Washington Allergy Season: Two States, One Mountain Range

Washington allergy season — the state, not the capital; everything below concerns the Washington of Seattle, Spokane and the Cascade rain shadow — starts earlier than almost anywhere else in America and then promptly splits in two. West of the mountains, alder and hazelnut can stir in January, and the tree season is reliably running by mid-February, weeks before much of the country thaws. East of them, Spokane keeps a continental clock: a later, slower spring, a hot dry summer, and a late weed act the wet side never really sees. One state line, two pollen calendars — and a mountain range is the reason.

Key takeaways

• West of the Cascades, alder and hazelnut can be releasing pollen in January, and UW Medicine specialists put the start of Seattle's tree pollen season at around mid-February in a typical year.
• East of the Cascades, Spokane runs on a later, continental clock: a slower spring, timothy through the summer, and sagebrush and tumbleweed weeds carrying into September.
• Rain crashes west-side counts toward zero under a day of steady rain, but the season restarts almost immediately behind the front, on the first dry, mild day.
• The Washington State Department of Health reports the state's pollen season now starts about 20 days earlier and lasts almost a month longer than 30 years ago.

Washington Allergy Season on the West Side: The January Head Start

The Puget Sound lowlands run the earliest tree season on the West Coast. Alder is the opener — a fast-growing waterside tree that flowers while the woods still look bare — with hazelnut alongside it, and in a mild winter both can be releasing pollen in January. UW Medicine specialists put the start of Seattle's tree pollen season at around mid-February in a typical year, with hazelnut, alder, birch, oak, cottonwood, ash and juniper forming the late-winter and spring cast. Birch deserves its billing: the Washington State Department of Health notes birch is among the most abundant pollen types recorded in Seattle. Maple and the planted street trees carry the season through April, and by the time the rest of the country is starting, Puget Sound is on its second act.

That second act is grass, and it is a long one. Ryegrass, bluegrass and the pasture grasses of the lowland valleys flower from May into July, in a climate mild enough to keep lawns — and pollen — growing through most of the summer. Pollen Atlas data shows the Seattle pollen count holding tree and grass indexes in the Moderate band for long stretches of late winter and spring, with High-band days arriving on the first warm, dry, breezy runs. Our tree pollen season guide walks the alder-to-oak succession the west side runs on fast-forward.

The planted landscape extends what nature started. Seattle's street and garden trees — flowering cherries and plums for show, maples for shade, birch planted where a European accent was wanted — layer a landscaped season over the native one, so the lowland spring reads as one long tree act from February into May rather than a series of separate ones. And within each day there is a smaller rhythm: pollen builds as the air warms and dries after a wet night, so a grey dawn can be the kindest hour of a west-side day, and a bright noon the rudest.

Then there is the rain, which is both the west side's mercy and its tease. A proper front scrubs the air — counts can crash toward zero under a day of steady rain — but the season restarts almost immediately behind it: the first dry, mild day releases a fresh charge from catkins that were only ever paused, not finished. West-side sufferers learn to read the forecast backwards, dreading not the wet day but the bright one after it.

Washington Allergy Season on the East Side: Spokane's Continental Clock

Cross Stevens or Snoqualmie Pass and the calendar changes its mind. Spokane sits in the rain shadow at elevation, with real winters, and its spring arrives weeks behind Seattle's — the same alder-and-oak programme, played later and faster once it starts. Pine joins the tree mix in the surrounding forests, timothy grass owns the summer fields and roadsides, and then the east side plays the card the west side barely holds: dry-country weeds. Sagebrush, Russian thistle — the plant that becomes tumbleweed — and pigweed build through the hot, dry late summer, so Spokane's season ends not with the west side's gentle fade but with a genuine weed act in August and September. Pollen Atlas data shows the Spokane pollen count running that pattern — a later tree start, then weed indexes in the Moderate band deep into late summer. The Boise pollen count, a few hours southeast and further inland, shows the same continental logic amplified: later, drier, weedier.

The split has a practical consequence Washingtonians know well: the second spring. Drive from Seattle to Spokane in April and you can leave a season that is nearly finished and arrive in one that is just beginning — the same trees, the same grasses, running weeks apart on the same highway. Sufferers who time a move, a campus choice or a work posting by the pollen calendar are not being precious; in this state, a few hours east genuinely buys a different year.

The east side adds one footnote the west mostly escapes. In late summer, wildfire smoke can drift over Spokane from fires across the inland Northwest. Smoke is not pollen — a different exposure entirely — but it lands on airways already primed by a summer of grass and weeds, which is why eastern Washingtonians, like Californians, have learned to read the air quality picture and the pollen count as one combined forecast.

Rain, Mold and the Restart Problem

The west side's third season is the one nobody puts on a calendar: mold. From the first autumn leaf-fall through the damp winter, decaying vegetation and saturated soil keep outdoor mold spores in the air around Puget Sound, and the same moisture works indoors wherever a house stays cold and wet. It is why some west-side sufferers insist their season never ends — the pollen chapters close, but the mold chapter runs underneath them from roughly October to March.

The region's counting infrastructure exists precisely because these seasons are so local. The AAAAI's National Allergy Bureau coordinates the counting stations whose readings anchor pollen reporting across North America — and in Washington, a station a few miles away can be reading a different season, because a shoreline breeze, a valley inversion or one range of hills rewrites the numbers. Statewide summaries flatten all of that; the city page nearest you does not.

A regional cheat sheet:

• Puget Sound and the west side: alder and hazel from January–February, maple and birch spring, grass May–July, mold riding the wet months.
• The Cascades: a late, compressed season timed to snowmelt; the state's pollen afterthought.
• Spokane and the east side: trees weeks later, timothy summer, sagebrush and tumbleweed carrying into fall.
• Statewide: the first dry week matters more than the first frost.

A Longer Season Than Your Parents Had

If Washington's seasons feel longer than they used to, the state has the receipt. The Washington State Department of Health reports that compared with 30 years ago, the pollen season in Washington now starts about 20 days earlier and lasts almost a month longer — more frost-free days, warmer temperatures and extra carbon dioxide doing the pushing, with birch pollen in particular projected to climb. The CDC's pollen and health guidance describes the same lengthening pattern nationally. Our pollen season calendar lays out the national year; Washington's entry is the one quietly expanding at both ends.

Set the state's two halves against each other on the city rankings in February and the split is unmissable: Seattle posting tree numbers while Spokane still reads winter. If your symptoms change character when you cross the Cascades, you are not imagining it — you crossed a climate line. Washingtonians who sneeze on a schedule deserve help that knows which side of the mountains they live on — find an allergist on your side of the pass. This article is general information only and is not medical advice.

Frequently Asked Questions

When does allergy season start in Washington?

On the west side, alder and hazelnut can release pollen in January, and the tree season is reliably running by mid-February. East of the Cascades, Spokane's spring arrives weeks later, so the state's two halves open their seasons on different dates.

Why are Seattle's and Spokane's allergy seasons so different?

The Cascades split the state into a wet, mild lowland west and a dry rain shadow east with real winters. The same trees and grasses grow on both sides, but they run weeks apart, and the east adds dry-country weeds the west side barely sees.

What is the main spring pollen in Seattle?

Alder opens the season in late winter, and the Washington State Department of Health notes birch is among the most abundant pollen types recorded in Seattle. Maple and planted street trees carry the season through April, and grass follows from May into July.

When does Washington allergy season end?

On the east side, the weed act runs through August and September. On the west side, the pollen chapters close sooner, but mold runs underneath from roughly October to March, which is why some west-side sufferers feel their season never fully ends.

Washington allergy season is two seasons sharing a name. Check yours before you head out — free counts and forecasts, worldwide, at PollenAtlas.com.