Allergy season in the United States typically runs from early spring through the first hard frost in fall, but the specific timing depends on where you live and which pollen triggers your symptoms. In warmer southern states, pollen can circulate nearly year-round, while northern regions experience a more compressed season from roughly March through September. Understanding the calendar for each type of pollen helps you prepare before symptoms hit.
Spring: Tree Pollen (February to May)
Tree pollen is the first major trigger of the year. For much of the country, it begins in March, but latitude makes an enormous difference. In Houston, Texas, oak pollen season starts as early as February 11 on average. In Rochester, New York, that same oak pollen doesn’t appear until around May 2, nearly three months later. West Coast cities like San Jose, Seattle, and Eugene also tend to see earlier start dates than inland cities at the same latitude.
Birch, cedar, and oak are the most allergenic tree species. If your eyes start itching and your nose runs in early spring before any grass has started growing, tree pollen is likely the cause. In parts of the South, tree pollen can even appear in January.
Late Spring and Summer: Grass Pollen (April to July)
Grass pollen picks up as tree pollen winds down, creating an overlap that can make late spring feel especially brutal. The Asthma and Allergy Foundation of America places the main grass pollen window from April through early June, though it stretches later in cooler climates. In southern states, grass pollen can appear throughout the year rather than following a neat seasonal pattern.
If your symptoms seem to worsen when you’re around freshly mowed lawns or open fields during late May and June, grass is the likely culprit. Grass pollen grains are small and travel easily on the wind, so you don’t need to be standing in a field to be affected.
Late Summer and Fall: Weed Pollen (August to October)
Ragweed dominates the fall allergy season. It starts releasing pollen around August and peaks in mid-September, continuing until the first hard frost kills the plants. Depending on your location, that means ragweed season lasts six to ten weeks. In northern states where frost comes early, the season wraps up in October. In the South, it can stretch well into November.
Ragweed is remarkably productive. A single plant can release a billion pollen grains over its lifetime, and those grains can travel hundreds of miles on the wind. This is why you can have ragweed allergies even in areas where the plant doesn’t grow nearby.
Mold: The Overlooked Fall and Summer Trigger
Outdoor mold spores follow a different calendar than pollen. Mold concentrations peak in summer and fall, driven largely by rainfall and higher temperatures. Research has found that heavier precipitation leads to increased mold spore counts for weeks or even months afterward. If your allergy symptoms persist after pollen counts drop in late fall, or flare up after rainy stretches, mold is a likely explanation.
Mold spore counts are measured on a different scale than pollen. A “high” mold count starts at 13,000 spores per cubic meter, compared to just 90 for tree pollen or 20 for grass pollen. The numbers sound dramatic, but the thresholds reflect how differently your immune system responds to each type of particle.
How Your Region Changes the Calendar
The single biggest factor in your personal allergy calendar is latitude. Warmer climates have longer seasons and earlier start dates. Here’s how the major regions break down:
- The South (Texas, Florida, Gulf Coast): Pollen is present in the atmosphere virtually year-round. Tree pollen can start in January, grass pollen appears across all seasons, and ragweed season extends deep into fall. The lowest pollen concentrations in cities like Waco, Texas, come in July, the opposite of northern patterns.
- The Northeast and Great Lakes: The main pollen season runs from March through September. Tree pollen starts later (April or May), grass peaks in June, and the first frost in October or November provides a clear endpoint.
- The Pacific Northwest: Seattle has a relatively short pollen season. Very little pollen is detected after July, making it one of the better locations for fall allergy sufferers.
- California: Pollen circulates year-round in cities like San Jose, similar to southern states. The mild climate means there’s no hard frost to shut down weed pollen in fall.
Allergy Seasons Are Getting Longer
If it feels like your allergies have gotten worse over the years, the data backs you up. A 2021 study found that human-caused warming was the primary driver of North American pollen seasons lengthening by 20 days on average between 1990 and 2018. An analysis of 172 U.S. cities by Climate Central found their freeze-free seasons had lengthened by 20 days on average, which directly extends the window for ragweed and other late-season allergens.
Longer warm seasons mean plants have more time to produce pollen. Rising carbon dioxide levels also cause many plants to produce more pollen per plant. The result is seasons that start earlier, end later, and carry heavier pollen loads than they did a generation ago.
What Happens in Your Body During Allergy Season
When you inhale pollen, your immune system can mistake it for a threat. Immune cells in your nasal lining process the pollen and trigger a chain reaction: specialized white blood cells start producing antibodies tailored to that specific pollen. Those antibodies attach to mast cells, which are packed throughout your nose and airways. The next time that pollen shows up, the mast cells recognize it immediately and release histamine, along with other inflammatory chemicals.
Histamine is what causes the four hallmark symptoms: runny nose, nasal congestion, itching, and sneezing. This initial reaction happens within minutes of exposure. A second wave of inflammation often follows hours later, which is why your symptoms can worsen in the evening even if pollen exposure happened that morning.
How to Check Pollen Counts
Local pollen counts are reported daily by monitoring stations and available through weather apps, the National Allergy Bureau, and sites like pollen.com. The numbers represent grains per cubic meter of air, and each pollen type has its own scale. For tree pollen, anything above 90 is considered high. For grass, that threshold is just 20. For weed pollen, it’s 50.
Pollen counts tend to be highest in the morning and on warm, dry, windy days. Rain temporarily washes pollen out of the air, so the hours after a rainstorm are often the most comfortable for outdoor activity. Counts drop to near zero after the first hard frost in fall.
Starting Medication Before Symptoms Hit
One of the most practical things you can do is start allergy medication one to two weeks before your local season begins, not after symptoms are already in full swing. Nasal steroid sprays in particular take one to two weeks to reach their full effect. If you wait until you’re already congested and miserable, you’re playing catch-up rather than prevention.
Knowing your specific triggers helps you time this right. If tree pollen is your problem and you live in the Northeast, that means starting medication in mid-to-late March. If ragweed is your main issue, mid-July is the time to begin. Checking historical pollen data for your city can give you a reliable estimate of when counts typically start climbing each year.