European Frogbit – One to Watch Out For

      There is a nice mixture of lily pad-like leaves floating off the end of my dock – huge round serving platter-sized lotus leaves, the smaller dinner plate-sized split leaves of water lilies and the split ovals of spatterdock.  One I hope never to see in the mix was discovered for the first time in the wild in Wisconsin in Oconto County this past July.  Those are the much smaller quarter-sized leaves – only ½” – 2 ¼” – of free-floating European frogbit.  They look just like tiny water lily leaves and have petite white three-petaled white flowers that are rarely seen.  The undersides of the leaves are purplish-red and have a spongy coating.

      While European frogbit might add visual beauty to the mix of foliage off my dock, it is a bad actor.  It forms huge floating mats of vegetation that severely affects aquatic life and recreational activities.  The mats reduce the light and nutrients that native plants need and deter movement of large fish and diving ducks.  In fall, the huge amounts of frogbit vegetation decompose and deplete water oxygen levels, causing fish and other aquatic organisms to die.  And forget trying to swim, motor or paddle through it.

      European frogbit is listed as Prohibited under Wisconsin’s Invasive Species rule, NR40.  Prohibited species are illegal to transfer, sell, possess, transport, or introduce into the state. But suddenly, here it is.  So how did it get into this stream just north of the city of Oconto and quickly spread into numerous neighboring marches, tributaries and drainage ditches?  One of two ways.  Either someone launched their boat at the nearby boat launch without cleaning it properly when they last took it out of the water, or somebody cleaned out their water garden or aquarium and dumped it in the stream.
      The last time European frogbit was found in Wisconsin was at a nursery in the southern part of the state but since it is now listed as Prohibited it can no longer be sold.  The closest wild population is in the U.P. near Sault Ste. Marie which seems quite a distance from Oconto County, but the boat launch near where it was discovered is heavily used by boaters throughout the Midwest who fish in Green Bay and adjacent waters.

      European frogbit has short-lived flowers that rarely produce seeds. If you are unlucky enough to see the plant, you would be “lucky” to ever see the flower as each one lasts only one day.  It blooms from mid-July to mid-August.  The plant has developed ways other than seed to spread though.  Plant fragments can sprout new plants so running a motorboat through a colony of plants wouldn’t kill them.  It would just make more of them.  If you’ve ever grown strawberries, you are familiar with how they produce runners that develop new plants along their length.  European frogbit does the same.  They also form turions, buds that drop to the bottom of the water in autumn and wait patiently until conditions are right to grow, surviving even harsh winters and drought. In spring, they rise to the surface and begin growing. One plant can produce up to 100 turions. 

      While the plant does have roots, they are free-floating and tangle with each other to reinforce the dense matting that stabilized the colony.

      European frogbit makes its home in marshes, ditches, swamps, edges of lakes, rivers and streams, really any still, shallow, quiet or slow-moving water.  It is highly competitive and becomes the dominant plant quickly, spreading at a maximum rate of 9.7 miles per year.

      The only way to control European frogbit at this time is mechanical removal by hand-pulling, so you can see that it’s not a once-and-done project with these plant parts and turions in wait.  Once it is discovered and removed, the area needs to be monitored for several years into the future.

      To give you an idea of how serious invasive species experts take the appearance of European frogbit in Wisconsin, here is what happened when its presence became known:  as soon as it was reported to the DNR, staff visited the site to verify the identification of the species and to collect specimens and photos.  Then they partnered with Oconto and Marinette counties, Timberland Invasive Partnership, Wild Rivers Invasive Species Coalition and the Forest Langlade Oconto Waterways Program to conduct removal efforts and future monitoring.  They also began outreach to sportsmen, local garden clubs, and bait shops. 

      Unfortunately, just after the DNR issued a press release in August about the discovery of European frogbit in Oconto County, it was discovered that numerous places along the Marinette County coastal area are also infested.  This means that the current known extent ranges from the Oconto Sportsman’s Club marsh all of the way up to Seagull Bar State Natural Area in the City of Marinette.


If you are a boater, paddler, angler, or waterfowl hunter, you can help reduce the spread of European frogbit and other aquatic native species by doing the following:

  • Inspect boats, trailers, equipment and clothing for attached aquatic plants or animals.
  • Remove all attached plants, animals and sediment.
  • Drain all water from boats, motors, livewells and other equipment.
  • Never move live fish away from a waterbody.

If you think you see European frogbit or any other invasive species, please report it, even if you aren’t sure, by following the instructions on the DNR website at or email


European frogbit can be confused with the similar appearing American frogbit.  Here are the differences:  American frog-bit’s leaves have lateral veins at a 30-80ﹾ angle with the midvein and the leaves contain large air pockets throughout.  European frogbits lateral veins are arching and make a 75-90ﹾ angle with the midvein.  Leaf tissue containing air pockets are located only along the midvein. 

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