What does USDA hardiness zone 6b mean in Washington?

Zone 6b means an average annual extreme minimum winter temperature of −5 to 0 °F — the standard USDA measure of which perennials survive winter. 27 locations in Washington fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in Washington's zone 6b is the growing season longest?

Waterville runs the longest season on this page at about 187 days; Davenport is shortest at about 94 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.

When should I plant tomatoes in Washington's zone 6b?

Using Inchelium's frost dates (this page's median-season location): start seeds indoors about Mar 7 – Mar 21, then transplant outside about May 9 – May 16. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.

Does every location in Washington's zone 6b have the same frost dates?

No — they do not all share a weather station, so frost dates vary by a week or more across the 27 locations listed (see the table above for each location's own dates).

USDA zone 6b in Washington

−5 to 0 °F avg. annual extreme minimum · 27 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6b covers 27 locations across Washington (average annual extreme minimum −5 to 0 °F): Airway Heights, WA, Banks Lake South, WA, Danville, WA, Davenport, WA, Disautel, WA, Ephrata, WA, and 21 more. Average last spring frost across this group ranges April 17 (Waterville) to June 6 (Davenport), and growing seasons run 94–187 days (Davenport to Waterville) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Washington location's own frost dates; the printable planting calendar uses Inchelium, this group's median-season location.

USDA zone: 6b−5 to 0 °F
Last frost range: Apr 17–Jun 6avg, 32°F
First frost range: Sep 10–Oct 22avg, 32°F
Growing season range: 94–187days

Locations in this group

Every location below shares this zone and a nearby weather station, but keeps its own frost dates — find your town for its own numbers, or use the ZIP planner on the home page for an instant personalized answer.

Own frost dates per location.
Location	Also covers	Nearest station	Last frost	First frost	Season (days)
Airway Heights	Cheney, Four Lakes	Spokane Intl Ap	Apr 26	Oct 12	168
Banks Lake South	Hartline	Hartline	May 2	Oct 12	162
Danville	—	Republic	May 30	Sep 10	102
Davenport	—	Davenport	Jun 6	Sep 11	94
Disautel	Malott, Tonasket, Riverside, Conconully	Omak Ap	Apr 30	Oct 7	160
Ephrata	Wilson Creek	Ephrata Muni Ap	Apr 22	Oct 17	175
Green Bluff	Deer Park	Deer Park Ap	May 22	Sep 14	112
Inchelium *	Marcus, Boyds	Colville	May 2	Oct 2	153
Inchelium	Twin Lakes, Springdale, Valley, Addy	Chewelah	May 16	Sep 15	120
Kittitas	—	Ellensburg	May 8	Oct 2	146
Mansfield	—	Chief Joseph Dam	Apr 26	Oct 11	167
Nespelem	—	Coulee Dam 1 Sw	Apr 22	Oct 21	180
Northport	Orient, Laurier	Northport	May 8	Oct 1	143
Odessa	Krupp	Odessa	May 14	Sep 26	135
Pullman	Palouse	Pullman 2 Nw	May 6	Sep 29	143
Ritzville	—	Ritzville 1 Sse	May 7	Oct 10	153
Spokane	Spokane Valley, Otis Orchards-East Farms, Green Bluff, Liberty Lake, Fairwood, Hauser, +3 more	Spokane Felts Fld	Apr 26	Oct 15	170
Sprague	Reardan, Harrington	Harrington	May 31	Sep 12	101
Steptoe	St. John, Lamont	St John	Apr 29	Oct 4	161
Suncrest	Fairchild AFB, Medical Lake, Reardan	Spokane Wfo	May 10	Sep 26	137
Tekoa	Oakesdale, Malden, Rosalia, Fairfield, Waverly, Spangle, +1 more	Rosalia	May 7	Sep 28	144
Twisp	—	Methow 2 S	Apr 21	Oct 15	177
Twisp	Winthrop	Winthrop 1 Wsw	May 15	Sep 22	127
Waterville	—	Chelan	Apr 17	Oct 22	187
Wilbur	Almira, Creston	Wilbur	Jun 1	Sep 22	112
Wilson Creek	—	Moses Lake Grant Co Ap	Apr 27	Oct 8	165
Winthrop	—	Stehekin 4 Nw	Apr 27	Oct 17	173

* Inchelium is this group's median-season location — the calendar below is computed from its data.

Data: NOAA NCEI U.S. Climate Normals 1991–2020 (public domain) and USDA Plant Hardiness Zone Map 2023. Planting windows synthesized from U.S. Cooperative Extension guides.

Planting calendar (Inchelium, representative)

Computed from Inchelium's average frost dates — the median-season location in this Washingtonzone 6b group. hatched = start seeds indoors, solid green = plant out, teal = a fall sowing, and the terracotta dot marks the estimated first harvest. Ranges are extension-guide planning guidance, not guarantees — your own location's dates may run a week or two earlier or later (see the table above).

Crop

JanFebMarAprMayJunJulAugSepOctNovDec

Tomato

Pepper

Cucumber

Summer squash / zucchini

Start indoors
Plant out
Fall sowing
First harvest

Planting windows for Inchelium (Washington, zone 6b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 7 – Mar 21	May 9 – May 16	Jul 8 – Jul 28	—	matures comfortably
Pepper	Very tender	Feb 21 – Mar 7	May 16 – May 23	Jul 15 – Aug 14	—	matures comfortably
Cucumber	Tender	Apr 4 – Apr 11	May 9 – May 16	Jun 28 – Jul 18	—	matures comfortably
Summer squash / zucchini	Tender	—	May 9 – May 16	Jun 23 – Jul 8	—	matures comfortably
Bush bean	Tender	—	May 9 – May 16	Jun 28 – Jul 8	Aug 3 – Aug 13	matures comfortably
Sweet corn	Tender	—	May 2 – May 16	Jul 1 – Jul 31	—	matures comfortably
Basil	Very tender	Mar 21 – Apr 4	May 9 – May 16	Jun 8 – Jun 23	—	matures comfortably
Lettuce	Half-hardy	Mar 21 – Apr 4	Apr 4 – Apr 18	May 19 – Jun 3	Jul 20 – Aug 4	matures comfortably
Pea	Hardy	—	Mar 21 – Apr 4	May 15 – May 30	Jul 10 – Jul 25	matures comfortably
Spinach	Hardy	—	Mar 21 – Apr 4	Apr 30 – May 10	Jul 30 – Aug 9	matures comfortably
Carrot	Half-hardy	—	Apr 11 – Apr 18	Jun 10 – Jun 30	Jun 30 – Jul 20	matures comfortably
Broccoli	Half-hardy	Mar 7 – Mar 21	Apr 4 – Apr 18	May 29 – Jun 18	Jul 5 – Jul 25	matures comfortably

Which crops fit this growing season

Using Inchelium's own 153-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 12 of 12 crops mature here with comfortable margin. A different location in the table above with a longer or shorter season will get a different mix — the ZIP planner on the home page gives your own.

Frost & freeze dates (Inchelium)

From NOAA's 1991–2020 Climate Normals at station USC00451630. The median (p50) is the average date; the 90%-safe column is the date the freeze has passed in about 9 years out of 10 — the conservative date to plant after or harvest before.

Freeze probabilities by temperature threshold (MM/DD, NOAA 1991–2020), Inchelium.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 17	Jun 3	Sep 22	Oct 4	127
32°F (freeze)	May 2	May 17	Oct 2	Oct 17	153
28°F	Apr 15	May 2	Oct 14	Nov 2	183
24°F	Mar 22	Apr 14	Oct 28	Nov 18	217

Growing degree days

Growing degree days (GDD) accumulate warmth above a base temperature over the year — a better predictor of crop development than the calendar alone. This group's median GDD (base 50°F) across all 27 locations is 2,191; Inchelium's own reading is shown below.

Annual growing degree days for Inchelium (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	2,357	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	4,419	cool-season crops (brassicas, greens)

Hardiness zone 6b in Washington

Zone 6b means an average annual extreme minimum winter temperature of about −5 to 0 °F on the USDA's 2023 map — the number that tells you which perennials, shrubs, and trees reliably survive an average winter here. It does not set your planting dates, which come from the frost calendar above. Washington spans 8 hardiness zones in this dataset — see all Washington locations for the full range, including zones6a, 7a, 7b, 8a, 8b, 9a, 9b.

Explore zone 6b in other states at zone 6b, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 6b mean in Washington?: Zone 6b means an average annual extreme minimum winter temperature of −5 to 0 °F — the standard USDA measure of which perennials survive winter. 27 locations in Washington fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in Washington's zone 6b is the growing season longest?: Waterville runs the longest season on this page at about 187 days; Davenport is shortest at about 94 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.
When should I plant tomatoes in Washington's zone 6b?: Using Inchelium's frost dates (this page's median-season location): start seeds indoors about Mar 7 – Mar 21, then transplant outside about May 9 – May 16. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.
Does every location in Washington's zone 6b have the same frost dates?: No — they do not all share a weather station, so frost dates vary by a week or more across the 27 locations listed (see the table above for each location's own dates).

Sources & method

Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020, one reading per location's own nearest station (see the table above). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to each location's ZIP. Planting windows are computed by counting from the average last and first frost using per-crop offsets synthesized from U.S. Cooperative Extension guides — the full method and citations are on the methodology page.