What does USDA hardiness zone 6a mean in Washington?

Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 9 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 6a is the growing season longest?

Loomis runs the longest season on this page at about 168 days; Curlew Lake is shortest at about 102 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 6a?

Using Winthrop's frost dates (this page's median-season location): start seeds indoors about Mar 14 – Mar 28, then transplant outside about May 16 – May 23. 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 6a 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 9 locations listed (see the table above for each location's own dates).

USDA zone 6a in Washington

−10 to −5 °F avg. annual extreme minimum · 9 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6a covers 9 locations across Washington (average annual extreme minimum −10 to −5 °F): Clayton, WA, Colville, WA, Curlew Lake, WA, Ione, WA, Loomis, WA, Loon Lake, WA, and 3 more. Average last spring frost across this group ranges April 26 (Loomis) to May 30 (Curlew Lake), and growing seasons run 102–168 days (Curlew Lake to Loomis) — 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 Winthrop, this group's median-season location.

USDA zone: 6a−10 to −5 °F
Last frost range: Apr 26–May 30avg, 32°F
First frost range: Sep 10–Oct 10avg, 32°F
Growing season range: 102–168days

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)
Clayton	—	Deer Park Ap	May 22	Sep 14	112
Colville	—	Colville	May 2	Oct 2	153
Curlew Lake	Curlew, Pine Grove, Malo	Republic	May 30	Sep 10	102
Ione	Metaline, Metaline Falls	Boundary Dam	May 4	Oct 4	153
Loomis	—	Tonasket 4 Nne	Apr 26	Oct 10	168
Loon Lake	Chewelah	Chewelah	May 16	Sep 15	120
Newport	Oldtown, Cusick	Newport	May 12	Sep 21	130
Oroville	—	Tonasket 11ne	May 6	Oct 6	151
Winthrop *	—	Mazama	May 9	Sep 29	142

* Winthrop 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 (Winthrop, representative)

Computed from Winthrop's average frost dates — the median-season location in this Washingtonzone 6a 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 Winthrop (Washington, zone 6a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 14 – Mar 28	May 16 – May 23	Jul 15 – Aug 4	—	matures comfortably
Pepper	Very tender	Feb 28 – Mar 14	May 23 – May 30	Jul 22 – Aug 21	—	matures comfortably
Cucumber	Tender	Apr 11 – Apr 18	May 16 – May 23	Jul 5 – Jul 25	—	matures comfortably
Summer squash / zucchini	Tender	—	May 16 – May 23	Jun 30 – Jul 15	—	matures comfortably
Bush bean	Tender	—	May 16 – May 23	Jul 5 – Jul 15	Jul 31 – Aug 10	matures comfortably
Sweet corn	Tender	—	May 9 – May 23	Jul 8 – Aug 7	—	matures comfortably
Basil	Very tender	Mar 28 – Apr 11	May 16 – May 23	Jun 15 – Jun 30	—	matures comfortably
Lettuce	Half-hardy	Mar 28 – Apr 11	Apr 11 – Apr 25	May 26 – Jun 10	Jul 17 – Aug 1	matures comfortably
Pea	Hardy	—	Mar 28 – Apr 11	May 22 – Jun 6	Jul 7 – Jul 22	matures comfortably
Spinach	Hardy	—	Mar 28 – Apr 11	May 7 – May 17	Jul 27 – Aug 6	matures comfortably
Carrot	Half-hardy	—	Apr 18 – Apr 25	Jun 17 – Jul 7	Jun 27 – Jul 17	matures comfortably
Broccoli	Half-hardy	Mar 14 – Mar 28	Apr 11 – Apr 25	Jun 5 – Jun 25	Jul 2 – Jul 22	matures comfortably

Which crops fit this growing season

Using Winthrop's own 142-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 (Winthrop)

From NOAA's 1991–2020 Climate Normals at station USC00455133. 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), Winthrop.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 25	Jun 10	Sep 17	Sep 29	113
32°F (freeze)	May 9	May 24	Sep 29	Oct 10	142
28°F	Apr 25	May 9	Oct 10	Oct 25	170
24°F	Apr 3	Apr 18	Oct 23	Nov 9	203

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 9 locations is 1,839; Winthrop's own reading is shown below.

Annual growing degree days for Winthrop (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	2,072	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,885	cool-season crops (brassicas, greens)

Hardiness zone 6a in Washington

Zone 6a means an average annual extreme minimum winter temperature of about −10 to −5 °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 zones6b, 7a, 7b, 8a, 8b, 9a, 9b.

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

Frequently asked questions

What does USDA hardiness zone 6a mean in Washington?: Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 9 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 6a is the growing season longest?: Loomis runs the longest season on this page at about 168 days; Curlew Lake is shortest at about 102 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 6a?: Using Winthrop's frost dates (this page's median-season location): start seeds indoors about Mar 14 – Mar 28, then transplant outside about May 16 – May 23. 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 6a 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 9 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.