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

Planting zone for 99346 — Plymouth, WA

ZIP code 99346 (Plymouth, Benton County) is in USDA zone 7b: the coldest winter night averages 5 to 10 °F. The median last spring frost is March 27 and the first fall frost November 4.

Where ZIP 99346 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.

Zone7b5 to 10 °F

Coldest night−15 to −12.2 °C

Last frostMar 27

First frostNov 4

2012 zone7b

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 7b means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Plymouth, 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 Plymouth, WA

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostMarch 27median — 50% chance of a later frost

Safe for tender plantsApril 14only 10% chance of frost after

First fall frostNovember 4median — 10% chance by Oct 21

Growing season221 daysfrost-free, median

A hard freeze (28 °F) usually ends around March 6 and returns around November 18.

From the NOAA station Mcnary Dam, WA — 6 miles away, elevation 361 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 99346

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Feb 13 – Feb 27

SpinachSow outdoors · Feb 13 – Mar 6

Broccoli, cabbageStart indoors · Jan 16 – Jan 30

Broccoli, cabbageTransplant · Mar 6 – Mar 20

Onion setsPlant outdoors · Feb 27 – Mar 13

PotatoesPlant outdoors · Mar 6 – Mar 20

LettuceSow outdoors · Feb 27 – Mar 13

Carrots, beetsSow outdoors · Mar 6 – Mar 20

TomatoesStart indoors · Jan 30 – Feb 13

TomatoesTransplant · Apr 3 – Apr 17

Peppers, eggplantStart indoors · Jan 16 – Jan 30

Peppers, eggplantTransplant · Apr 10 – Apr 24

BeansSow outdoors · Apr 3 – Apr 17

Sweet cornSow outdoors · Apr 3 – Apr 17

Cucumbers, squashSow outdoors · Apr 3 – Apr 17

Fall lettuce, spinachSow outdoors · Sep 9 – Sep 23

GarlicPlant cloves · Oct 21 – Nov 25

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Mar 27) and first fall frost (Nov 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 99346 came on Mar 8 — 6 days earlier than the 1991–2020 average (Mar 14).

average
Mar 142026
Mar 8

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 7b in Washington

592 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 7b. 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 Plymouth, WA.

Soil in Plymouth, WA

The soil mapped at the center of ZIP 99346 is Burke — very fine sandy loam, well drained.

TextureVery fine sandy loamdrains and warms fast; dries out and needs more feeding

pH7.9alkaline; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter1.5%low — add compost

Hydrologic groupCwater soaks in slowly

Farmland classFarmland of statewide importanceUSDA classification

Map unit: Burke very fine sandy loam, 0 to 15 percent slopes, eroded.

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 99346; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Plymouth, WA

Summer high89 °Faverage high, July

Winter low30 °Faverage low, January

Hot days34days a year at 90 °F or more

Freezing nights67nights a year at 32 °F or less

Growing degree days3,180base 50 °F — the season’s usable heat

Longest day15 h 45 mindaylight on June 21

Shortest day8 h 38 mindaylight on December 21

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

Precipitation7.8 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: Mcnary Dam, WA (6 mi away); precipitation: Mcnary Dam, WA (6 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

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

978827b

Umatilla, OR · 9 mi

993377b

Kennewick, WA · 10 mi

993387b

Kennewick, WA · 10 mi

978387b

Hermiston, OR · 12 mi

993367b

Kennewick, WA · 14 mi

Stanfield, OR · 17 mi

Washington planting zones →

Questions about 99346

What planting zone is ZIP code 99346?

ZIP code 99346 (Plymouth, WA) is in USDA hardiness zone 7b on the 2023 map: the coldest night of an average winter reaches 5 to 10 °F (−15 to −12.2 °C).

When is the last frost in Plymouth, WA?

The median last spring frost (32 °F) near 99346 is March 27; after April 14 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Mcnary Dam station.

When is the first frost in Plymouth, WA?

The median first fall frost near 99346 is November 4, giving a median frost-free growing season of 221 days.

When should I plant tomatoes in 99346?

Start tomato seeds indoors around Jan 30 – Feb 13 and set transplants outside around Apr 3 – Apr 17, one to three weeks after the median last frost.

How much rain and snow does 99346 get?

About 7.8 inches of precipitation a year (NOAA 1991–2020 normals, Mcnary Dam station).

How hot do summers get in Plymouth, WA?

The average high peaks at 89 °F in July, with about 34 days a year at 90 °F or more.

What kind of soil does 99346 have?

The USDA soil survey maps Burke at the center of ZIP 99346: very fine sandy loam, well drained, pH about 7.9 (alkaline).

Did spring come early in Plymouth, WA in 2026?

Spring leaf-out near 99346 came on Mar 8 in 2026, 6 days earlier than the 1991–2020 average (Mar 14), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Plymouth, WA?

Long-day onions: the longest day near 99346 is 15 h 45 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).