Ambrosia psilostachya

Native

Dry. Prairies, disturbed sites.

August to September

Green

12″ to 40″, usually 12″ to 24″

This is a 12″ to 40″, usually 12″ to 24″ tall, erect, perennial forb that rises from creeping, fibrous roots and rhizomes. It is colonial, often forming dense patches. The root system releases chemical toxins that inhibit the growth or germination of nearby competing plants (allelopathy).

The stems are erect, bushy, branched at least above the middle, and covered with white hairs.

Leaves are opposite near the base, alternate above. They are narrow and once pinnatifid, the lobes deep, widely spaced, and bluntly pointed. They are ¾″ to 5½″ long, usually no longer than 2⅓″, and ⅓″ to 2″ wide, usually no wider than 1⅓″. The upper and lower surfaces are rough to the touch, gland-dotted, and covered with stiff hairs that are either straight or appressed. The leaves near the base are on stalks up to 1″ long, the stalks progressively shorter as they ascend the stem. Upper leaves are attached to the stem without stalks.

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 5 to 30, usually 5 to 15 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 and have no floral scent.

The fruit is an achene without tufts of hair.

Common ragweed (Ambrosia artemisiifolia) leaves are bipinnately lobed.

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. It has been recorded only in Ramsey County.

Annual bursage (Ambrosia acanthicarpa) leaves are bipinnately lobed and are covered with white to gray, short, bristly hairs.