About Ambrosia pumila (Nutt.) A.Gray
Ambrosia pumila is a hairy perennial herb that grows no taller than 0.5 metres (1.6 ft). Its leaves are gray-green, fuzzy, and divided into multiple subdivided segments. Excluding their winged petioles, the leaves reach up to 13 centimeters in length. In the plant's inflorescence, staminate (male) flower heads grow at the tip, positioned above several larger pistillate (female) flower heads. Each pistillate head typically produces one fruit: a fuzzy burr only a few millimeters wide, covered with short, soft spines. This plant rarely produces seeds, and reproduces vegetatively instead, growing new sprouts from an elongated rhizome system. Ambrosia pumila is adapted to dry habitats, but only grows on upper floodplain fringes, or adjacent to depressions that hold vernal pools or similar features. It grows in open habitat and cannot tolerate heavy shade. Currently, 19 known populations of the species exist: 14 are located in San Diego County, two are in Riverside County, and three are found south of the United States border in Baja California and Baja California Sur, Mexico. Most of the species' native habitat has been lost to urbanization and development, and it is also threatened by agricultural activity. It is a federally listed endangered species in the United States.
