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Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Avenue, Room 1000, Portland, OR 97204

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Purple Loosestrife

photo of purple loosestrifeWhat is the species?

  • Lythrum salicaria (purple loosestrife)
  • ODA “B” rank

Purple loosestrife prefers moist, marshy areas, affecting streams and wetlands nationwide.

Why are we concerned?

Purple loosestrife is known to form dense clumps which fill in wetlands and slow-moving water bodies. Purple loosestrife infestations can lead to:

  • Reduced plant diversity as other species are choked out
  • Reduced drainage flows as clumps slow water and accumulate sediment

photo of purple loosestrifeHow does it spread?

Purple loosestrife is spread only by seed, but produces large amounts of it (over 100,000 per plant). Purple loosestrife seeds are moved by:

  • Water
  • Waterfowl
  • Hiking boots

What does it look like?

Purple loosestrife is a perennial that can grow to be over 6 feet tall, with hundreds of small, magenta flowers. Seeds are tiny and dark brown. Roots survive winter flooding, re-sprouting in late spring when water levels drop.

Are there any lookalikes?

Fireweed, a common native, resembles it, but is rarely found in the wet areas more likely infested by loosestrife.

How do we deal with it?

  • Manual: Hand pulling often leaves roots fragments behind, which can regrow as new plants. For the same reason, cutting reduces seed production but does not kill the plant.
  • Herbicide: Herbicide treatments in August and September have effectively managed small patches of purple loosestrife.

How can folks help?

If you suspect you’ve found purple loosestrife within the City of Portland, feel free to contact us for more information. Purple loosestrife is known along the Willamette and throughout Oaks Bottom.