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Finding frog-bit

September 4, 2012
By Hilary Smith ( , Adirondack Park Invasive Plant Program

Just when you think that an invasive species is limited in its distribution - BAM - you get a call from an eagle-eyed volunteer and the situation suddenly changes.

Eurasian watermilfoil and Japanese knotweed are familiar plants in the Adirondacks, but other plants that are less well known are making inroads. European frog-bit (Hydrocharis morsus-ranae) is one of those plants. It isn't making headlines, but it is quietly making its way through calm coves, bays and wetlands around the region.

While many invasive species are introduced unintentionally, such as through ship's ballast water or by hitchhiking on recreational gear, European frog-bit was intentionally introduced in the 1930s as an ornamental planting in a pond in Ontario. This plant didn't stay put and its range now extends throughout the St. Lawrence River, eastern Lake Ontario and Lake Champlain.

Article Photos

European frog-bit infestations result in dense, tangled clumps of plants.
(Photo — Meghan Johnstone, APIPP)

In 2005, a volunteer first detected frog-bit in the Adirondacks in a temporary floodplain pool along the Grasse River, on the Park's western boundary, and reported it to the Adirondack Park Invasive Plant Program. We mobilized a response. Extensive surveys of the area determined that the infestation was not widespread and appeared to be contained to several small adjacent pools.

Our prescription? Pluck the plants from the water, place them in a 5-gallon bucket with holes punched in the bottom to let the water drain out and then dispose of them on dry land away from the water to decompose. Also, monitor annually to track the number of plants removed and whether native plants recovered following the removal of the invasive. Though hand harvesting reduces the number of frog-bit plants each year and native plants are rebounding, six years later control efforts are still under way.

Finding frog-bit deep in the Grasse River Wild Forest perplexed us. Its arrival on recreational gear seemed unlikely since the infestation was located off the main river, and the area was not conducive to boating or angling. Wildlife was the most logical explanation. Waterfowl and signs of beaver are prominent there. Their activity plus water currents can carry plants to new locations.

Meanwhile, in 2011 in Essex County, on the other side of the Park, another volunteer reported a frog-bit infestation on a private, artificial pond. Again, we visited the site to determine the extent, possible source and potential management action. This time the infestation was larger, but again, its source was confounding. The pond was not hydrologically connected to another infected waterway and not one where recreation occurred.

Last week, yet another volunteer reported frog-bit at various marshes near Westport. The situation was the same - not hydrologically connected and not conducive to recreation.

While human activity typically is the primary culprit for introducing and spreading invasive species, wind, water and wildlife also can transport them to new locations. In the case of the frog-bit sightings in the Park to-date, waterfowl and other wetland wildlife must be the suspects.

Since frog-bit is a free-floating plant (not anchored to the bottom), it may be easier to transport from the water's surface by a passing animal. It has an extensive root system resulting in dense, tangled clumps of plants that form dense mats that smother native vegetation, limit light penetration and can inhibit recreation. When large masses of frog-bit die in the fall, decomposition results in low oxygen levels that can cause fish and other organisms to die.

Frog-bit is an annual plant with leaves that are heart-shaped, leathery and about the size of a quarter, with undersides that may be dark purple. Three-petaled, white flowers with yellow centers bloom in the summer. In mid to late summer, each plant develops up to 100 turions, or buds, which drop into the water, overwinter in the mud and emerge in the spring when the ice melts.

Volunteers are important in the fight against invasive species. If you are an aquatic enthusiast, learn to identify European frog-bit and report sightings to Meghan Johnstone, APIPP's aquatic invasive species project coordinator, at 518-576-2082. Take a photograph of the suspicious plant (include an object in the photo as a reference of scale), or take a sample and place it in a baggie with a damp paper towel and send it along with your contact information to APIPP at P.O. Box 65, Keene Valley, NY 12943.


Eye on Invasives is a biweekly column that spotlights a top invader when it is easiest to identify. Hilary Smith directs the Adirondack Park Invasive Plant Program, a partnership program housed at the Adirondack Chapter of The Nature Conservancy in Keene Valley. Find out more about this award-winning program online at



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