The pollen count in St. Louis 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 Oak — oak trees release enormous amounts of fine pollen — a leading cause of spring allergies across the US. On warm, dry, and windy days the pollen count in St. Louis 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.