Oregon Pollen Calendar: Season by Species

Q: When does ragweed season start and end in Oregon?

Where a weed row appears above, its dates and the institution that published them are on the card. In Oregon the sources either give the weed season its own explicit dates, or ragweed is not the state's fall story the way it is east of the Rockies, where EPA's ragweed indicator documentation has the season ending at the first fall frost.

Q: What pollen is high in October in Oregon?

Mid October falls outside every sourced window on this page for Oregon, which is as close to an off season as a pollen calendar gets. Individual warm or windy days can still stir up something.

Q: Where can I check today's pollen count for Oregon?

Certified daily counts are published by the National Allergy Bureau at pollen.aaaai.org, which lists counting stations run under the American Academy of Allergy, Asthma and Immunology, and search engines print a daily pollen card for most locations. This site publishes the calendar: which species, which months, and the frost date that ends weed season.

Q: Do the Oregon pollen windows change from year to year?

The published windows are climatological typicals and stay put; the year moves around them. ACAAI notes that "Mild winter temperatures can cause plants to pollinate early." That is exactly why the tree season panel shows the Spring Index anomaly, and why the ragweed end is tied to the first frost rather than to a fixed date.

Q: Where does this data come from, and can I check it?

Yes, and you should. Every species window names its institution and links its page, with the sentence we relied on quoted under the card. The frost medians behind the derived ragweed ends are the NOAA 1991 to 2020 normals in our first frost pages. The Spring Index proxy is read live from the USA National Phenology Network under CC BY 4.0.

Grasses runs May 25 to July 10. Below: every tracked species, its months, and who published them.

Grasses pollen typically runs May 25 to July 10 here. In October, the calendar has none of the tracked species in season.

Grasses season

May 25 to Jul 10

In season this October

Sourced windows

Where the Oregon pollen year stands

Climatological windows, resolved against today

quiet

groups in season, mid October

no tracked species in window

May 25 to Jul 10

grasses season

per Oregon Public Broadcasting (quoting Oregon Allergy Associates, Eugene)

October 28

median first frost

the date that ends ragweed season here (derived)

The Willamette Valley is the center of United States grass seed production, and Oregon Public Broadcasting reports Eugene grass pollen counts above 200 are rated very high yet reached 410 in late May 2024.

The calendar

What pollinates when in Oregon

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 Oregon’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.

HazelnutTreeJanuary 1 to February 28

Red alderTreeFebruary 1 to April 15

Oregon white oakTreeMarch 1 to June 30

GrassesGrassMay 25 to July 10

trees grasses weeds today❄ end derived from median first frost

Windows are climatological typicals for Oregon as published by the sources on this page. They say when a season normally runs, not how strong any given day is.

Orchard grass flower heads with pink anthers spread for wind pollination — The species that leads a state’s calendar decides when its worst weeks land, which is why every state page opens with different months. “Dactylis glomerata - Pink with pollen” by Bob Harvey, CC BY-SA 2.0

Species by species

The windows, with their sources

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.

HazelnutTree

Corylus avellana

January 1 to February 28

The source puts the peak around February 1.

“Peak pollination season is from January through February, but this can vary depending on weather conditions.”

Oregon State University Extension (EM 9074, Growing Hazelnuts in the Pacific Northwest)

An orchard publication rather than an allergy page, and it is explicit about the mechanism: hazelnut trees are wind pollinated and each catkin produces more than 1 million functional pollen grains. Nearly all US hazelnuts grow in the Willamette Valley, so this is the valley midwinter pollen.

Red alderTree

Alnus rubra

February 1 to April 15

“Flowering occurs in late winter or early spring”

USDA Forest Service, Silvics of North America

Rangewide Pacific Northwest source, not Oregon specific; window approximates late winter through early spring.

Oregon white oakTree

Quercus garryana

March 1 to June 30

“Flowering has been noted in March, April, May, and June (72,74), but the seasonal span is probably greater over the wide range of latitudes and elevations where this species occurs.”

USDA Forest Service, Silvics of North America

GrassesGrass

Poaceae

May 25 to July 10

Midpoint of the published window falls around June 15; the source does not name a peak.

“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)

News source quoting the allergist who runs the certified Eugene pollen counting station; no OSU or state page with explicit months was found, so this is a flagged non university source. End date derived from the quoted four to six week duration after Memorial Day.

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.

The frost connection

Why ragweed season in Oregon ends on a frost date

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.

Oregon'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.

Common ragweed flower spike held upright against a clear blue sky — Common ragweed in flower. Each spike sheds wind-borne pollen from late summer until the plant is killed by freezing temperatures, which is why the end of its season is a frost statistic rather than a botany fact. “Ambrosia artemisiifolia flowerhead7” by Harry Rose, CC BY 2.0

Live proxy · USA-NPN

Is tree season running early or late in Oregon?

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 Oregon’s tree pollen season is ahead of or behind schedule this year.

First Leaf Index

…

Leaf-out of lilacs and honeysuckles, the earliest woody plants to break bud.

First Bloom Index

…

Flowering of lilacs and honeysuckles, which USA-NPN associates with blooming of early-spring shrubs and leaf out of deciduous trees.

What this is

A proxy, stated as one. The index tracks lilac and honeysuckle leaf-out and bloom, not pollen. Wind-pollinated trees flower in the same early-spring window, so an early index spring usually means an early tree pollen season. It says nothing about grass or ragweed.

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.

Pine branch with yellow pollen cones releasing a visible cloud of pollen into the wind — A pine shedding pollen into the wind. Wind-pollinated trees release enormous quantities because almost none of it lands on another flower, which is also why it lands on everything else. “Pine releasing pollen into the wind in Tuntorp 1” by W.carter, CC BY-SA 4.0

How this page is built

A calendar, not a count

Sourced windows

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.

Derived ragweed ends

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.

A labelled proxy, not a forecast

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.

No counts, by design

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.

Common questions

Oregon pollen season FAQ

When is grasses pollen season in Oregon?

When does ragweed season start and end in Oregon?

What pollen is high in October in Oregon?

Is pollen season early or late this year?

For spring tree pollen there is a usable proxy. USA-NPN's Extended Spring Indices measure how early or late spring leaf-out and bloom are running against the 1991 to 2020 normal, at 4 km resolution, updated daily. Wind-pollinated trees flower in that same early spring window, so an early index spring usually means an early tree pollen season. It is a proxy and this page labels it as one: the index models lilacs and honeysuckles, not pollen, and it says nothing about grass or ragweed timing.

Where can I check today's pollen count for Oregon?

What do the allergy organizations say about pollen exposure?

The allergy societies publish the practical guidance. AAAAI: "Keep your windows closed at home and if possible, use air conditioning, which cleans, cools and dries the air." AAAAI: "Pollen tends to travel more with hot, dry and windy weather, which can increase your allergy symptoms." ACAAI: "Take a shower, wash your hair and change your clothes after you've been working or playing outdoors." For anything about symptoms or treatment, talk to a clinician.

Do the Oregon pollen windows change from year to year?

Where does this data come from, and can I check it?

The rest of the Oregon calendar

Plan around the Oregon seasons

Oregon frost datesThe median first frost is the date that ends ragweed season here. Same number, two pages.Spring bloom timingThe same Spring Index that flags an early tree pollen year also moves the wildflower calendar.Oregon lawn calendarGrass pollen comes from grass allowed to flower. The mowing calendar is the other half of that story.

More West pollen calendars

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