is usually found along trails and roadsides. Male flowers hang in umbels along
the top of the stem. The involucres that cover the umbels have a black rib, which
is an identifying characteristic. Female flowers are inconspicuous and are located
in the leaf axils. Large plants have leaves with three lobes, reflected in the
species name trifida. Smaller plants, however, may lack three-lobed leaves
and have simple leaves with entire margins. Ragweed pollen causes hay fever.
Compare to Ambrosia psilostachya,
which has dissected leaves. Involucres
of the male flower umbels lack a black rib.
Compare to Ambrosia tomentosa, which
has finely dissected leaves. Female flower heads are spiny burrs.