The pollen count in San Francisco peaks in March–May, though allergy sufferers can experience symptoms from as early as late winter. California's Mediterranean climate produces a long pollen season from February through June.
The primary driver is Grass — grass pollen affects more Americans than any other allergen — over 40 million people are sensitive to it. On warm, dry, and windy days the pollen count in San Francisco can spike dramatically, while rain temporarily brings it down.
Tree pollen starts in late winter and runs through spring. Grass pollen peaks during spring. Weed pollen is less of a concern in the west region.