Ambrosia psilostachya

The plant is widespread across much of North America (United States, Canada, and northern Mexico).^[5] It is also naturalized in parts of Europe, Asia, Australia, and South America.^[6] It is a common plant in many habitat types, including disturbed areas such as roadsides.^{[7][8][9][10][11]}

Ambrosia psilostachya is an erect perennial herb growing a slender, branching, straw-colored stem to a maximum height near two meters, but more often remaining under one meter tall. Leaves are up to 12 centimeters long and vary in shape from lance-shaped to nearly oval, and they are divided into many narrow, pointed lobes. The stem and leaves are hairy.^[7]

The top of the stem is occupied by an inflorescence which is usually a spike. The species is monoecious, and the inflorescence is composed of staminate (male) flower heads with the pistillate heads located below and in the axils of leaves.^[7] This bloom period is from June through November.

The pistillate heads yield fruits which are achenes located within oval-shaped greenish-brown burs about half a centimeter long. The burs are hairy and sometimes spiny. The plant reproduces by seed and by sprouting up from a creeping rhizome-like root system.^[12]

Ambrosia psilostachya is a host plant for the caterpillars of Bucculatrix transversata,^[13] Cosmopterix opulenta, Exaeretia gracilis, Gnorimoschema saphirinella,^[14] Schinia sexplagiata; the beetles Calligrapha disrupta, Calligrapha suturalis;^[15] and the grasshopper Spharagemon collare.^[16]

This plant had a number of medicinal uses among several different Native American tribes, including the Cheyenne, Kumeyaay (Diegueno), and Kiowa people.^[17]

Ambrosia psilostachya contains a group of phytochemicals called psilostachyins.^[18]