Ludwigia hyssopifolia

Native to tropical America and northern Australia, this erect wetland plant grows as an annual herb or sometimes becomes woody and perennial. Its branching stems can reach about 10 feet tall. Small yellow flowers fade toward orange-yellow and have four petals. Plants rooted in water may develop elongated breathing roots on submerged branches. It commonly occurs beside streams, in ditches, and in other open, damp habitats. It is a serious weed of rice fields in South and Southeast Asia. A single plant can produce about 250,000 seeds, which can float for more than two weeks before sinking. It has become established in Spain. Photographed in Colombia.