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General: Evening Primrose Family (Onagraceae). Ludwigia decurrens, a native, is the tallest water primrose, reaching a height of 3-6 feet. This perennial is somewhat “woody”, which is unusual for species of water primroses. It is considered an annual to perennial. It also stands upright in contrast to most Ludwigia species, which trail along the ground or water. The leaves are usually linear (hence the name willow primrose), alternate, and entire. The flowers are yellow with 4-5 petals (usually 4). Also, common to the Ludwigia genus is the “seed box” located immediately below the flower. In fact, some refer to the entire genus as “seedboxs.” Willow primrose has a very distinctive stem that is 4-winged and can be distinguished from all other water primroses by this unique characteristic.
There are approximately 11 species of Ludwigia in Texas. All species are found in wet sites and all but one species have yellow flowers. Six species have erect growth habits. Vegetatively, Ludwigia alternifolia is the most similar, but it does not have the distinctive winged stems and the seedbox is not nearly as long. Ludwigia decurrens has one of the longer “seedboxs” of all Ludwigia, attaining lengths of up to ¾ inch.