Ambrosia trifida (Giant Ragweed) Family: Asteraceae (Sunflower)
     
Ambrosia trifida 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.


Vegetation zone:  Plains
Time of bloom:  July - September
Origin:  Introduced
Eastern Colorado Wildflowers