Changing Planet

Purple Loosestrife: Freshwater Species of the Week

Purple loosestrife Photo: Meggar, Wikimedia Commons

Today I flipped through a reference book called A Handbook of Global Freshwater Invasive Species (always a cheerful read). My eye was caught by purple loosestrife (Lythrum salicaria L.), since I have often encountered it on wetlands hikes around the country.

Purple loosestrife is native to Eurasia, but has become an aggressive invader in North America, where it chokes out some local species. According to my Handbook, the plant with pretty purple flowers has also arrived in Australia and New Zealand.

The plant is perennial (meaning individuals last more than one season) and typically grows to a height of 5 feet (1.5 meters), although it has been known to reach 10 feet (3 meters). Thirty to 50 stems often emerge from a single root, so to say the thing is weedy is an understatement. freshwater species of the week

According to the Handbook, the first scientific mention of purple loosestrife in North America was in 1814, when it was described in the harbors of New York, Philadelphia, Boston, and New Bedford. Based on that, scientists estimate that the plant first arrived at the end of the 18th century or beginning of the 19th century. Experts theorize that the plant arrived in the form of seeds stuck to sheep wool or carried by ship ballast.

Purple loosestrife then spread along the Erie Canal and as roads were built. By the 1850s, people began to appreciate the plant for landscaping, and it may have been purposely planted in the Great Lakes and farther west, according to the Handbook.  Purple loosestrife has long been used medicinally as an astringent and as a bug repellent too.

In the 1930s, the plant began aggressively spreading across North America. Some scientists think it was spurred by the extensive public works programs of the New Deal. Highway construction may also have played a role by helping disperse seeds. Bee keepers are also known to have purposely spread the plant, since the flowers are favored by bees.

The plant usually needs a disturbance to take hold in a wetland, but once it gets established it often takes over, crowding out natives. To combat this invader, managers often employ a mix of hand pulling and chemical and biological control (beetles and weevils). While there often can be success on local stands, overall the plant remains quite ubiquitous and aggressive.

As a result, purple loosestrife is listed as a “noxious weed” in at least 32 states, where it is illegal to sell or plant it. Many ecologists have suggested that the plant hurts native plants and wildlife, although a review of the scientific literature in the Handbook found that actual evidence of ecological harm is inconclusive. More studies are needed.

Is purple loosestrife in your community?


Brian Clark Howard covers the environment for National Geographic. He previously served as an editor for and E/The Environmental Magazine, and has written for Popular Science,,,, Yahoo!, MSN, and elsewhere. He is the co-author of six books, including Geothermal HVACGreen LightingBuild Your Own Small Wind Power System, and Rock Your Ugly Christmas Sweater.

  • Christopher Clarke

    Just take a drive down the NY State Thruway! If purple loosetrife spread along the Erie Canal, it stuck around long enough to populate the moist low ground on either side of NY’s east-west thoroughfare, which more or less parallels the route of the Canal. I suspect that most New Yorkers would be surprised to learn that p.l. had arrived in the country during the early 1800s.

  • Bonnie L. Harper-Lore

    purple loosestrife is already on 25 State noxious weed lists due to the environmental costs of the plant. Some $45M is spent on restoration and control annually across the U.S. Economic costs include loss of hay meadows, recreational hunting and trapping, and wetland pastures to farmers. It also threatens wild rice production. First researched as issue in New York in Montezuma Refuge where they flooded it for control…only spread it more. 1500A. of the refuge were infested before biocontrol beetle was released circa 1997.
    Although arrived in 1800s, not a problem until 1900s when some say it hybridized with a native Lythrum alatum (native) the is a wetland species. My coalition put it on our MN State noxious weed list in the ’80s….first State to do so, I believe.
    The biocontrol beetles are effective, but probably will require reintroduction after their food source is sparce and their populations plummet. No other control at this time.

  • Bonnie L. Harper-Lore

    P.L. is on 25 State noxious weed lists due to environmental and agricultural harm caused. Loss include: wildlife habitat, native plants; hay meadows and pastures; recreational hunting/trapping, possible wild rice yields. P.L. quiet in 1800s,but likely hybridized with Lythrum alatum, a wetland-loving native and went rogue. Montezuma Refuge in NY
    has a long history. Biocontrols only control at this time. MN was first to put P.L. on Noxious Weed list in circa 1987.

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