 Farmington, WA

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostMay 15median — 50% chance of a later frost

Safe for tender plantsJune 2only 10% chance of frost after

First fall frostSeptember 21median — 10% chance by Aug 31

Growing season126 daysfrost-free, median

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

From the NOAA station Potlatch 3 Nne, ID — 14 miles away, elevation 2,759 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 99128

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Apr 3 – Apr 17

SpinachSow outdoors · Apr 3 – Apr 24

Broccoli, cabbageStart indoors · Mar 6 – Mar 20

Broccoli, cabbageTransplant · Apr 24 – May 8

Onion setsPlant outdoors · Apr 17 – May 1

PotatoesPlant outdoors · Apr 24 – May 8

LettuceSow outdoors · Apr 17 – May 1

Carrots, beetsSow outdoors · Apr 24 – May 8

TomatoesStart indoors · Mar 20 – Apr 3

TomatoesTransplant · May 22 – Jun 5

Peppers, eggplantStart indoors · Mar 6 – Mar 20

Peppers, eggplantTransplant · May 29 – Jun 12

BeansSow outdoors · May 22 – Jun 5

Sweet cornSow outdoors · May 22 – Jun 5

Cucumbers, squashSow outdoors · May 22 – Jun 5

Fall lettuce, spinachSow outdoors · Jul 27 – Aug 10

GarlicPlant cloves · Sep 7 – Oct 12

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (May 15) and first fall frost (Sep 21) 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 99128 came on Mar 23 — 14 days earlier than the 1991–2020 average (Apr 6).

average
Apr 62026
Mar 23

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 Farmington, WA.

Soil in Farmington, WA

The soil mapped at the center of ZIP 99128 is Thatuna — silt loam, moderately well drained.

TextureSilt loama good balance for most vegetables

pH6.5neutral; most vegetables like 6.0–7.0

DrainageModerately well drainednatural drainage class

Organic matter4.5%moderate

Hydrologic groupCwater soaks in slowly

Farmland classFarmland of statewide importanceUSDA classification

Map unit: Thatuna silt loam, 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 99128; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Farmington, WA

Summer high84 °Faverage high, August

Winter low25 °Faverage low, December

Hot days17days a year at 90 °F or more

Freezing nights153nights a year at 32 °F or less

Growing degree days1,676base 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)

Precipitation24.5 inper year, rain and melted snow

Snowfall41 inper year

Average high and low, by month

Precipitation by month inches · ❄ = snowfall

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Potlatch 3 Nne, ID (14 mi away); precipitation: Potlatch 3 Nne, ID (14 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 6b.

Washington planting zones →

Questions about 99128

What planting zone is ZIP code 99128?

ZIP code 99128 (Farmington, 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 Farmington, WA?

The median last spring frost (32 °F) near 99128 is May 15; after June 2 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Potlatch 3 Nne station.

When is the first frost in Farmington, WA?

The median first fall frost near 99128 is September 21, giving a median frost-free growing season of 126 days.

When should I plant tomatoes in 99128?

Start tomato seeds indoors around Mar 20 – Apr 3 and set transplants outside around May 22 – Jun 5, one to three weeks after the median last frost.

How much rain and snow does 99128 get?

About 24.5 inches of precipitation a year and 41 inches of snowfall (NOAA 1991–2020 normals, Potlatch 3 Nne station).

How hot do summers get in Farmington, WA?

The average high peaks at 84 °F in August, with about 17 days a year at 90 °F or more.

What kind of soil does 99128 have?

The USDA soil survey maps Thatuna at the center of ZIP 99128: silt loam, moderately well drained, pH about 6.5 (neutral).

Did spring come early in Farmington, WA in 2026?

Spring leaf-out near 99128 came on Mar 23 in 2026, 14 days earlier than the 1991–2020 average (Apr 6), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Farmington, WA?

Long-day onions: the longest day near 99128 is 15 h 55 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 99128 change?

Yes. The 2012 USDA map put 99128 in zone 6a; the 2023 map puts it in zone 6b. The newer map uses 1991–2020 data and more weather stations.