Red alder runs February 1 to April 15. Below: every tracked species, its months, and who published them.
Red alder pollen typically runs February 1 to April 15 here. In October, the calendar has none of the tracked species in season.The Washington State Department of Health reports that compared to 30 years ago the pollen season in Washington is starting 20 days earlier and lasting almost a month longer.
Every bar is a sourced season window; the institution and sentence behind each one are printed in the species cards below. Bars marked ❄ end at Washington’s median first frost, a date we compute from NOAA normals rather than copy from a source, because frost is what physically stops frost-tender weeds like ragweed.
Windows are climatological typicals for Washington as published by the sources on this page. They say when a season normally runs, not how strong any given day is.
Each card shows the months, the institution that published them, and the verbatim sentence the window came from. Where a date is computed by us rather than published, the card says so in as many words.
February 1 to April 15
Midpoint of the published window falls around March 1; the source does not name a peak.
“Flowering occurs in late winter or early spring”
USDA Forest Service, Silvics of North America
Rangewide Pacific Northwest source, not Washington specific; window approximates late winter through early spring. Regional reporting (KUOW, quoting an allergy specialist) identifies alder and birch as the two biggest pollen culprits in the Pacific Northwest.
April 15 to June 10
“Paper birch flowers from mid-April through early June depending on location.”
USDA Forest Service, Silvics of North America (paper birch)
Rangewide source for paper birch, not Washington specific; the Washington State Department of Health notes birch is the third most abundant pollen type found in Seattle.
May 25 to July 10
“Jacobson said grass pollen season usually starts right around Memorial Day in the Eugene area. It usually lasts four to six weeks weeks.”
Oregon Public Broadcasting (quoting Oregon Allergy Associates, Eugene)
Neighboring state source: window is from the Willamette Valley in Oregon; no Washington .edu or .gov page with explicit grass months was found.
What the allergy bodies say, in their words. AAAAI: “Seasonal allergic rhinitis is often caused by tree pollen in the early spring. During the late spring and early summer, grass pollen often cause symptoms. Late summer and fall hay fever is caused by weed pollen.” ACAAI: “Tree pollination begins earliest in the year followed by grass pollination later in the spring and summer and ragweed in the late summer and fall.” For anything about symptoms or treatment, see a clinician.
Ragweed is a short-day annual: it starts shedding pollen as days shorten in late summer and keeps going until cold kills it. The US EPA’s technical documentation for its ragweed pollen indicator puts it plainly: the season “ends in response to cold weather in the fall (i.e., first frost)”, and ragweed “will not continue flowering once temperatures dip below freezing.” EPA’s own analysis truncates every station’s ragweed season at the first fall frost.
Washington's calendar above carries no frost-derived weed bar: its sources either publish the weed season's own dates or do not make ragweed the state's fall story. The state's median first frost, October 28, still marks where any frost-tender weed season would close.
USA-NPN’s Extended Spring Indices measure how early or late spring leaf-out and bloom are running against the 1991 to 2020 normal. The index knows nothing about pollen; it tracks cloned lilacs and honeysuckles. But wind-pollinated trees flower in the same early-spring window it measures, so it is a usable, clearly labelled proxy for whether Washington’s tree pollen season is ahead of or behind schedule this year.
The live grid currently holds the finished spring of 2026, shown as history. Tree windows above are not shifted by it; readings for the coming spring start filling in from late January.
Every season window names the extension service, agency or university that published it, with the sentence quoted on the card. Where nobody publishes a window, the page says so instead of inventing one.
Weed bars marked with the frost symbol end on the state’s median first frost, computed from NOAA 1991 to 2020 normals and labelled as derived, on the strength of EPA’s ragweed indicator documentation.
The Spring Index anomaly is a measurement of the spring that has already run. It shifts nothing except spring tree windows, and only while the year’s spring is actually in progress.
Daily counts belong to the certified counting stations of the National Allergy Bureau, and search engines already print them. This site publishes the part nobody prints: the calendar.
Season windows are climatological typicals, each attributed on its card to the institution that published it, with the supporting sentence quoted. Frost-derived weed season ends are computed by us from NOAA 1991 to 2020 median first frost dates and labelled as derived wherever they appear; the frost link is documented by the US EPA’s ragweed pollen indicator technical documentation. Spring Index anomalies are read live in your browser from the USA National Phenology Network (www.usanpn.org, CC BY 4.0) and shown as a labelled proxy for spring tree pollen timing. National picture: every state’s pollen calendar →
Hero image: “Dactylis glomerata - Pink with pollen” by Bob Harvey, CC BY-SA 2.0