Pollen Types, Seasons & Counts Explained

Seasonal allergies are usually triggered by three kinds of pollen — tree, grass, and weed — plus outdoor mold spores. Each has its own season and its own way of being measured. This guide explains what each type is, when it peaks, and how to read the numbers RadarBug's pollen layers show.

Why pollen matters

Pollen is the fine powder plants release to reproduce. Plants that rely on the wind, rather than insects, release enormous amounts of small, light grains that can travel for miles. Those wind-carried grains cause most pollen allergies. Showy flowering plants pollinated by bees usually cause fewer problems, because their pollen is heavier and stickier and stays out of the air.

Tree pollen

Season: late winter through spring, roughly February to May in much of the US. It starts earlier in the South; mountain cedar in Texas sheds pollen as early as December.

When it peaks: trees pollinate in a relay, so the season is a series of overlapping waves rather than one peak. Early bloomers such as elm, maple, alder, and juniper start it off; birch follows; and oak, usually the heaviest producer, peaks last, in March and April in the South and late April through May farther north. Pine and mulberry overlap the oak peak. Once trees leaf out fully, tree pollen drops off quickly, usually by early June in the North.

Common sources: oak, birch, maple, elm, ash, cedar and juniper, mulberry, cottonwood, hickory, and pecan.

Tree pollen is the first big wave of the year, and a single day can bring very high counts. The yellow dust that coats cars in spring is mostly pine pollen, which is large and rarely the main allergy trigger.

Grass pollen

Season: late spring through summer, roughly May to July. In warm southern climates grass can pollinate for much of the year.

When it peaks: cool-season grasses (Timothy, bluegrass, orchard grass, ryegrass) peak in late May and June across the northern half of the country and the Pacific Northwest, then fade in the summer heat. Warm-season grasses (Bermuda, Bahia, Johnson grass) take over in the South and Southwest, pollinating from spring well into fall and nearly year-round in South Florida and South Texas. A smaller second flush of grass pollen often follows late-summer rain.

Common sources: Timothy, Bermuda, Kentucky bluegrass, orchard grass, ryegrass, and Johnson grass.

Grass counts are usually much lower than tree counts, but grass pollen is potent: even a small number of grains per cubic meter can trigger symptoms. That is why its rating bands start so low.

Weed pollen

Season: late summer through fall, roughly August until the first hard frost.

When it peaks: ragweed usually starts shedding in mid-August, peaks in early to mid-September, and continues until the first hard frost ends it, typically in October in the North and as late as November in the South. In the West, sagebrush, Russian thistle, and other weeds run from July or August into October. Earlier in the year, plantain, nettle, and pigweed add smaller amounts of weed pollen from late spring through summer.

Common sources: ragweed above all, plus pigweed and amaranth, lamb's quarters, sagebrush and mugwort, Russian thistle, plantain, and nettle.

Ragweed is the classic fall allergy trigger. A single plant can release up to a billion grains in a season, and its pollen travels long distances on the wind, so it can affect places where little ragweed grows.

Mold spores

Season: mainly summer and fall, and nearly year-round in warm, humid climates. Counts often climb after rain and during damp weather.

When it peaks: in most of the country outdoor mold climbs through July, peaks in late summer and early fall, and gets another boost from fallen, decaying leaves in October before a hard freeze knocks it down. Along the Gulf Coast and in Florida it stays elevated all year, highest in the wet, humid months from June to November. In the Pacific Northwest the damp fall and winter months are the main mold season.

Common sources: outdoor molds such as Cladosporium and Alternaria, which grow on soil, leaves, and decaying plants.

Mold is not pollen, but its spores float in the air and trigger the same kinds of symptoms. Spore counts are routinely in the thousands, far higher than pollen, so RadarBug reports mold separately instead of mixing it into the pollen chart.

Major mold types

Station counts list molds by name, often in groups, because some spores can't be told apart under a microscope. These are the ones you are most likely to see when you tap the Mold spores row in a station's dialog:

• Cladosporium — the most common outdoor mold in the world and often the largest share of a day's spore count. Olive-green to black, it grows on dead leaves, grasses, and other decaying plant material, and indoors on damp window frames and bathroom surfaces. A frequent allergy trigger.
• Alternaria — a dark mold that grows on grains, grasses, soil, and decaying plants. One of the most potent mold allergens and the one most closely tied to asthma, including severe attacks; counts jump during grain harvest and on dry, windy days.
• Aspergillus / Penicillium — counted together because their small round spores look alike. They grow in compost, leaf piles, soil, stored food, and damp buildings; Penicillium is the familiar blue-green mold on bread and citrus. Both are common allergens, and Aspergillus can also cause lung infections in people with weakened immune systems.
• Epicoccum — grows on dead grasses and plant debris and is released in dry, windy weather from summer into fall. A moderate allergen.
• Drechslera / Bipolaris / Helminthosporium — a group of grass and grain pathogens with large spores, most common in summer and fall and in warm southern climates.
• Curvularia — grows on grasses and grains in warm, humid regions such as the Gulf Coast and Florida, and is a known allergen there.
• Fusarium — a crop pathogen found on corn, wheat, and other grains; its spores are released in wet weather.
• Smuts and rusts — parasitic fungi that infect grasses, grains, and weeds. Their spores are counted together, sometimes with slime molds (myxomycetes), and peak in summer around farmland and grassy areas.
• Ascospores — not one mold but a large group of sac fungi that shoot their spores into the air when rain or dew wets them. Counts can rise sharply during and right after rain.
• Basidiospores — spores from mushrooms, bracket fungi, and puffballs. Released on damp nights and mornings, they often make up the largest share of the spore count in late summer and fall, and some people are allergic to them.

You won't see Stachybotrys, the “black mold” often in the news, in outdoor counts: it grows indoors on materials that stay wet, such as drywall after a leak or flood. Any mold growing indoors, whatever its color, is worth cleaning up and fixing the moisture behind it.

Mold seasons in detail

Mold doesn't follow a bloom calendar the way plants do. It follows moisture and temperature, and different molds release their spores in different kinds of weather, so the mold season is really several overlapping ones. AAAAI stations count each mold separately; tap the Mold spores row in a station's dialog to see which ones were in the air.

Molds fall into two broad groups. Dry-air spores such as Cladosporium and Alternaria are shaken loose by wind and peak on sunny, breezy afternoons. Wet-air spores, the ascospores and basidiospores, are launched by moisture and peak overnight and in the early morning after rain. That is why a rainy spell can bring pollen down and mold up at the same time, and why the dry, windy days that follow often bring a second rise.

Mold through the year

• Winter: in the North, freezing temperatures and snow cover keep outdoor mold low. Along the Gulf Coast, in Florida, and in coastal California, mild, wet winters keep spores in the air, and in the Pacific Northwest winter rain makes this the main mold season.
• Early spring: melting snow uncovers wet, matted leaves, so counts make a first, smaller rise as soon as temperatures stay above freezing.
• Summer: counts climb steadily through June and July, especially after thunderstorms and during humid heat waves.
• Late summer and fall: the main peak in most of the country, usually August through October. Grain harvest in the Midwest and Plains stirs up Alternaria and Cladosporium, and fallen leaves feed another rise in October and November until a hard freeze ends it.

Tips for mold season

• Avoid raking leaves, mowing, and turning compost on high mold days, or wear a well-fitting N95 mask if you have to.
• Expect wet-air spores to be highest in the morning after an overnight rain, and dry-air spores on windy afternoons.
• Keep indoor humidity below about 50% and fix leaks promptly, so outdoor season isn't followed by an indoor one.

Pollen season calendar

This calendar shows a typical year in the central and eastern US. Bright bars mark the usual peak months; faded bars mark months when that type is in the air at lower levels. Use the regional table below to adjust for where you live.

Seasons by region

Pollen seasons follow the weather, so they arrive earliest in the South and at low elevations and latest in the North and in the mountains. These are typical ranges; a warm early spring can move the start several weeks earlier, and a late frost can push it back.

Many studies have found pollen seasons starting earlier and lasting longer than they did a few decades ago, with the biggest change in the northern US, where ragweed season now runs noticeably later into the fall. Treat the dates above as a starting point and use the map to see what's actually in the air this week: the heatmap shows the forecast for the type you pick, and the nearest station shows what was counted.

Reading measured counts

Pollen stations collect air samples and count the grains under a microscope. Counts are grains (or spores) per cubic meter of air, averaged over 24 hours. RadarBug's pollen station dialogs rate each count using the AAAAI National Allergy Bureau scale:

Tap a slice in a station's pie chart to see which plants or molds were counted. Some rows group several plants together, such as "All identified weed pollen not counted elsewhere."

Reading the forecast index

The pollen heatmap and tap-for-forecast dialog use Google's Universal Pollen Index, a 0 to 5 scale that combines expected pollen levels with how strongly each type tends to cause symptoms:

The index is a forecast from a model, while station counts are measurements from the past day or two. Use both: the forecast tells you what to expect, and the nearest station tells you what was actually in the air.

How weather affects pollen

• Warm, dry, breezy days spread the most pollen.
• Steady rain washes pollen out of the air, so counts usually drop during and after it.
• Thunderstorms can do the opposite: strong gusts and moisture can break grains into smaller pieces that reach deeper into the lungs, a pattern known as thunderstorm asthma.
• Time of day matters too. Many plants release pollen in the morning, and levels often stay elevated into the afternoon on dry days.
• Damp, humid weather favors mold, so mold counts can rise just as pollen falls.

Pollen mode turns on the NWS fronts layer so you can see weather systems moving in: a passing cold front often brings wind ahead of it and rain that clears the air behind it. See Weather Fronts Explained for what the symbols mean.

Tips for high pollen days

• Check the forecast and your nearest station before spending time outdoors.
• Keep windows closed at home and in the car; use air conditioning if you have it.
• Shower and change clothes after time outside, and dry laundry indoors.
• Consider a HEPA air purifier for the bedroom.