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water primrose (Ludwigia ssp.)
ODA rating: B
Oregon water primrose distribution
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  Images courtesy of Glenn Miller, Oregon Dept. of Agriculture.
If images are downloaded and used from the ODA web site please be sure to credit the photographer.

Creeping waterprimroses, are perennial, occurring in marshes, swamps, ditches, ponds, and around lake margins, where they form dense floating mats up to 3 ft. tall, crowding out native species. The stems root freely at the nodes either in the water or in damp soil. Reproduction occurs both by seeds and vegetatively through fragmentation. Creeping waterprimroses produce light green, floating stems early in the season with rosettes of smooth, shiny, rounded leaves. Later in the season, the stems become erect, reddish-brown, and produce elongated, willow-like, pointed leaves arranged alternately along the stems. Wiry, branched roots form at the nodes giving the root system a feathery appearance. Emergent leaves and stems usually are slightly to extremely hairy, giving the plant another common name, "hairy waterprimrose". Flowers appear during early summer on stalks attached in the upper leaf axils of emergent stems. They are solitary, up to an inch in diameter, have five to six bright yellow petals, and may be covered with hairs, particularly on the stalks. Flowering occurs from mid to late summer and continues until a killing frost. Many small, yellowish seeds are produced during the summer in elongated, woody capsules. Long distance dispersal can be linked to human factors such as plant marketing and ornamental plantings, it may also be linked to waterfowl feeding and transport during migrations. Short distance dispersal occurs through flood events, waterfowl movement and through human disturbances.
Significant clogging of irrigation canals and drainage ditches occurs in California where Ludwidgia has established a foothold. Due to the potential for crop damage and environmental concerns, plant removal is often limited to mechanical means. Such practices are expensive and time consuming. Recreation is also impacted due to the loss of fish habitat, fishing access, clogging of boating waterways, and swimming areas. Wildlife habitat can become degraded by monoculture infestations of Ludwigia in a number of ways.  Infested waterways suffer drops in dissolved oxygen, which can kill fish and invertebrates reducing productivity. Waterfowl loose preferred food plants and feeding grounds. Species richness of all species drops significantly. Infested waterways often build up significant populations of mosquitoes because of the improved habitat conditions for them, which provides protection for developing larvae.
Creeping waterprimroses, are plants native to Central and South America. Water primrose species have flourished in the United States for many decades, but only in the last 10-15 years have they rapidly expanded their range and density. There are multiple species and sub-species of primrose willows on the west coast of the United States. Many are native to Central and South America and are now demonstrating highly invasive growth habits. It is noted that these species were sold in the nursery trade as aquatic garden plants which may explain their distribution nationwide.
Distribution in Oregon

Biological controls
There are no biological controls available in Oregon at this time. 
Informational links
Printable trifold water primrose brochure (pdf)