The pollen count in Denver peaks in April–September, though allergy sufferers can experience symptoms from as early as early spring. High altitude compresses pollen seasons in mountain states, but counts can be intense during peak weeks.
The primary driver is Mixed Trees — a mix of oak, maple, birch, and elm pollen peaks throughout spring. On warm, dry, and windy days the pollen count in Denver 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.