Sagebrush runs August 15 to September 28. Below: every tracked species, its months, and who published them.
Sagebrush pollen typically runs August 15 to September 28 here. In October, the calendar has none of the tracked species in season.Boise allergists describe grass pollen as the predominant summer pollen on the Snake River Plain, with sagebrush taking over from mid-August (sources quoted in the grass and hero entries; both are private clinic pages, fetched 2026-08-27).
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 Idaho’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 Idaho 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.
April 1 to May 15
“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)
Regional Rockies source: Colorado State Extension, applied to Idaho. No Idaho-specific tree pollen month statement was found from University of Idaho Extension.
June 1 to August 31
“Grass pollen is the predominant pollen in the air during the summer here in Idaho.”
Private allergy clinic source, used as a last resort. Source says summer without months; the Jun through Aug window is our reading. The same page lists timothy, rye, orchard, sweet vernal, red top and blue grasses as the northern grasses involved.
August 15 to September 28 ❄ end derived from median first frost
“Sagebrush flowers usually from Mid-August to late September generating pollen in North-western US.”
The Allergy Group (Boise, Idaho) · end date computed by us from NOAA 1991 to 2020 frost normals, not published by the source
Private allergy clinic source, used as a last resort; no Idaho .edu or .gov page stating sagebrush months was found. Sourced end is late September; end computed from first frost. USU range phenology for the same species (Flowers August to September) corroborates.
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: Idaho's median first fall frost is September 28 (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 Idaho’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