The plants triggering your allergies depend on the time of year and where you live, but the calendar breaks into three distinct waves: trees dominate from late winter through spring, grasses take over in late spring and early summer, and weeds (especially ragweed) run from August until the first hard frost. Knowing which wave you’re in helps you pinpoint exactly what’s in the air.
Tree Pollen: Late Winter Through Spring
Trees are the first major pollen producers each year, releasing pollen from roughly March through May in most of the United States. In warmer southern regions, tree pollen can appear as early as December or January. Oak pollen season, for example, starts around February 11 in Houston but doesn’t begin until early May in Rochester, New York, a nearly three-month gap driven by latitude. West Coast cities like San Jose, Eugene, and Seattle also see earlier tree pollen than inland cities at the same latitude, thanks to milder coastal winters.
The trees most likely to cause allergy symptoms include oak, birch, cedar, elm, maple, ash, juniper, mulberry, cottonwood, poplar, walnut, willow, hickory, pecan, and olive. Of these, oak and birch are among the most widespread triggers. Birch pollen is especially worth knowing about because it cross-reacts with a long list of foods, causing tingling or itching in your mouth when you eat raw apples, cherries, plums, pears, peaches, apricots, hazelnuts, and even soy. This reaction, called pollen-food allergy syndrome, only happens with fresh or raw forms of these foods. Cooking breaks down the proteins responsible, so a baked apple pie won’t cause the same problem.
Research tracking pollen data from 2002 to 2019 found that tree pollen seasons are getting longer, extending by roughly half a week on average. That may sound small, but it means more cumulative exposure each year. Growing seasons across Europe have increased by about 11 days since the 1960s, and similar trends are playing out in North America.
Grass Pollen: April Through Early June
Grass pollen picks up right as tree pollen starts fading, typically peaking from April through early June. In warmer parts of the country, particularly the Deep South and Gulf Coast, grass pollen can circulate nearly year-round. A study comparing tree and grass allergy patients found that symptom severity actually peaks during grass season: while symptoms begin climbing with tree pollen in mid-March, medication use hits its highest point at the height of grass pollination in early June.
The grasses most likely behind your symptoms are Timothy, Kentucky bluegrass, Bermuda, ryegrass, orchard grass, fescue, Bahia, Johnson grass, and sweet vernal grass. Timothy and orchard grass share similar proteins, so if one bothers you, the other likely will too. These grasses can also cross-react with certain foods, triggering mouth and throat tingling when you eat cantaloupe, honeydew, watermelon, oranges, tomatoes, or white potatoes.
Weed Pollen: August Through First Frost
Ragweed is the dominant allergy trigger from late summer into fall, affecting close to 50 million people in the United States. The season starts around August, peaks in mid-September, and can last six to ten weeks depending on where you live. It ends only when the first hard frost kills the plants. Ragweed grows in every state except Alaska, but it’s most concentrated in the Eastern and Midwestern states.
Seventeen types of ragweed grow in North America, with tooth-leaved ragweed causing the most symptoms. Several related plants in the same botanical family also spread wind-borne pollen and can trigger identical reactions. These include mugwort, sagebrush, marsh elder, rabbit brush, and groundsel bush. If ragweed bothers you, any of these could as well.
How Location Changes Your Allergy Calendar
The farther south you live, the earlier your allergy season starts and the longer it lasts. Cities with earlier start dates consistently have longer pollen seasons overall. In San Jose, California, oak pollen lingers for about 103 days. In Seattle, it lasts just 23 days. Southern cities like Waco, Texas, see their lowest pollen counts in July (the brief gap between spring and weed seasons), while northern cities get their break during winter months.
The Great Lakes region, including cities like London, Ontario, sees a main pollen season running from March through September, covering all three waves with little interruption. If you’ve recently moved to a new region and your allergies seem worse or different, the shift in local plant life and timing is likely the reason.
When Pollen Levels Are Highest Each Day
Pollen doesn’t hit you evenly throughout the day. Research from the American College of Allergy, Asthma and Immunology found that pollen counts are lowest between 4:00 a.m. and noon, then climb through the afternoon, reaching their highest levels between 2:00 and 9:00 p.m. This pattern flips the common assumption that morning is the worst time for allergies. If you want to exercise outdoors or open windows, late morning is your best window. Save indoor time for late afternoon and evening when concentrations peak.
Pollen-Food Allergy Syndrome
If you’ve ever bitten into a fresh apple or slice of melon and felt your lips tingle or your throat itch, your pollen allergy may be the cause. Your immune system mistakes proteins in certain raw fruits and vegetables for pollen proteins, triggering a localized allergic reaction. About 50% of people allergic to peaches will also react to other related fruits like apples, cherries, plums, pears, and apricots. Among people allergic to melons, roughly 90% also react to kiwi, avocado, peach, and banana.
The reactions are almost always limited to your mouth and throat, and they only happen with raw or uncooked foods. Heat destroys the proteins involved, so cooked, canned, or baked versions of the same foods are typically fine. If you notice these symptoms consistently during your worst pollen months, the connection is likely pollen-food cross-reactivity rather than a true food allergy.
Why Your Allergies May Be Getting Worse
If your symptoms feel more intense than they used to, you’re not imagining it. Pollen seasons are measurably longer than they were two decades ago, with tree pollen seasons extending by nearly half a week between 2002 and 2019 based on monitoring station data. Warmer temperatures cause plants to bloom earlier and produce pollen for a longer stretch. Higher carbon dioxide levels also stimulate plants to produce more pollen per plant. The result is more pollen in the air over a longer period, which means more cumulative exposure for anyone who’s sensitized. Mold spore seasons have extended by a similar margin, adding another layer of airborne allergens during the warmer months.