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

Planting zone for 99350 — Prosser, WA

ZIP code 99350 (Prosser, Benton County) is in USDA zone 7b: the coldest winter night averages 5 to 10 °F. The median last spring frost is April 14 and the first fall frost October 22.

Where ZIP 99350 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 frostApr 14
First frostOct 22
2012 zone7a

Was 7a 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 7b means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Prosser, 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 Prosser, WA

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostApril 14median — 50% chance of a later frost
Safe for tender plantsMay 7only 10% chance of frost after
First fall frostOctober 22median — 10% chance by Oct 5
Growing season189 daysfrost-free, median

A hard freeze (28 °F) usually ends around March 23 and returns around November 2.

From the NOAA station Prosser, WA — 4 miles away, elevation 830 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 99350

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 3 – Mar 17
SpinachSow outdoors · Mar 3 – Mar 24
Broccoli, cabbageStart indoors · Feb 3 – Feb 17
Broccoli, cabbageTransplant · Mar 24 – Apr 7
Onion setsPlant outdoors · Mar 17 – Mar 31
PotatoesPlant outdoors · Mar 24 – Apr 7
LettuceSow outdoors · Mar 17 – Mar 31
Carrots, beetsSow outdoors · Mar 24 – Apr 7
TomatoesStart indoors · Feb 17 – Mar 3
TomatoesTransplant · Apr 21 – May 5
Peppers, eggplantStart indoors · Feb 3 – Feb 17
Peppers, eggplantTransplant · Apr 28 – May 12
BeansSow outdoors · Apr 21 – May 5
Sweet cornSow outdoors · Apr 21 – May 5
Cucumbers, squashSow outdoors · Apr 21 – May 5
Fall lettuce, spinachSow outdoors · Aug 27 – Sep 10
GarlicPlant cloves · Oct 8 – Nov 12
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 14) and first fall frost (Oct 22) 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 99350 came on Mar 10 — 7 days earlier than the 1991–2020 average (Mar 17).

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.

Trees 31

Alaska cedar75 ft · blooms mid spring · low shade tolerance · to −22 °FAmerican sycamore100 ft · blooms spring · medium shade tolerance · to −34 °Fbalsam poplar80 ft · blooms late spring · high shade tolerance · to −79 °Fbigleaf maple60 ft · blooms mid spring · medium shade tolerance · to −14 °Fblack locust60 ft · blooms spring · high shade tolerance · to −37 °Fboxelder60 ft · blooms early spring · low shade tolerance · to −46 °F

Shrubs 105

alderleaf buckthorn3 ft · blooms late spring · low shade tolerance · to −33 °FAllegheny serviceberry35 ft · blooms early spring · low shade tolerance · to −38 °Falpine laurel1 ft · blooms late spring · low shade tolerance · to −28 °FAmerican plum24 ft · blooms mid spring · high shade tolerance · to −38 °Fantelope bitterbrush6 ft · blooms early spring · medium shade tolerance · to −36 °FBarclay's willow15 ft · high shade tolerance · to −43 °F

Wildflowers & perennials 284

alkali buttercup1 ft · blooms late spring · low shade tolerance · to −38 °Falpine bistort1 ft · blooms mid spring · high shade tolerance · to −38 °Falpine mountainsorrel1 ft · blooms mid summer · medium shade tolerance · to −43 °FAmerican alpine speedwell1 ft · blooms summer · medium shade tolerance · to −28 °FAmerican bistort1.5 ft · blooms mid spring · high shade tolerance · to −38 °FAmerican eelgrass1 ft · blooms mid summer · high shade tolerance · to −33 °F

Grasses 157

Alaska brome5 ft · blooms late spring · low shade tolerance · to −18 °FAlaska oniongrass3 ft · blooms mid spring · low shade tolerance · to −33 °Falkali cordgrass3 ft · blooms late summer · high shade tolerance · to −38 °Falkali sacaton3 ft · blooms mid summer · high shade tolerance · to −38 °Falpine timothy1.4 ft · blooms late spring · medium shade tolerance · to −43 °FAmerican beachgrass2.5 ft · blooms spring · high shade tolerance · to −28 °F

Vines 3

riverbank grape45 ft · blooms late spring · medium shade tolerance · to −33 °Ftrumpet creeper2 ft · blooms summer · high shade tolerance · to −13 °Fwestern white clematis1 ft · blooms mid summer · medium shade tolerance · to −38 °F

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 Prosser, WA.

Soil in Prosser, WA

The soil mapped at the center of ZIP 99350 is Willis — silt loam, well drained.

TextureSilt loama good balance for most vegetables
pH7.2neutral; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter1.5%low — add compost
Hydrologic groupCwater soaks in slowly
Farmland classPrime farmland if irrigatedUSDA classification

Map unit: Willis silt loam, 0 to 5 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 99350; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Prosser, WA

Summer high90 °Faverage high, July
Winter low26 °Faverage low, December
Hot days47days a year at 90 °F or more
Freezing nights109nights a year at 32 °F or less
Growing degree days3,098base 50 °F — the season’s usable heat
Longest day15 h 46 mindaylight on June 21
Shortest day8 h 37 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation8.7 inper year, rain and melted snow

Average high and low, by month

20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 40 °F, low 27 °F 40°27° JanFebruary: average high 47 °F, low 29 °F 47°29° FebMarch: average high 56 °F, low 35 °F 56°35° MarApril: average high 65 °F, low 40 °F 65°40° AprMay: average high 74 °F, low 48 °F 74°48° MayJune: average high 81 °F, low 54 °F 81°54° JunJuly: average high 90 °F, low 60 °F 90°60° JulAugust: average high 89 °F, low 58 °F 89°58° AugSeptember: average high 80 °F, low 50 °F 80°50° SepOctober: average high 65 °F, low 41 °F 65°41° OctNovember: average high 49 °F, low 32 °F 49°32° NovDecember: average high 40 °F, low 26 °F 40°26° Dec

Precipitation by month inches

0″1″2″January: 1.24 in precipitation 1.2 JanFebruary: 0.90 in precipitation 0.9 FebMarch: 0.69 in precipitation 0.7 MarApril: 0.74 in precipitation 0.7 AprMay: 0.88 in precipitation 0.9 MayJune: 0.68 in precipitation 0.7 JunJuly: 0.21 in precipitation 0.2 JulAugust: 0.16 in precipitation 0.2 AugSeptember: 0.27 in precipitation 0.3 SepOctober: 0.77 in precipitation 0.8 OctNovember: 0.86 in precipitation 0.9 NovDecember: 1.33 in precipitation 1.3 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Prosser, WA (4 mi away); precipitation: Prosser, WA (4 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 7b.

989307b
Grandview, WA · 11 mi
993457b
Paterson, WA · 15 mi
993207b
Benton City, WA · 17 mi
978447b
Irrigon, OR · 20 mi
993537b
West Richland, WA · 20 mi
993467b
Plymouth, WA · 22 mi
993387b
Kennewick, WA · 22 mi
993527b
Richland, WA · 22 mi

Washington planting zones →

Questions about 99350

What planting zone is ZIP code 99350?

ZIP code 99350 (Prosser, 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 Prosser, WA?

The median last spring frost (32 °F) near 99350 is April 14; after May 7 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Prosser station.

When is the first frost in Prosser, WA?

The median first fall frost near 99350 is October 22, giving a median frost-free growing season of 189 days.

When should I plant tomatoes in 99350?

Start tomato seeds indoors around Feb 17 – Mar 3 and set transplants outside around Apr 21 – May 5, one to three weeks after the median last frost.

How much rain and snow does 99350 get?

About 8.7 inches of precipitation a year (NOAA 1991–2020 normals, Prosser station).

How hot do summers get in Prosser, WA?

The average high peaks at 90 °F in July, with about 47 days a year at 90 °F or more.

What kind of soil does 99350 have?

The USDA soil survey maps Willis at the center of ZIP 99350: silt loam, well drained, pH about 7.2 (neutral).

Did spring come early in Prosser, WA in 2026?

Spring leaf-out near 99350 came on Mar 10 in 2026, 7 days earlier than the 1991–2020 average (Mar 17), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Prosser, WA?

Long-day onions: the longest day near 99350 is 15 h 46 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 99350 change?

Yes. The 2012 USDA map put 99350 in zone 7a; the 2023 map puts it in zone 7b. The newer map uses 1991–2020 data and more weather stations.