Indiana Pollen Calendar: Season by Species

Q: What pollen is high in October in Indiana?

Based on the sourced windows on this page, mid October in Indiana falls inside the season for Ragweed. Whether the air is actually thick with it on a given day depends on weather; the windows say what is possible, not what today's count is.

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

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 Indiana 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.

Oak and mixed hardwoods runs March 1 to May 31. Below: every tracked species, its months, and who published them.

Oak and mixed hardwoods pollen typically runs March 1 to May 31 here. In October, the calendar has Ragweed in season.

Oak and mixed hardwoods season

Mar 1 to May 31

In season this October

Sourced windows

Where the Indiana pollen year stands

Climatological windows, resolved against today

Weed

groups in season, mid October

Ragweed

Mar 1 to May 31

oak and mixed hardwoods season

per University of Illinois Extension (ILRiverHort)

October 16

median first frost

the date that ends ragweed season here (derived)

Purdue Extension flags common ragweed as a major hay fever contributor across Indiana in late summer and early fall (see ragweed entry source).

The calendar

What pollinates when in Indiana

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 Indiana’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.

Oak and mixed hardwoodsTreeMarch 1 to May 31

GrassesGrassMay 1 to August 31

RagweedWeedAugust 15 to October 16 ❄ derived

trees grasses weeds today❄ end derived from median first frost

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

Common ragweed flower spike held upright against a clear blue sky — The species that leads a state’s calendar decides when its worst weeks land, which is why every state page opens with different months. “Ambrosia artemisiifolia flowerhead7” by Harry Rose, CC BY 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.

Oak and mixed hardwoodsTree

Quercus spp. and others

March 1 to May 31

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

“Allergenic trees are usually a problem from March through May and include the following (in flowering order): Maple, Willow, Poplar, Elm, Birch, Mulberry, Ash, Hickory, Oak, and Walnut.”

University of Illinois Extension (ILRiverHort)

Neighboring-state source: Illinois Extension, applied to Indiana. No Indiana-specific extension page stating tree pollen months was found. Purdue sources confirm the same species mix statewide.

GrassesGrass

Poaceae (orchardgrass, bluegrass, timothy)

May 1 to August 31

“Grasses are more powerful allergens than trees and bloom from May through summer.”

University of Illinois Extension (ILRiverHort)

Neighboring-state source: Illinois Extension, applied to Indiana. Source says May through summer; Aug 31 end is our reading of through summer.

RagweedWeed

Ambrosia spp.

August 15 to October 16 ❄ end derived from median first frost (in season now)

“Pollen from common ragweed is carried in the wind and is a major contributor to hay fever in the late summer and early fall.”

Purdue University Cooperative Extension (Vegetable Crops Hotline) · end date computed by us from NOAA 1991 to 2020 frost normals, not published by the source

Purdue wording is late summer and early fall; Aug 15 start is our reading of late summer. End computed from first frost per EPA (see 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.

The frost connection

Why ragweed season in Indiana 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.

This page derives the end date from that rule: Indiana's median first fall frost is October 16 (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.

Live proxy · USA-NPN

Is tree season running early or late in Indiana?

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 Indiana’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

Indiana pollen season FAQ

When is oak and mixed hardwoods pollen season in Indiana?

University of Illinois Extension (ILRiverHort) supports a window of roughly March 1 to May 31. Their words: "Allergenic trees are usually a problem from March through May and include the following (in flowering order): Maple, Willow, Poplar, Elm, Birch, Mulberry, Ash, Hickory, Oak, and Walnut." That is a climatological typical, not a forecast; any single year commonly runs a week or two either side of it.

When does ragweed season start and end in Indiana?

The start comes from the source on the card. The end is different: we compute it from Indiana's median first fall frost, October 16, because the US EPA's ragweed indicator documentation states that ragweed pollen season "ends in response to cold weather in the fall (i.e., first frost)" and that ragweed "will not continue flowering once temperatures dip below freezing." The end date is therefore derived by us from NOAA 1991 to 2020 frost normals, and the page labels it that way. A late first frost stretches the season; an early one cuts it short.

What pollen is high in October in Indiana?

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 Indiana?

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 Indiana pollen windows change from year to year?

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

Orchard grass flower heads with pink anthers spread for wind pollination — Orchard grass in flower, anthers out. Grass pollen is the midsummer act of the calendar almost everywhere, and a lawn kept mowed never reaches this stage, which is the whole connection to the mowing schedule. “Dactylis glomerata - Pink with pollen” by Bob Harvey, CC BY-SA 2.0

The rest of the Indiana calendar

Plan around the Indiana seasons

Indiana 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.Indiana lawn calendarGrass pollen comes from grass allowed to flower. The mowing calendar is the other half of that story.

More Midwest 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: “Ambrosia artemisiifolia flowerhead7” by Harry Rose, CC BY 2.0