Spring is the season most associated with high pollen, but pollen levels can be elevated from early February all the way through October depending on where you live and which plants are releasing pollen. The U.S. pollen calendar breaks into three overlapping waves: tree pollen in spring, grass pollen in late spring and summer, and weed pollen in late summer and fall. For many parts of the country, there’s barely a break between them.
Spring: Tree Pollen Starts the Year
Trees are the first major pollen producers each year and are responsible for most spring allergy symptoms. Across the U.S., trees release the most pollen from February through April. In southern states, some trees begin pollinating as early as December or January and can peak multiple times throughout the year.
The list of allergenic trees is long: oak, birch, cedar, maple, elm, ash, walnut, pecan, juniper, and cottonwood are among the biggest offenders. Tree pollen counts are considered high once they reach 90 grains per cubic meter of air, and very high at 1,500 or more. During peak weeks in spring, counts in many cities regularly exceed those thresholds.
Late Spring and Summer: Grass Pollen Takes Over
As tree pollen tapers off, grass pollen rises. Grasses like Timothy, Bermuda, Kentucky Bluegrass, rye, and fescue begin releasing pollen in May and June, then hit their peak in July and August. Grass pollen doesn’t need to reach the same raw numbers as tree pollen to cause problems. Counts as low as 20 grains per cubic meter are classified as high, and anything above 200 is very high. That’s why even a moderate-looking grass pollen day can trigger significant symptoms.
Fall: Ragweed Dominates
Weed pollen, especially ragweed, takes over starting in August and can persist for six to ten weeks. In most of the U.S., ragweed peaks in mid-September and lingers through October. Your symptoms typically won’t let up until the first hard frost kills the plants. Weed pollen counts hit the “high” category at 50 grains per cubic meter and “very high” at 500.
Ragweed is the single biggest driver of fall allergies. A single ragweed plant can release billions of pollen grains in a season, and those grains travel hundreds of miles on the wind.
Winter Isn’t Always Safe
Most people think of winter as a pollen-free window, but that’s not true everywhere. Mountain cedar trees (a type of juniper) pollinate during cold weather, typically from December through February, with symptoms sometimes stretching from November into March. This phenomenon, known as cedar fever, is most intense in central Texas, where Ashe juniper trees grow in dense concentrations. Cedar trees also pollinate in parts of Arkansas, Arizona, New Mexico, Oklahoma, and Missouri. Juniper pollen tends to spike right after a cold front passes through.
Pollen Peaks Later in the Day Than You’d Think
A common belief is that pollen is worst first thing in the morning. Research tells a different story. The lowest pollen concentrations occur between 4 a.m. and noon. After that, levels climb steadily and reach their maximum between 2 p.m. and 9 p.m. If you’re trying to exercise outdoors or open your windows, mornings are generally the better bet.
Weather plays a role too. Dry, windy days push more pollen into the air, while rain temporarily washes it away. A stretch of warm, breezy days after a rain event tends to produce some of the highest counts of the year.
Pollen Seasons Are Getting Longer
If it feels like allergy season lasts longer than it used to, that’s backed by data. A study published in the Proceedings of the National Academy of Sciences found that between 1990 and 2018, pollen seasons across North America started about 20 days earlier and lasted roughly 8 days longer. Over that same period, total annual pollen concentrations increased by about 21%. Warmer temperatures extend the growing season and allow plants to produce more pollen, meaning the trend is likely to continue.
What Happens in Your Body
When pollen grains land in your nose, eyes, or throat, your immune system may identify them as a threat. In people with pollen allergies, immune cells release histamine and other inflammatory chemicals in response. Histamine is what causes the familiar cascade of sneezing, itchy eyes, nasal congestion, and a runny nose. The reaction can start within minutes of exposure and worsen with continued contact throughout the day.
Timing Your Allergy Medications
If you know which pollen season triggers your symptoms, starting your allergy medication about two weeks before that season begins can make a real difference. This gives antihistamines and nasal sprays time to build up their effect before your body is flooded with pollen. For spring tree pollen allergies, that means starting in mid-January in southern states or early February farther north. For ragweed sufferers, mid-July is a reasonable time to begin. Checking local pollen forecasts as the season approaches helps you fine-tune the timing for your area.