Purple loosestrife in Lake Leelanau, Michigan.

Combating Purple Loosestrife in Lake Leelanau

Driving along the lake, you might soon notice a purple flower in bloom along Lake Leelanau’s shoreline and wetland areas. While beautiful, purple loosestrife is a dangerous invasive species threatening our wetlands. We invite you to read the following to learn more about the impacts of purple loosestrife in Lake Leelanau and how you can help protect the watershed.

What is purple loosestrife?

Originating in Europe and Asia, purple loosestrife (PL) is a perennial herb that has invaded Michigan’s wetlands. The plant can grow 4-10 feet tall and its flowers can produce over two million seeds the size of ground pepper, making it highly capable of spreading. It also spreads via its woody taproot, rhizomes, and vegetative growth. Purple loosestrife thrives in wet soils, marshes, and ponds; along streams and riverbeds; and on lakesides. 

Purple loosestrife's woody roots can have 30-50 square, woody stems and opposite leaves. This invasive plant is in bloom from July through October, and the flowers are a vivid purple making it easy to spot from a distance.

Image collage of purple loosestrife

What problems does purple loosestrife cause? 

Purple loosestrife forms monocultures that replace native plants, reducing critical food resources for birds, butterflies, and other wild creatures. Not only do purple loosestrife seeds germinate very rapidly, it grows faster than almost any wetland plant. This makes it very easy for it to out-compete native species.

When purple loosestrife enters an area, its stiff stems can collect debris such as silt (sedimentation). This can dry up a shallow water habitat and make it into a terrestrial site, destroying the habitat for native aquatic animals. Furthermore, the stems of purple loosestrife are unwelcoming to waterfowl, so waterfowl do not frequent these areas.

How is purple loosestrife managed?

Once established, purple loosestrife is difficult to control and likely will only partially disappear if in a large patch. The best thing to do is monitor natural areas and work to remove purple loosestrife as soon as it is spotted.

  1. Dig it up. The plant can be difficult to dig up, and any remaining roots can start a new plant, so the removal must be complete to be effective.
  2. Obligate beetles. Literature suggests that Galerucella beetles can be a moderately effective biological control. They are considered a loosestrife obligate insect meaning they only eat loosestrife. The beetle damages the plant and prevents it from spreading further or creating a monoculture of loosestrife. The beetles can not eliminate the plant entirely but can help lessen its spread.
  3. Herbicides. While many people object to herbicide use, and for good reason, invasive species experts often turn to herbicides when the infestation is extensive, or digging is not an option. Unlike herbicide application to plants submerged in the water (like Eurasian watermilfoil), herbicide for purple loosestrife is applied directly to the plant. This direct application minimizes its unintentional impact. However, seeds in the soil will continue to germinate for some time, requiring repeated treatment.

What is LLLA doing about purple loosestrife?

In 2023, LLLA contracted an environmental consulting firm, One Less Consulting LLC, to complete a terrestrial invasive species (TIS) survey of Lake Leelanau and the Leland River. This was one of the 2023 goals set by the Lake Association to continue to broaden our work to protect the lake.

The survey indicated there is a substantial infestation of the invasive plant along the northern shores of South Lake Leelanau and in the Narrows in addition to small patches of purple loosestrife around the shoreline and the Leland River.

We are employing the Integrated Pest Management (IPM) approach recommended by State Natural Resources agencies and invasive species experts.  The IPM approach is used to manage pest damage by the most economical means and with the least possible hazard to people, property, and the environment.  For homeowners with purple loosestrife infestations, our suggested course of action is as follows:

  1. If a property has less than 10 purple loosestrife plants, we are asking the property owner to dig them up and dispose of them in their garbage.
  2. If a property has more than 10 purple loosestrife plants and digging is problematic, we are asking that the property owner consider using an herbicide to kill the plant ONLY if there is no standing water immediately surrounding the plant. Only Certified Pesticide Applicators may spray herbicide in areas with standing water surrounding the plant.
  3. If a property has more than 10 purple loosestrife plants and there is standing water immediately surrounding the infestation, we request that you consider contacting a Commercial Business Pesticide Applicator to perform the work.
  4. If you live in an area where many properties have purple loosestrife plants, LLLA is working with property owners to deploy the Galerucella beetle that exclusively feeds on purple loosestrife plants. This technique is used extensively in the Midwest to keep the plant in check.  A permit from EGLE to release the beetles is not required, as they have been found to be safe and effective at controlling the plant.
2023 Purple Loosestrife shoreline survey results
2023 Purple Loosestrife shoreline survey results

Combating purple loosestrife in Lake Leelanau

LLLA has contacted all those whose properties have been identified as being infested with purple loosestrife on the shoreline with suggested treatment options and a request to contact us for more information. If you have not yet contacted us, please get in touch with us at [email protected] to discuss your options.

We greatly appreciate your support in eradicating this highly invasive species. For more information on purple loosestrife and other invasive species threatening the Lake Leelanau watershed, please visit the Terrestrial Invasive Species Page on our website.

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