The involucre encloses the fruit, which is thus dispersed as a hard, nut-like package. Ambrosia, Cyclachaena, Iva, and Xanthium, with their copious wind-dispersed pollen, all can cause hay-fever. They tend to avoid some of the blame by blooming inconspicuously at the same time of year as more colorful but innocent genera, e.g., Solidago (goldenrod), which have showier displays, but with insect-dispersed pollen, and thus do not cause hayfever.

1. Leaves 3 (-5)-lobed (± palmate) or simple, all opposite.

A. trifida

1. Leaves once or more pinnatifid, the upper ones tending to be alternate.

2. Leaves once- to thrice-pinnatifid, smooth to scabrous above with tiny appressed hairs, any longer hairs spreading; plants annual, from a taproot.

A. artemisiifolia

2. Leaves once-pinnatifid, rough above with ± appressed stiff hairs; plants perennial, with deep, horizontal roots.

A. psilostachya