Quick Answer
For most Americans, allergy season runs from February through October — nearly 9 months. Tree pollen season begins in late winter and peaks in spring; grass pollen dominates late spring and summer; weed pollen (primarily ragweed) peaks in late summer and fall. Patients sensitized to multiple allergens may experience nearly year-round symptoms.
The Allergy Season Calendar: Month by Month
February–April (Tree Pollen Season): In the South, tree pollen can begin as early as January or February with cedar and juniper (particularly mountain cedar in Texas, which starts in December). Birch and oak pollen peak in April–May in the Northeast and Midwest. Maple and elm are early producers in the mid-Atlantic. Spring tree pollen season lasts approximately 6–8 weeks in any given region.
May–July (Grass Pollen Season): Timothy grass, Kentucky bluegrass, Bermuda grass, and orchard grass dominate this period. Grass pollen season in the continental US runs from late April through July in northern states and continues through September in southern states where Bermuda grass grows year-round. Grass pollen is the most significant summertime trigger globally.
August–October (Weed Pollen Season): Ragweed is the primary weed allergen across the eastern and central US. Sagebrush (Artemisia) is the dominant late-season trigger in the West. Weed pollen season ends with the first hard frost — in southern states this may extend to November. Mold spores from Alternaria and Cladosporium also peak in late summer, extending total allergen burden.
How Climate Change Is Extending Allergy Season
Research from the University of Utah and other institutions shows that North American pollen seasons have lengthened by approximately 20 days since 1990 and total pollen production has increased by 21%. Warmer spring temperatures advance tree pollen release by days to weeks. Delayed autumn frost extends weed pollen season into November in regions where it previously ended in October.
Higher atmospheric CO2 concentrations directly increase ragweed pollen production — in elevated CO2 environments, ragweed plants grow larger and produce significantly more pollen than in pre-industrial CO2 conditions. These trends are projected to continue, with allergy seasons potentially becoming 4–6 weeks longer by 2050 under current emissions scenarios.
Year-Round Allergies: When Seasons Overlap
Patients sensitized to multiple allergen types (polysensitized) experience overlapping seasonal symptoms and may have nearly continuous allergy symptoms from February through October. In southern and coastal regions with mild winters, even modest pollen production can continue year-round, and mold spores remain elevated in humid environments.
True perennial (year-round) allergy is caused by non-seasonal indoor allergens: house dust mites (present year-round in bedding and carpets), pet dander (constant exposure if pets are in the home), cockroach proteins, and indoor molds. Patients with both seasonal and perennial allergies may have continuous symptoms for 10–12 months annually.
Key Takeaways
- Allergy season spans February through October for most Americans — nearly 9 months of pollen exposure.
- Tree pollen: winter-spring. Grass pollen: spring-summer. Weed pollen: late summer-fall.
- Climate change has extended pollen seasons by 20 days and increased pollen production by 21% since 1990.
- Patients allergic to dust mites, pet dander, and mold may have year-round (perennial) symptoms.
- Starting preventive medication 1–2 weeks before your peak season begins significantly improves control.
Related Guide
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