Peirson´s Milkvetch (Astragalus magdalenae vat. peirsonii) is a silvery, short-lived perennial plant that is somewhat broom like in appearance. A member of the pea and bean family, it can grow to 2.5 feet tall and is notable among milkvetches for its greatly reduced leaves. Peirson´s milkvetch produces attractive, small purple flowers , generally in March or April, with 10 to 17 flowers per stalk. It yields inflated fruit similar to yellow-green pea pods with triangular beaks.
Peirson´s milkvetch has the largest seeds of any milkvetch. Large seeds are an important adaptation in dune plants. While small seeds can readily germinate under several inches of moist sand, they may exhaust their stored food before the seedling can emerge from the sand at such depths and begin producing its own food. Large seeds provide a greater reservoir of stored food and enable a seedling to grow a greater distance before emergence and/or depletion of their stored energy.
Peirson´s milkvetch is named for Frank Peirson, an early California plant collector who was the first to prepare a scientific collection of the plant back in 1927. In the United States, the plant is known only from the Algodones Dunes (Imperial Sand Dunes), and in nearby Mexico from a limited area of dunes within the Gran Desierto, in the northwestern portion of the state of Sonora. Peirson´s milkvetch is listed as endangered under the California Endangered Species Act and threatened under the federal Endangered Species Act.
View photo of Peirson´s Milkvetch