For most of the United States, allergy season starts in February or March, when trees begin releasing pollen. But the real answer depends on where you live and what you’re allergic to. In parts of the Southwest, tree pollen circulates as early as December, while in the Deep South, January can bring the first wave of symptoms. Allergy season then rolls through several phases, with different triggers peaking at different times, all the way into November.
Tree Pollen: The First Wave
Tree pollen is what kicks off allergy season for most people. In the Northeast, birch, cedar, and maple trees start releasing pollen as early as February. The Pacific Northwest follows a similar timeline, with alder, birch, and cedar producing pollen from February through April. In the Southeast, oak, elm, and pine are the main offenders during spring. The Midwest sees oak, birch, and cedar pollen arrive in spring as well, though generally a few weeks later than warmer regions.
The outlier is the Southwest. Juniper, cedar, and oak tree pollens can start circulating as early as December in parts of Arizona, New Mexico, and Texas. If you live in this region and notice congestion or sneezing around the holidays, tree pollen is a likely culprit. Across the South more broadly, tree pollen can be prominent by January.
The core of tree pollen season runs from March through May nationwide. This is the stretch when pollen counts tend to be highest and when people who’ve never had allergies before sometimes notice symptoms for the first time.
Grass Pollen: Late Spring Into Summer
Just as tree pollen starts to taper off, grass pollen picks up. Grass pollen season generally runs from late April through June, with counts peaking in late May. In warmer southern states, Bermuda grass and Johnson grass are particularly problematic. In northern states, Timothy grass and Kentucky bluegrass are more common triggers.
This overlap between the tail end of tree season and the start of grass season, roughly late April through mid-May, is when many people feel their worst. If your symptoms seem to drag on well past spring, you may be reacting to grass rather than trees.
Ragweed and Fall Allergies
Ragweed season starts in late summer and runs through early fall, with symptoms typically most intense in August and September when pollen counts peak. A single ragweed plant can release up to a billion grains of pollen over a season, and the lightweight pollen can travel hundreds of miles on the wind. Depending on location, ragweed pollen persists from August all the way into November.
Climate change is making ragweed worse. Rising temperatures and higher carbon dioxide levels cause ragweed plants to grow faster and produce more pollen, and that pollen is more potent in terms of its ability to trigger allergic reactions. For people with ragweed allergies, fall has become a longer, more miserable stretch than it was a generation ago.
Outdoor Mold: The Overlooked Trigger
Mold spores are active outdoors primarily in spring and fall, though they behave differently from pollen. Counts tend to soar one to four days after a rainstorm during those seasons. In late summer and fall, dying and decomposing plants create concentrated pockets of mold spores, which is why your symptoms might flare on damp autumn days even after ragweed counts drop.
One useful distinction: in northern latitudes, freezing temperatures effectively clear outdoor air of mold spores during winter. So if your symptoms disappear completely once the ground freezes, outdoor mold (or pollen) is likely the cause rather than something inside your home.
Year-Round Allergies Feel Different
If your symptoms don’t follow a seasonal pattern at all, the trigger is probably indoors. Year-round (perennial) allergies are caused by things like dust mite droppings, pet dander, cockroach particles, and indoor mold. These allergens are present in homes throughout the year, though they often cause worse symptoms during cold months when you spend more time inside with windows closed.
The simplest way to tell the difference: seasonal allergies start and stop with predictable timing, usually correlating with specific outdoor activities or weather patterns. Perennial allergies flare in response to indoor activities like vacuuming, petting a cat, or spending time in a dusty basement. If you can connect your symptoms to a season or a specific month each year, you’re almost certainly dealing with pollen or outdoor mold.
Allergy Season Is Getting Longer
If it feels like your allergies last longer than they used to, you’re not imagining it. A 2021 study found that human-caused warming was a primary driver of North American pollen seasons lengthening by an average of 20 days between 1990 and 2018. That means allergy season now starts earlier in late winter and stretches further into fall than it did 30 years ago. Climate Central has tracked this shift across 172 U.S. cities, confirming the pattern is widespread.
This lengthening hits from both ends. Warmer late-winter temperatures prompt trees to pollinate sooner, while warmer falls allow ragweed and grasses to keep producing pollen later. For people allergic to multiple types of pollen, the gap between seasons has shrunk or disappeared entirely.
Month-by-Month Allergy Calendar
- December through January: Tree pollen in the Southwest and Deep South. Most of the country is relatively pollen-free.
- February through March: Tree pollen season begins across the Northeast, Northwest, and Midwest. This is when most Americans first notice spring allergy symptoms.
- April through May: Tree pollen peaks nationwide. Grass pollen begins in late April, creating an overlap that intensifies symptoms.
- June through July: Grass pollen dominates. Tree pollen fades. Outdoor mold spores increase after summer storms.
- August through September: Ragweed season peaks. Mold spores rise as plants begin to decay. This is the worst stretch for fall allergy sufferers.
- October through November: Ragweed tapers off. Mold remains active until the first hard freeze. Indoor allergens become more relevant as windows close for winter.
Starting Medication at the Right Time
Knowing your specific trigger month matters because allergy medications, particularly nasal steroid sprays, work best when you start them before symptoms appear. The American College of Allergy, Asthma & Immunology recommends beginning allergy medications about two weeks before your symptoms normally start. If tree pollen reliably hits you in March, that means starting treatment in mid-February. If ragweed is your main problem, mid-July is the time to prepare.
Antihistamine pills can help even when taken after symptoms begin, but nasal sprays need time to reduce inflammation in your nasal passages before pollen exposure ramps up. Tracking which months your symptoms appear, and which allergens are active during those months, lets you time your treatment precisely rather than reacting after you’re already miserable.