Marsh Elder
Cyclachaena xanthiifolia (Iva xanthifolia)

Family: Sunflower (Asteraceae)
AKA: False Ragweed, Giant Sumpweed, Carelessweed
Photo taken on: August 29, 2014
Location: Taylor Canyon, GV, CO
Life Zones: Plains to foothills
Habitat: Moist areas, stream banks, roadsides

Grows 4 to 6 feet tall with robust, un-branched stems. Leaves have long stalks and are large and coarsely toothed. Tiny, inconspicuous, greenish cream flowers grow in long clusters. Contact with the plant can cause irritation and the pollen is a cause of hay fever.

Flora of North America reference.