What is the species?
- Ludwigia peploides ssp. montevidensis (water primrose)
- ODA “B” rank
Water primrose has become a substantial problem in the drainage and irrigation canals of central California. They tend to settle in slow-moving waters and make them even slower. Populations in Oregon tend to be focused in ponds and wetlands and were likely initially introduced from the water garden trade.
Why are we concerned?
Water primrose is known to form dense plant mats which fill in wetlands and slow-moving water bodies. Shoreline root mats can be 1-2 feet deep, preventing all other plants from rooting. Water primrose infestations can lead to:
- Reduced plant and animal diversity as other species are displaced
- Reduced drainage flows as mats slow water and accumulate sediment
- Reduced water quality as dissolved oxygen is used up
- Reduction recreation opportunities, like fishing or swimming
How does it spread?
Water primrose is primarily spread by fragments, but can spread by seed, as well. Water primrose seeds and fragments are likely moved around by:
- Running water
What does it look like?
Water primrose has waxy, oblong leaves with pronounced veins and smooth edges. Its stems have a reddish tinge, especially in later summer, and often stand up off the water’s surface. In August and September, water primrose produces yellow flowers about 1 inch across.
Are there lookalikes?
There are native species of water primrose, as well as several varieties of non-native invasive primrose. Native species have small, inconspicuous flowers and opposite leaves versus alternate leaves and showy flowers for the invasive Ludwigia species. Invasives on the West Coast include not only L. peploides, but also L. grandiflora and L. hexapetala.
How do we deal with it?
- Manual: Digging up water primrose requires a great deal of effort. It must also be dried before it is removed, to prevent further spread. Excavated plants can be composted on-site, if space allows. Specialized tools such as the “LakeRake”™ for mechanically removing this and other aquatic plant species from the water surface, are available.
- Herbicide: Trials are currently being conducted in Portland and in the Eugene area to determine the most effective herbicide and rate. Aquatic treatment of herbicide must only be done by licensed applicators.
How can folks help?
If you suspect you’ve found water primrose, contact either the City of Portland's Bureau of Environmental Services or the Oregon Department of Agriculture. Water primrose should be positively identified before management begins, to prevent unintended damage to related native species.