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Des Moines sits in central Iowa's humid continental climate zone, where a mix of prairie, agricultural land, and deciduous forest creates a prolonged and intense allergy season stretching from early spring through the first hard frost.
Tree pollen kicks off the year in late March and peaks through April and May, with oak, maple, birch, elm, cottonwood, ash, and cedar among the most prolific local offenders. As tree counts taper, grass pollen takes over from mid-May through July, driven primarily by Kentucky bluegrass, timothy, orchard grass, and Bermuda grass common to Iowa's lawns and pastures.
By mid-August, weed season dominates and typically runs through October, with ragweed being the region's most notorious allergen—Iowa's agricultural landscape produces exceptionally high ragweed counts—alongside pigweed, lamb's quarters, sagebrush, and Russian thistle. The late-spring overlap between tree and grass pollen, and the August transition between grasses and weeds, often intensify symptoms for multi-sensitive sufferers.
Beyond pollen, Des Moines residents contend with high summer humidity that fuels outdoor mold spores, indoor dust mites, and agricultural dust drifting in from surrounding farmland.
Overall, Des Moines's allergy profile is defined by a long, layered season marked by heavy ragweed exposure and persistent mold driven by Midwestern humidity.