Ludwigia hexapetala - Water Primrose - A Probem Aquatic Plant in the Western USA

Water Primrose

The Western Aquatic Plant Management Society

Description and Variation

Water Primrose is a perennial herb that can be found creeping along the shoreline, floating on the water surface, or growing upright. It is a robust plant with bright yellow, showy flowers and willow-like leaves. It is a non-native species originally from South America and the s ludwigiaflower.jpg (56122 bytes) outhern United States and has been introduced into Europe and northern North America.

Because of its showy yellow flowers, this plant is being sold as an ornamental species. Lake residents are strongly discouraged from planting water primrose in lakes, private ponds with an outlet or in a flood zone, or in natural waterbodies. These plants are very invasive and aggressive and will form very dense mats of vegetation.

Growth Habit

Water primrose grows in dense mats along shorelines and out into the water. Water primrose mats along the drainage ditches in Longview/Kelso It favors the margins of lakes, ponds, ditches, and streams. It blooms throughout the summer. This photograph shows water primrose growing along a ditch bank in Longview, Washington. It reproduces by seeds and by plant fragments.

Management

Methods such as mechanical removal, cutting, covering with opaque materials, and using the aquatic herbicide RodeoŽ may be effective. It is not likely that grass carp would find water primrose to be palatable.

Identification

Water primrose can be identified by its sprawling growth habit and showy yellow flowers. Look for the following characteristics:

Forms a vegetative mat sprawling across the water surface, rooted in the mud, with flowering stems held above the water.
Stem glabrous to sparsely pubescent, grows horizontally on water or mud. At flowering the stems lengthen and grow upright. Flowering stems can rise to 3 ft. above the water surface.
Feathery roots at the nodes often dangle in the water, also often has white, spongy aerenchymous roots at the nodes.
Leaves are alternate. Each growth consists of rosette-like clusters of rounded (suborbicular to spatulate) leaves on the water surface. At flowering they lengthen to lanceolate or elliptic shaped (willow like).

Flowers are bright yellow, 5 or 6 petals, arise from leaf axils on peduncles 2-3 cm long. Petals 15 to 30 mm, sepals are persistant, 8-19 mm long.
Fruit is a cylindrical capsule.