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NATURAL HISTORY

PEIRSON’S MILK VETCH } Astragalus magdalenae var. peirsonii
FAMILY: Fabaceae


DESCRIPTION: The Peirson’s milk vetch is a perennial herb with erect stems, eight to 36 inches long. Flower clusters contain five to 20 flowers with pink-purple petals, often white tipped. The fruit is an oval, inflated pod covered in stiff, straight, sharp hairs. To survive the harsh climate of the Algodones, Peirson’s has a long taproot extending deep into the sand that enables the plant to find hidden moisture and serves as an anchor in the face of strong winds.

HABITAT: The milk vetch is found in desert dunes at elevations between 180 and 820 feet.

RANGE: The only known occurrences of the Peirson’s milk vetch in the United States are in the Algodones (also known as Imperial) Sand Dunes of Imperial County. The plant is also found on the sand dunes of the Gran Desierto of Sonora, Mexico.

LIFE CYCLE: Peirson’s produces delicate purple flowers and large, papery fruit pods that contain black seeds. As the fruits dry out, they fall from the plant and open slightly; winds scatter the seeds and plants generally flower and produce new seeds between October and May, although germination in any given year is dependent on rainfall. Rainfall in the Algodones Dunes is scarce and irregularly distributed; likewise, Peirson’s milk vetch tends to have scattered occurrences within the dunes. Typically, plants in their first year produce about five fruits per plant, but plants older than one season produce more than 170 fruits. For this reason, it is important for Peirson’s milk vetch plants to survive more than one season. The higher seed yield of older plants makes them crucial for maintaining the seed bank.

THREATS: The Peirson’s milk vetch is threatened by habitat destruction due to off-road vehicle use.

POPULATION TREND: Population size is highly variable from year to year and fluctuates according to precipitation.

Photo © Jim Dice