Western Washington’s last spring frost typically lands between February 27 and March 18, and the first fall frost arrives between November 13 and November 30, based on NOAA’s 1991-2020 Climate Normals for a reference station in the Seattle-Tacoma area. That’s a long, mild window by national standards. But it only describes the Puget Sound lowlands. Cross the Cascades into eastern Washington and the growing season shrinks fast, so treat this figure as a starting point, not a statewide rule.
The frost dates for Washington
The numbers above come from NOAA NCEI’s 1991-2020 Climate Normals, calculated for station USW00024233 in the Seattle-Tacoma area. NOAA doesn’t publish a single “the frost happened on this day” date. Instead, it tracks the probability that the temperature drops to 32°F on any given calendar day, then reports the window where that probability sits between 30% and 70%. That’s why you get a range instead of one number.
For spring, that 30-70% band runs from February 27 to March 18. Read it this way: by February 27, there’s roughly a 30% chance a 32°F freeze still happens. By March 18, that chance has dropped to around 30% as well, meaning the odds have already flipped toward “probably safe” earlier in the range and “still risky” later in it. For fall, the same math produces a window from November 13 to November 30, when freeze risk climbs back through that same 30-70% band as the season turns.
| Season | Frost window (32°F, 30-70% probability) |
|---|---|
| Last spring frost | February 27 to March 18 |
| First fall frost | November 13 to November 30 |
The critical caveat: this is western Washington data, specifically the mild, maritime Puget Sound lowlands. Eastern Washington, beyond the Cascade crest, is high desert climate with far colder winters, hotter summers, and a much shorter frost-free stretch. A gardener in Yakima, Spokane, or the Palouse working from a Seattle-area frost date would be planting weeks too early and risking a hard freeze weeks earlier in fall than this window suggests. No single station can represent a state that spans a mountain range and a full climate shift within its borders.
There’s also a second wrinkle specific to this region: a mild last-frost date does not mean an early planting date. Cold, wet soil and low late-winter light hold spring crops back long after the freeze risk has passed, which matters more here than in drier climates. More on that below.
How long the growing season actually is here
Stack the two windows together and you get a rough frost-free season, but the length depends on which end of each range you use. Using the more conservative combination (the latest spring frost date, March 18, paired with the earliest fall frost date, November 13) gives a season of roughly 240 days. Using the more optimistic combination (February 27 in spring, November 30 in fall) stretches it closer to 276 days. The real season for any given year falls somewhere inside that spread, since freeze timing shifts year to year even at the same station.
That’s a wide season by national standards, and it’s the reason western Washington supports such a long list of crops, from cool-season greens that tolerate the damp shoulder seasons to heat-lovers that need the full stretch of summer to mature. But raw day count doesn’t tell the whole story. Growing season length measures freeze risk, not growing conditions. A frost-free day in a shaded, waterlogged yard in February does nothing for seedlings that need warm soil and strong light to germinate.
Where hardiness zone fits in
Washington’s USDA Plant Hardiness Zone, per the USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals), spans zone 4a to 9a across the state. That’s a five-zone range, which is enormous for a single state and reflects the same east-west divide that shows up in frost dates. The USDA system measures one thing only: the average annual extreme minimum winter temperature at a location, used mainly to judge whether a perennial plant or shrub survives winter there. It says nothing about frost timing, growing season length, or when to plant a garden bed. Treat hardiness zone and frost dates as two separate tools measuring two separate risks, and don’t assume a mild hardiness zone number means an early spring.
Turning these dates into a sowing date
Frost date and planting date are not the same thing anywhere, but the gap between them is wider in western Washington than in most of the country. The region’s combination of cold, slow-draining soil and short, weak daylight in late winter means that even after the statistical freeze risk drops, the ground often isn’t warm enough to germinate seed reliably. Gardeners who plant on the frost date alone frequently see seeds rot in cold soil rather than sprout, especially in heavier clay-based yards common around the lowlands.
The more useful approach starts with soil temperature rather than the calendar. Most warm-season vegetables need soil consistently above roughly 60°F to germinate well, and in a maritime climate that threshold can lag weeks behind the last frost date. Cool-season crops are more forgiving and can go in earlier, once soil is workable rather than saturated. A basic soil thermometer, checked at planting depth over a few mornings, tells you more about readiness than any date on a seed packet.
Counting backward for transplants
For anything started indoors and transplanted out, work backward from your local last-frost window rather than a fixed date. Seed packets typically list a number of weeks to start before the last frost; apply that to the later end of the February 27 to March 18 range if your yard tends to run cold, damp, or shaded, and to the earlier end if it’s a warm, sheltered, south-facing bed. That built-in slack matters more here than in a climate with a tighter, more predictable frost window.
Local, county-specific guidance is where this gets sharpened into something actionable. Washington State University’s Extension Service, at extension.wsu.edu, publishes region-specific planting calendars and soil guidance built around local conditions rather than a single statewide date. Since frost timing and soil behavior vary so much between the coast, the lowlands, the foothills, and eastern Washington, checking with the county-level extension office in your area is a more reliable next step than applying any single frost date across the whole state.
Why your own garden differs from this figure
The February 27 to March 18 and November 13 to November 30 windows come from one weather station in the Seattle-Tacoma area. Your yard is not that station, and the gap between the two can be significant even within the same metro region. Elevation is one of the biggest factors: gaining a few hundred feet as you move into the foothills east of the lowlands can push frost dates later in spring and earlier in fall, shortening the effective season compared to sea-level readings.
Proximity to water changes things too. Puget Sound and the region’s rivers moderate temperature swings, so waterfront and near-water properties tend to run milder than inland spots just a few miles away. Urban heat also plays a role: paved, built-up areas hold heat overnight and often see a later first fall frost and earlier last spring frost than surrounding rural land, a pattern documented broadly in urban climate studies.
Cold air drainage matters at the yard scale. Cold air is denser than warm air and flows downhill like water, settling in low spots, valley bottoms, and the base of slopes. A garden at the bottom of a slope can see frost on nights when a garden partway up the same hill stays clear. Fences, walls, and dense hedges can trap or redirect that cold air, creating frost pockets in spots that seem otherwise unremarkable.
Tree canopy and structures add another layer. A south-facing bed against a heat-retaining wall warms earlier and holds heat later than an open, north-facing plot nearby. None of these local effects show up in a single station’s climate normals, which is exactly why they’re worth tracking yourself, ideally with a simple min-max thermometer placed at plant height in the specific bed you’re planning to use.
Common questions
What is the coldest hardiness zone in Washington?
The USDA Plant Hardiness Zone Map places Washington’s overall range at 4a to 9a. The map doesn’t single out one city as the coldest point in this article’s sourced data, but the range itself confirms that parts of the state sit far colder than the mild Puget Sound lowlands the frost dates above describe.
Are frost dates the same on both sides of the Cascades?
No. The February 27 to March 18 and November 13 to November 30 windows apply to the western, maritime side of the state, based on a Seattle-Tacoma area station. Eastern Washington’s high-desert climate produces a shorter frost-free season, though a specific eastern Washington frost date isn’t published in the data behind this article.
How reliable are these probability-based frost dates?
They’re a statistical estimate, not a guarantee. The 30-70% band means freeze risk on the early end of the spring window (or the late end of the fall window) still sits close to a 30% chance, so a frost outside the stated range remains entirely possible in any given year.
Where can I get frost and planting advice specific to my county?
Washington State University’s Extension Service, at extension.wsu.edu, is the land-grant university extension resource for the state and publishes region-specific planting guidance that accounts for the local variation a single statewide frost date can’t capture.