Photo from USDA

Tall Marsh-Elder
Iva xanthifolia  Nutt.

An annual that grows 2 to 10 feet tall. The leaves are mostly opposite, but the uppermost leaves are alternate with long petioles with leaf blades are ovate to broadly ovate, 3 to 12 inches long. Flowering heads are stalk less, crowded on long, branching spikes at the top of the stem and upper leaf axils and the flowers are greenish white, with 8 to 20 staminate.  The leaves can produce a skin rash in some people. Flowering occurs from late July through September. It is found in sandy, damp or drying sites, especially streambeds and flood plains.  Found throughout the West and on the Fort Hall Indian Reservation of southeastern Idaho.

Medicinal Uses
An infusion or decoction of the plant has been drunk and used as a lotion in the treatment of coughs and influenza. A poultice of the plant has been used in the treatment of boils.