Ambrosia artemisiifolia

Native

Disturbed sites

August to September

Green

12″ to 40″

This is a 12″ to 40″, usually 12″ to 24″ tall, erect, annual forb that rises from a taproot.

The stems are erect, bushy, frequently branched at least above the middle, and hairy.

Leaves are opposite near the base, alternate above. They are deeply cut into lobes that are spaced out along the midrib but not cut to the midrib. The lobes are themselves deeply cut into lobes that are spaced out along the midrib but not cut to the midrib (the lobes are pinnatifid, the leaves are bipinnatifid). The leaves are 1″ to 3½″ long, usually no longer than 2⅛″, and ¾″ to 2″ wide, usually no wider than 1⅛″. They are wider at the base than at the tip. The upper and lower surfaces are gland-dotted and, on younger leaves, covered with stiff hairs that are either straight or appressed. Mature leaves are relatively hairless. The leaves are on stalks that can be 1″ to 2⅓″ long, but are usually no more than 1⅓″ long.

There are separate male and female flowers, both borne on the same plant. Male flowers are borne in spike-like inflorescences at the ends of stems and branches. The spikes are up to 4″ long with 12 to 20 or more flower heads. The heads are green, bead-like, and about ⅛″ in diameter. They hang downward at the end of short stalks. Female flowers appear singly from upper leaf axles. Both flowers are rayless.

The fruit is an achene without tufts of hair.

Annual bursage (Ambrosia acanthicarpa) also has bipinnately lobed leaves and closely resembles common ragweed. However, its leaves are covered with white to gray, short, bristly hairs.

Giant ragweed (Ambrosia trifida var. trifida) is a much taller plant. Its leaves are palmately lobed.

Lance-leaved ragweed (Ambrosia bidenta) leaves, as the latin name suggests, have two teeth, one on each side of the leaf.

Western ragweed (Ambrosia psilostachya) leaves are pinnatifid (once cut), not bipinnatifid (twice cut).