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An Illinois River Almanac

Jason Haupt's Energy and Environment Blog

Invasive Species- Purple Loosestrife

If you were to ask people to name an invasive species, Purple Loosestrife would be one of the top five species named.  It originally was carried to the US as a medicinal plant and was also brought over in ballast of ships coming from Europe.  It established itself on the northeastern seaboard in the early 1800’s and spread inland slowly.  It established itself in Illinois in the 1930’s and is currently listed as an exotic weed in Illinois and is therefore prohibited from sale in Illinois.

Purple Loosestrife (Lythrum salicaria) is an annual native to Europe and has a number of medicinal uses and is easily noticeable.  The showy spike of rosy-purple flowers is present in mid to late summer.  It differs from the native winged loosestrife (Lythrum alatum) most notably in its size; the native loosestrife only growing to a height of about two feet where the invasive species can be anywhere from three to 10 feet tall (typically five feet in height).  Purple Loosestrife has entire (smooth margin) opposite leaves or whorls of three with a four sided stem that can look and feel woody at the base, especially in larger plants.  Purple Loosestrife can be found in wet habitats including roadside ditches, river banks, and along the side of lakes and reservoirs.

Purple Loosestrife has been identified as a significant threat to native wetlands in Illinois.  Purple Loosestrife is highly aggressive and quickly crowds out native wetland species creating a monoculture in these areas.  This monoculture provides little to no food and shelter to the local animal species that rely on the native wetland species for food, nectar, pollen, and shelter.

Controlling Purple Loosestrife is difficult due primarily to its habitat and its level of seed production.  A single stalk can produce as many as 300,000 seeds.  In heavy infestations a stand of Purple Loosestrife can produce between 12 billion seeds to, in extreme cases 24 billion seeds per acre.  In addition to seed production, Purple Loosestrife also reproduces through vegetation left in the wetland.  Both roots and stem segments can produce new flowering stalks.  This means that mowing, burning, and flooding have proven to be ineffective control methods as the seeds remain viable even after 20 months of being submerged.  The most effective control is mechanical pulling and complete removal of all vegetation.  Herbicides have also proven to be effective in controlling Purple Loosestrife.  However, herbicide control is indiscriminate and many are water soluble and therefore highly mobile in a wetland.   The key to controlling Purple Loosestrife is early detection and control.  Searching wetlands each year in late July and August before the plants have begun seed production is the most effective control method as small infestations are more successfully controlled than large infestations.

For more information please contact Jason Haupt (


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