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

Planting zone for 99320 — Benton City, WA

ZIP code 99320 (Benton City, Benton County) is in USDA zone 7b: the coldest winter night averages 5 to 10 °F. The median last spring frost is April 6 and the first fall frost October 24.

Where ZIP 99320 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 6
First frostOct 24
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 Benton City, 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 Benton City, WA

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostApril 6median — 50% chance of a later frost
Safe for tender plantsApril 24only 10% chance of frost after
First fall frostOctober 24median — 10% chance by Oct 10
Growing season200 daysfrost-free, median

A hard freeze (28 °F) usually ends around March 18 and returns around November 4.

From the NOAA station Richland, WA — 12 miles away, elevation 374 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 99320

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Feb 23 – Mar 9
SpinachSow outdoors · Feb 23 – Mar 16
Broccoli, cabbageStart indoors · Jan 26 – Feb 9
Broccoli, cabbageTransplant · Mar 16 – Mar 30
Onion setsPlant outdoors · Mar 9 – Mar 23
PotatoesPlant outdoors · Mar 16 – Mar 30
LettuceSow outdoors · Mar 9 – Mar 23
Carrots, beetsSow outdoors · Mar 16 – Mar 30
TomatoesStart indoors · Feb 9 – Feb 23
TomatoesTransplant · Apr 13 – Apr 27
Peppers, eggplantStart indoors · Jan 26 – Feb 9
Peppers, eggplantTransplant · Apr 20 – May 4
BeansSow outdoors · Apr 13 – Apr 27
Sweet cornSow outdoors · Apr 13 – Apr 27
Cucumbers, squashSow outdoors · Apr 13 – Apr 27
Fall lettuce, spinachSow outdoors · Aug 29 – Sep 12
GarlicPlant cloves · Oct 10 – Nov 14
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 6) and first fall frost (Oct 24) 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 99320 came on Mar 6 — 8 days earlier than the 1991–2020 average (Mar 14).

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 Benton City, WA.

Soil in Benton City, WA

The soil mapped at the center of ZIP 99320 is Kiona — very stony silt loam, well drained.

TextureVery stony silt loama good balance for most vegetables
pH7.6slightly alkaline; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter0.75%low — add compost
Hydrologic groupBwater soaks in moderately
Farmland classNot prime farmlandUSDA classification

Map unit: Kiona very stony silt loam, 0 to 30 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 99320; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Benton City, WA

Summer high89 °Faverage high, July
Winter low28 °Faverage low, December
Hot days39days a year at 90 °F or more
Freezing nights91nights a year at 32 °F or less
Growing degree days3,125base 50 °F — the season’s usable heat
Longest day15 h 48 mindaylight on June 21
Shortest day8 h 36 mindaylight on December 21
Onions to growLong-daysummer days reach 15+ hours, enough for long-day varieties (intermediate types work too)
Precipitation8.6 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 41 °F, low 29 °F 41°29° JanFebruary: average high 48 °F, low 30 °F 48°30° FebMarch: average high 57 °F, low 35 °F 57°35° MarApril: average high 65 °F, low 41 °F 65°41° AprMay: average high 74 °F, low 48 °F 74°48° MayJune: average high 80 °F, low 54 °F 80°54° JunJuly: average high 89 °F, low 60 °F 89°60° JulAugust: average high 88 °F, low 59 °F 88°59° AugSeptember: average high 79 °F, low 51 °F 79°51° SepOctober: average high 64 °F, low 42 °F 64°42° OctNovember: average high 49 °F, low 34 °F 49°34° NovDecember: average high 40 °F, low 28 °F 40°28° Dec

Precipitation by month inches

0″1″2″January: 1.17 in precipitation 1.2 JanFebruary: 0.78 in precipitation 0.8 FebMarch: 0.70 in precipitation 0.7 MarApril: 0.67 in precipitation 0.7 AprMay: 0.84 in precipitation 0.8 MayJune: 0.76 in precipitation 0.8 JunJuly: 0.18 in precipitation 0.2 JulAugust: 0.23 in precipitation 0.2 AugSeptember: 0.29 in precipitation 0.3 SepOctober: 0.71 in precipitation 0.7 OctNovember: 1.04 in precipitation 1.0 NovDecember: 1.26 in precipitation 1.3 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Richland, WA (12 mi away); precipitation: Richland 0.8 Sw, WA (10 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 7b.

993537b
West Richland, WA · 6 mi
993547b
Richland, WA · 10 mi
993527b
Richland, WA · 12 mi
989447b
Sunnyside, WA · 15 mi
993387b
Kennewick, WA · 15 mi
993507b
Prosser, WA · 17 mi
989307b
Grandview, WA · 19 mi
993467b
Plymouth, WA · 23 mi

Washington planting zones →

Questions about 99320

What planting zone is ZIP code 99320?

ZIP code 99320 (Benton City, 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 Benton City, WA?

The median last spring frost (32 °F) near 99320 is April 6; after April 24 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Richland station.

When is the first frost in Benton City, WA?

The median first fall frost near 99320 is October 24, giving a median frost-free growing season of 200 days.

When should I plant tomatoes in 99320?

Start tomato seeds indoors around Feb 9 – Feb 23 and set transplants outside around Apr 13 – Apr 27, one to three weeks after the median last frost.

How much rain and snow does 99320 get?

About 8.6 inches of precipitation a year (NOAA 1991–2020 normals, Richland 0.8 Sw station).

How hot do summers get in Benton City, WA?

The average high peaks at 89 °F in July, with about 39 days a year at 90 °F or more.

What kind of soil does 99320 have?

The USDA soil survey maps Kiona at the center of ZIP 99320: very stony silt loam, well drained, pH about 7.6 (slightly alkaline).

Did spring come early in Benton City, WA in 2026?

Spring leaf-out near 99320 came on Mar 6 in 2026, 8 days earlier than the 1991–2020 average (Mar 14), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Benton City, WA?

Long-day onions: the longest day near 99320 is 15 h 48 min, so summer days reach 15+ hours, enough for long-day varieties (intermediate types work too).

Did the zone for 99320 change?

Yes. The 2012 USDA map put 99320 in zone 7a; the 2023 map puts it in zone 7b. The newer map uses 1991–2020 data and more weather stations.