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Invasive Plant: Purple Loosestrife

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Photo shows a wetland filled with green plants with pink-purple flowers.
Purple loosestrife is an invasive plant with small, magenta flowers. While pretty, it takes over and is a particular problem in sensitive areas like wetlands, which protect water quality and provide critical habitat for many animals. Purple loosestrife threatens the delicate balance in wetlands.
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At a Glance

  • Photo shows tall magenta stalk of flowers against a grassy background.
    Look for tall magenta blooms in moist, marshy areas during the summer months.
    Scientific name: Lythrum salicaria
  • Annual, perennial, or biennial? Perennial
  • Conditions it likes best: Prefers moist, marshy areas, like streams and wetlands.
  • How it spreads: Each plant makes thousands of seeds, which are spread by water, birds, and hiking boots.
  • How it causes damage: Reduces plant diversity and keeps water from flowing.
  • Best time and way to manage: Dig when water levels are low enough, and get the whole plant out. Pieces of root can grow into new plants.
  • Oregon Department of Agriculture noxious weed rank: B
  • Report patches of this plant? No. If it’s on your property, find information below on how and when to remove it.

How to Identify Purple Loosestrife

Photo shows a bunch of plants with pink-purple flowers.
While it is pretty, purple loosestrife takes over an area, using up the water, space, and nutrients other plants need. It's particularly a problem in wetlands, which are sensitive areas that clean water, reduce flood risks, improve water quality, and provide habitat.

Winter–Spring. Dead brown stalks remain through the winter and roots survive winter flooding. Look for re-sprouting in spring.

Summer. Look for tall purple or magenta blooms in moist, marshy areas in the summer (July–September). Purple loosestrife can be over 6 feet tall, with many small, magenta flowers.

Fall. In early fall, the leaves dry out and turn red for a week or two before falling off.

How and When to Remove Purple Loosestrife

  • Manual. Dig when water levels are low enough.  Effective digging means getting all of the plant out. Pieces of root that aren’t removed can grow into new plants.
  • Herbicide. Herbicides used in July or August can be effective on small infestations. Use herbicides carefully to avoid damaging other plants.
  • Check. Check the site for new plants at least once a year, and especially for the first 2-3 years after removal.
  • Replant. After a year or two of managing this plant, consider planting native or less aggressive non-native plants.

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

Fireweed, a common native, looks a little like purple loosestrife but does not grow well in the wet areas that purple loosestrife grows well in.

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.

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