The invasive aquatic plants Ludwigia hexapetala (primrose-willow), L. grandiflora and L. peploides ssp. montevidensis (floating primrose-willow) have environmental, economic and social impacts, described in greater detail below. The California Invasive Plant Council Inventory Review Committee has given an "A" rating to these species with respect to environmental impact, the highest score possible. Economic impacts are measured in part by the millions of dollars spent in California on Ludwigia management in projects ranging from annual maintenance in the Potter Valley Irrigation District, Mendocino County to massive suppression efforts in the Laguna de Santa Rosa, Sonoma County. Social impacts include displacement of traditional-use species, aesthetic modification in riparian areas frequented by outdoor enthusiasts and obstruction of mosquito abatement efforts.

Images:

Ludwigia hexapetala monoculture Ludwigia hexapetala forms monocultures where no other types of vegetation can compete. These patches can become quite large in slow-moving, shallow waters such as lakes, ponds and reservoirs.	Riverfront Park sign with weeds Recreational use is not an issue at this reservoir, but water quantity and wildlife habitat are. Evapotranspiration from Ludwigia infestations lead to water loss. The non-native Ludwigia hexapetala provides little benefit to wildlife and displaces native plants that support wildlife. Other weeds, such as Phalaris aquatica (Harding grass), appear in the upland.
Ludwigia in understory Unlike other Ludwigia species, L. hexapetala can survive dessication and so will establish in the understory of riparian woodland, displacing native vegation that supports wildlife.	Tules amid Ludwigia Tule (Schoenoplectus acutus), is a native plant that supports wildlife and is used by Native Americans for baskets, boats and many other items. Shown here amid a lush swath of Ludwigia hexapetala, tules would probably be more prevalent on the shores of this reservoir if Ludwigia were absent.
Ludwigia on rocky banks Aquatic plants that invade rocky shores and open waters introduce vegetation into areas where there had been none. Living plants cause more water to evaporate than would occur if plants were not present. Therefore there is a loss of water in the system that otherwise would have been available for wildlife and habitat support, recreational uses, drinking water or agricultural irrigation.

References Cal-IPC Invasive Plant Inventory, Plant Assessment Form for Ludwigia Hexapetala  , Cal-IPC Invasive Plant Inventory, Plant Assessment Form for Ludwigia peploides ssp. montevidensis