Oak (blue oak) runs March 15 to May 15. Below: every tracked species, its months, and who published them.
Oak (blue oak) pollen typically runs March 15 to May 15 here. In October, the calendar has Ragweed in season.A North American pollen calendar study on PubMed Central notes the San Jose station samples year round because pollen is present in the atmosphere throughout the year in central California.
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 California’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 California 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.
March 1 to May 31
“European olive, Olea europa, March-May”
University of Arizona Cooperative Extension, Yavapai County (Bulletin #2)
Neighboring state source: window is the High Pollen Months table entry from a University of Arizona extension bulletin, not a California source. UC IPM says of olive: The pollen is highly allergenic. A UC Master Gardener article says olive bloom time is in the spring.
March 15 to May 15
Midpoint of the published window falls around April 15; the source does not name a peak.
“Blue oak flowers from late March to mid-May, depending on elevation, aspect, climate, and reproductive capability of individual trees.”
USDA Forest Service, Silvics of North America
Timing is for blue oak, a widespread California foothill oak; other California oaks flower on similar spring schedules.
June 1 to September 30
“It may flower several times throughout the growing season, and can cause allergy problems from spring through fall.”
University of Arizona Cooperative Extension, Yavapai County (Bulletin #2)
Neighboring state source: months are from a University of Arizona extension bulletin (table lists June through September), not a California source; UC ANR lists bermudagrass among offending grasses for California gardeners.
August 15 to December 1 ❄ end derived from median first frost (in season now)
“Ragweed plants mature in mid-summer and produce small flowers that generate pollen. Ragweed pollen season usually peaks in late summer and early fall, but these plants often continue to produce pollen until the first frost.”
U.S. EPA Climate Change Indicators: Ragweed Pollen Season · end date computed by us from NOAA 1991 to 2020 frost normals, not published by the source
National EPA source, not California specific; UC ANR lists ragweed among offending herbaceous plants for California.
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.
This page derives the end date from that rule: California's median first fall frost is December 1 (NOAA 1991 to 2020 normals, the same number our first frost pages publish), and that is the derived end date on the ragweed bar above. It is a median: about half of years frost earlier, half later, and EPA's indicator work shows fall frosts arriving later than they used to across the northern states, which is exactly why ragweed seasons have been getting longer.
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 California’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