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The City of Portland, Oregon

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Ave, Suite 613, Portland, OR 97204

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Why is this species a problem?

Water primrose is known to form dense plant mats which fill in wetlands and slow-moving water bodies, slowing them down further or stopping the flow completely. Shoreline root mats can be 1-2 feet deep, preventing all other plants from rooting. This view of water primrose infestations is pretty typical:

water primrose

Do you see any open water? Right. Clogged waterways are associated with several problems:

  • Reduced plant and animal diversity as other species are displaced
  • Poorer drainage flows as mats slow water and accumulate sediment
  • Lower water quality as dissolved oxygen is used up
  • Fewer recreation opportunities, like fishing or swimming

Managing water primrose is also difficult. The plants are often too deeply rooted (and heavy!) to easily, or effectively, hand-pull. Ludwigia species spread primarily by fragments, and, to a lesser degree, by seed. Both seeds and fragments are moved around by running water and waterfowl.

seeds spread by running waterseeds spread by birds