Cottonwood runs April 1 to April 30. Below: every tracked species, its months, and who published them.
Cottonwood pollen typically runs April 1 to April 30 here. In October, the calendar has none of the tracked species in season.CSU PlantTalk notes the white June cottonwood cotton that blankets Colorado lawns is seed fluff, not pollen; the allergenic cottonwood pollen flies in April (see hero entry source).
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 Colorado’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 Colorado 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
The source puts the peak around April 10.
“pollinates from early March through May in Western Colorado”
Allergy and Asthma Center of Western Colorado (Grand Junction pollen count)
Private allergy clinic source, used as a last resort; no CSU or CDPHE page stating juniper months was found. The same page says landscape junipers erupt with huge amounts of pollen in early to mid April, which is the sourced peak.
April 1 to April 30
“Flowers on female trees, after being pollinated by wind-borne pollen from male trees in April, develop small green capsules that split open in June to shed small seeds carried by wind-borne "cotton".”
PlantTalk Colorado (Colorado State University Extension)
CSU stresses the June cotton is not pollen; people with symptoms during June cotton fall are probably reacting to grass or weed pollen instead.
April 1 to July 31
“As the year progresses, grasses will emerge and their pollen will cause symptoms through most of the summer - typically April to July.”
University of Colorado Anschutz Medical Campus
Colorado academic medical campus, but the timing is stated for the U.S. generally, not Colorado specifically. Original punctuation uses a dash between summer and typically.
August 15 to October 2 ❄ end derived from median first frost
“Then ragweed and other weeds release pollen that causes symptoms into the fall until a freeze stops their pollen production.”
University of Colorado Anschutz Medical Campus · end date computed by us from NOAA 1991 to 2020 frost normals, not published by the source
Source gives fall-until-freeze timing without a start month; Aug 15 start is our reading consistent with the EPA national indicator in the file header.
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: Colorado's median first fall frost is October 2 (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 Colorado’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: “Artemisia tridentata (common sagebrush)” by James St. John, CC BY 2.0