The pollen count in Portland peaks in March–June, though allergy sufferers can experience symptoms from as early as late winter. The Pacific Northwest is known for its intense alder and birch pollen seasons, plus some of the highest grass pollen counts in the US.
The primary driver is Alder — alder is one of the first trees to pollinate each year, sometimes releasing pollen as early as January. On warm, dry, and windy days the pollen count in Portland can spike dramatically, while rain temporarily brings it down.
Tree pollen starts in late winter and runs through spring. Grass pollen peaks during late spring through early summer. Weed pollen is less of a concern in the northwest region.