The pollen count in Ann Arbor peaks in April–September, though allergy sufferers can experience symptoms from as early as early spring. The Midwest has clearly defined seasons: tree pollen in spring, grass in early summer, and ragweed dominating 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 Ann Arbor 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.