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Floating primrose-willow identification and control
Ludwigia peploides, Family Onagraceae
Floating primrose-willow, also called creeping water primrose, is an herbaceous perennial that grows in water and wet areas, forming dense mats in water up to 10 feet deep or growing flat along the mud. Although it is native to parts of North, Central and South America, it can be very invasive out of its native range, including in Washington where it is a Class A noxious weed. It is very similar to a closely related plant called water primrose (Ludwigia hexapetala), a Class B noxious weed in Washington.
Legal status in King County, Washington
Public and private landowners are required by state law to eradicate this plant when it occurs on their property. Floating primrose-willow is a Class A Noxious Weed in Washington due to its limited distribution in the state and the potential for significant impact to state resources. For more information see Noxious Weed Lists and Laws.
This species is also on the Washington quarantine list (known as the prohibited plants list) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts of this species, into or within the state of Washington. It is further prohibited to intentionally transplant wild plants and/or plant parts of this species within the state of Washington. See Noxious Weed Lists and Laws.
Because of the difficulty in distinguishing this plant from native water purslane (Ludwigia palustris) and other species, we recommend contacting the noxious weed program for a positive identification before removing the plants. There is currently only one record of this plant in King County, so if you do find floating primrose-willow in King County, please report the location right away.
Identification (see below for more photos)
Showy yellow flowers with five petals on stalks attached at leaf axils (where leaf attaches to stem)
Petals are about 1/2 inch long
Blooms late July to August
Leaves are alternate on the stem (not opposite like the native water purslane)
Leaves variable in size and shape, lance to egg-shaped and up to 3.5 inches long
Leaves have smooth margins (edges) and are either hairless or with long, soft hairs
Leaf base tapers to a stalk (1-1.5 inches long)
Leaf veins are light green and pinnate (arranged like a feather)
Stems tend to float along water's surface or grow prostrate along the mud
Stems fleshy, reddish, 8 inches to 2 feet long
Roots two types: either attached to substrate or adventitious on stems (to get oxygen)
Can be confused with native water purslane (Ludwigia palustris), which has inconspicuous green flowers and opposite leaves, and also with the class B noxious weed water primrose (Ludwigia hexapetala), which tends to have more upright flowering stems, but is otherwise very similar to floating primrose-willow.
Habitat and impact
Floating primrose-willow grows in freshwater wetlands, ponds, shorelines and streams. It can tolerate depths up to 10 feet and can also grow up onto land and can tolerate dry spells. In Washington state, the one known infestation is in a ponded area along a tributary to the Cedar River in King County.
This plant has the ability to create dense floating mats that can clog waterways and reduce habitat for native wildlife and plants. It is fast-growing. In France, a small patch of 400 square feet grew to cover 320 acres in five years. Dense growth can impede navigation, clog irrigation canals and streams, increase sedimentation and reduce water flow. It is unpalatable and can displace native vegetation used by waterfowl and other wildlife. When established, it can reduce pH and dissolved oxygen in the water and harm fish habitat.
Floating primrose-willow has been introduced to and is considered invasive in France, Belgium, Italy, Netherlands, Australia and in the United States in Delaware, New York and Washington state.
Growth and reproduction
Floating primrose-willow spreads mostly through plant fragmentation, where pieces of plant break off and can form roots. It also appears to produce viable seeds.
Control
Prevention: Floating primrose-willow may be sold as a water plant so be sure to check the scientific name of all plants before purchasing them, especially through the internet where companies may not be aware of Washington state laws. If you do spot this plant growing in a new place, be sure to act quickly to remove it entirely to keep it from spreading.
Small patches: Be sure to remove the entire plant and not leave any stem or root fragments behind. Dispose of plants in the garbage to avoid spreading the plant further.
Larger patches: Controlling this species is very challenging once it is established in a waterway. In addition, permits will be needed since this plant grows in water. Please check with your local permitting office for more information. Herbicides that are registered for aquatic use in Washington have shown some effectiveness although multiple years of application are needed for complete control. Contact King County Noxious Weed Control Program for more information on herbicide recommendations. Please refer to herbicide labels for site specific control information and refer to the PNW Weed Management Handbook for additional information on herbicide use.
Additional information on floating primrose-willow
What to do if you find this plant in King County, Washington
Please notify us if you see this plant growing in King County. Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. Also, because floating primrose-willow is not established in King County beyond the one known infestation, we have an opportunity to stop it from spreading if we act quickly. We map all known locations of regulated noxious weeds such as this in order to help us and others locate new infestations in time to control them.
Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact staff, see the Noxious Weed Control Program Directory, send an email, or call 206-477-WEED (206-477-9333).
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