Invasive Plant: Water Primrose

Photo shows a waterway filled with an invasive plant.
Water primrose forms dense mats that fill in wetlands, riverbanks, and lakeshores—slowing or stopping the water completely. This leads to poor drainage, loss of habitat, and low water quality. These factors hurt fish and activities like swimming, boating, and fishing.
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At a Glance

  • Scientific name: Ludwigia hexapetala, Ludwigia peploides
  • Annual, perennial, or biennial? Perennial
  • Conditions it likes best: Marshes, swamps, ditches, ponds, and lake edges
  • How it spreads: Roots, stems, and leaves break off of floating mats and develop into new infestations. Seeds can survive in the soil for seven or eight years or be moved by water and waterfowl.
  • How it causes damage: Slows down channels and streams by rooting deep into their banks. Once it gets started, Ludwigia grows floating strands that stretch to 20 feet across the surface of the water, slowing water even more.
  • Best time and way to manage: : Professional crews manage Ludwigia in August and September.
  • Oregon Department of Agriculture noxious weed rank: B
  • Report patches of this plant? Yes. Please report sightings of this plant to Portland's Environmental Services. Find contact information on this page. A photo and location are required.

How to Identify Water Primrose

Photo shows a green plant with a small yellow flower in the middle against a background that looks like a pond.
Water primrose produces yellow flowers from late July to early September. Because water primrose lives near and in water, this plant should only be managed by professionals.

Spring. Water primrose produces light green, floating stems as early as May, but also in June.  It has smooth, shiny, rounded leaves. Its waxy, oblong leaves have pronounced veins and smooth edges.

Summer. In late summer, its stems often turn red, and sometimes grow straight up from the surface of the water. Between late July and early September, water primrose produces pretty, 5-6 petaled yellow flowers about one inch across. Flowering continues until October.

Fall-Winter. Water primrose dies back in the fall and is not visible until spring.

How and When to Remove Water Primrose

  • Requires professional management. Water primrose is a difficult plant to manage, and because it lives near and in water, this plant should only be managed by or in collaboration with professionals.
  • Manual. Digging up water primrose is a lot of work and doesn’t work very well on large patches. It is easy to leave roots still in the soil or to break off a piece of stem. Both roots and stems can make new plants.
  • Herbicide. Herbicide treatments appear to be most effective. However, aquatic use of herbicide must only be done by licensed applicators. Do not attempt this method on your own. Contact Environmental Services (contact information on this page) or your county’s weed program contact.
  • Check: Monitor the site at least annually and especially 2-3 years after treatment to catch any new plants early.

Prevention is Best Practice

Clean your boots, pets, and maybe even your tires when you finish a hike or trail ride in Pacific Northwest natural areas, or if you have invasive plants on your own property. Cleaning boots and pets keeps invasive plants from spreading to new places. Be careful when trading plants with neighbors and fellow gardeners to make sure you’re not trading this or any other plant of concern.

Be Aware of Look-alikes

A native species of water primrose grows in the wet areas invaded by the non-native water primrose. However, the native plants have  small, inconspicuous flowers and leaves on opposite sides of the stems. Invasive water primroses have bigger (1-inch), pretty yellow flowers. The leaves that are not opposite each other on the stem. Invasive water primroses on the West Coast include L. peploides, L. grandiflora and L. hexapetala.

Properly Dispose of Invasive Plants

To prevent spreading invasive plants:

  • Never place plants or seeds in your compost or yard debris.
  • Never throw pulled plants on the ground or into the street or sidewalk.
  • Put all pulled plants, bulbs, or seeds into a plastic bag and put the bag in the trash.
  • Wash all garden tools and gloves.