WSU Extension

Hortsense

Weeds : Water primrose : Ludwigia uruguayensis (L. hexapetala)
(revision date: 6/9/2014)

Family: Onagraceae
Cycle: Perennial
Plant Type: Broadleaf aquatic

Use Integrated Pest Management (IPM) for successful weed management.

Biology
Water primrose is a perennial aquatic herb. Stems sprawl or grow horizontally to erect on water or mud, rooting freely at the nodes to form large, dense, tangled mats. Early growth consists of rosette-like clusters of rounded, smooth, shiny leaves on the water surface. At maturity, the alternate leaves are longer, hairy, and somewhat willow-like in appearance. The erect flowering stems are woody and reddish, often splitting along their length. They can rise to 3 feet above the water surface. The flowers are produced singly and have five (sometimes six) bright yellow petals. The flower is typically about one inch across. Water primrose reproduces by seed and also by plant fragments, which root freely in water or on damp soil. Water primrose is an aggressive and invasive aquatic species, forming extensive mats that impair water flow and shoreline activity. This species has the potential to dominate the shoreline vegetation if introduced to lakes, river, ponds, ditches or streams. It is very difficult to control once established. SPECIAL INFORMATION: In WASHINGTON and OREGON, water primrose is designated as a Class 'B' noxious weed. Management of this species may be required by law in your county. In addition, it is on the Washington noxious weed seed and plant quarantine list, which prohibits sale, purchase, and transport of plants, seeds, and plant parts. Floating primrose-willow (L. peploides) is a related species that is similar in appearance. It is a Class 'A' noxious weed in WASHINGTON; control is REQUIRED by state law. In OREGON, it is a Class 'B' noxious weed. Consult your local Noxious Weed Control Board for more information.
Habitat
Water primrose is an aquatic herb, found growing and rooted in areas of shallow water to about 3 feet deep, as well as along shorelines. It can withstand slowly flowing water. It favors the margins of lakes, ponds, ditches and streams.

Management Options

Non-Chemical Management
  • Hand-pull to eliminate weeds.
  • Inorganic mulches, such as plastic, commercial "weed barrier" fabrics and other materials such as roofing paper, is an effective weed management option. Cover inorganic mulches with a thin layer of soil or organic mulch.
Select non-chemical management options as your first choice!

Chemical Management

Digging and handpulling is only effective on very small infestations. Applications should be made by those holding a current aquatic pest control license. In Washington, a special permit is required for use of herbicides in aquatic sites. Contact the Washington Department of Ecology or the Washington State Noxious Weed Control Board for more information.

Images
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Caption: Water primrose with flowers
Photo by: Washington Noxious Weed Board slide collection
Caption: Water primrose on the margin of a creek
Photo by: Washington Noxious Weed Board slide collection