Ludwigia linearis

Native to the central and eastern United States, this perennial wetland herb has narrow leaves and upright stems that grow about 1 to 3 feet tall. Yellow flowers have four petals and four triangular sepals, appearing from June through September in Louisiana. The species occurs on muddy shores, in shallow water, and in wet pine flatwoods, prairies, savannas, ditches, and bogs. Ducks and other waterfowl eat its seeds. Photographed in Peru.