The pollen count in Philadelphia peaks in April–September, though allergy sufferers can experience symptoms from as early as early spring. Northeast allergy season runs April through October, with birch and oak leading in spring and ragweed peaking in fall.
The primary driver is Birch — birch pollen is among the most allergenic of all tree pollens, triggering symptoms in up to 25% of allergy sufferers. On warm, dry, and windy days the pollen count in Philadelphia can spike dramatically, while rain temporarily brings it down.
Tree pollen starts in early spring and runs through spring. Grass pollen peaks during late spring through early summer. Ragweed dominates from August through October.