Allergy season refers to the stretches of the year when plants release pollen into the air, triggering symptoms like sneezing, congestion, and itchy eyes in sensitized people. In most of the United States, it runs from early spring through late fall, roughly March to November, though the exact timing depends on where you live and which plants grow there.
How Your Body Reacts to Pollen
Seasonal allergies happen when your immune system misidentifies harmless pollen grains as a threat. The first time you’re exposed to a particular pollen, your body may quietly produce antibodies against it without you noticing anything. The trouble starts on subsequent exposures. Those antibodies sit on the surface of specialized immune cells lining your nasal passages, and when matching pollen lands there, the cells release a flood of inflammatory chemicals, including histamine. That’s what causes the sneezing, the runny nose, the watery eyes, and the swelling inside your sinuses.
This initial burst of symptoms can hit within minutes of breathing in pollen. But the reaction doesn’t always stop there. The same immune cells also recruit reinforcements to the site, which can keep inflammation going for hours or even days during heavy pollen exposure. That’s why symptoms often feel worse as the season progresses: your nasal tissue is already inflamed from repeated exposure, so each new wave of pollen hits harder.
Spring: Tree Pollen From March to May
Spring is when most people first notice their symptoms each year. Tree pollen is the dominant trigger from roughly March through May, though in southern states, trees can start releasing pollen as early as January. The most common offenders include oak, birch, elm, maple, ash, cedar, sycamore, and pecan. Oak pollen is particularly widespread and produces heavy yellow-green dust you can sometimes see coating cars and outdoor furniture.
Different tree species peak at slightly different times, so spring allergy sufferers often feel like they can’t catch a break. Elm and cedar tend to pollinate earlier, while oak and birch peak a bit later. If you’re allergic to multiple tree species, your spring season can effectively stretch from February into June depending on your region.
Summer: Grass Pollen Takes Over
As tree pollen winds down in late spring, grass pollen ramps up and can stay airborne from late spring through early autumn. Grasses are the main summer allergen across most of the country, with species varying by region. In California, for instance, Bermuda grass is the dominant pollen producer, while timothy and Kentucky bluegrass cause more trouble in northern and midwestern states.
Weather plays a big role in how miserable summer gets. Warm, breezy days push pollen counts to their highest levels, while rain temporarily washes pollen out of the air. Hot, humid conditions also encourage mold spore growth outdoors, which can layer additional allergy symptoms on top of grass pollen reactions. If you feel worse on dry, windy afternoons and better after a rainstorm, pollen counts are likely driving your symptoms.
Fall: Ragweed From August to November
Ragweed is the dominant fall allergen, and it’s remarkably productive. A single ragweed plant can release up to a billion pollen grains over the course of a season, and those grains can travel hundreds of miles on the wind. Ragweed typically pollinates from August through November depending on location, making fall allergy season longer than many people expect.
Ragweed affects roughly 15% of the U.S. population and is most problematic in the Midwest and East Coast. It’s less of an issue in parts of the West, including much of California, where ragweed is relatively uncommon. If your worst allergy symptoms consistently arrive in late summer and linger into October, ragweed is the most likely culprit.
Winter and Indoor Allergens
Outdoor pollen largely disappears once the first hard freeze arrives, but that doesn’t mean allergy symptoms stop. Many people shift from pollen reactions to indoor triggers during winter. Mold is one of the most common: it can grow in basements, behind walls, on damp surfaces, in carpet pads, and anywhere moisture accumulates. Only certain mold types cause allergies, with common varieties including alternaria, aspergillus, and cladosporium.
Dust mites and pet dander also peak indoors during winter, when homes are sealed up and ventilation drops. If your symptoms persist year-round or flare up when you’re indoors, these triggers are worth investigating with an allergist.
Why Location Changes Everything
Allergy season doesn’t follow one national schedule. In the southern U.S., tree pollen can begin in January and grasses may pollinate nearly year-round. In northern states, the season compresses into a shorter window but can be intense during peak months. Some parts of the country now experience pollen from trees, grasses, or weeds in every month of the year.
Climate change is making this worse. Warmer temperatures cause plants to start growing earlier, pollinate longer, and produce more pollen overall. Urban areas face an additional challenge: trapped heat stimulates pollen production while also increasing air pollution, which can worsen respiratory symptoms independently. The net result is that allergy seasons are starting earlier and lasting longer than they did a generation ago, with more potent pollen concentrations at peak times.
When Pollen Counts Are Highest and Lowest
Pollen concentrations shift throughout the day in a pattern that’s useful to know. Research from the American College of Allergy, Asthma and Immunology found that the lowest pollen levels occur between 4:00 a.m. and noon, while the highest levels hit between 2:00 and 9:00 p.m. This means morning hours are generally the better time for outdoor activities if you’re trying to minimize exposure. Evenings, especially warm and breezy ones, tend to be the worst.
Local pollen counts are tracked daily in most metro areas and are available through weather apps and sites like pollen.com. Checking the count before spending extended time outdoors can help you plan around the worst days.
Allergies vs. a Cold
Seasonal allergies and the common cold share several symptoms, including sneezing, a stuffy nose, and a runny nose. The key differences come down to itchiness, duration, and timing. Allergies cause itchy, watery eyes, which colds almost never do. Allergies also produce clear, thin nasal discharge, while colds tend to generate thicker mucus that may turn yellow or green after a few days.
Duration is the other reliable clue. A cold typically resolves within one to two weeks. Allergies last as long as you’re exposed to the trigger, which can mean weeks or months of continuous symptoms during pollen season. If your “cold” shows up at the same time every year and drags on for more than two weeks, it’s almost certainly allergies.