Thursday, August 1, 2013

European Frogbit


© Dave Spier

Among its many incursions into southeastern Wayne County, New York, European Frogbit (Hydrocharis morsus-ranae) can be found edging Black Creek as it leaves the Montezuma Marshes west of Savannah, just before passing under Route 31 and flowing on its way to Crusoe Creek. This Eurasian species was introduced into Ottawa, Canada in the 1930's and has since spread around the lower Great Lakes and St. Lawrence Seaway watersheds. Its spread northeastward has been limited only by water salinity. There is no limit to the south and west, so it has reached Vermont, New York and Michigan. Frogbit can form dense colonies on the surface and threatens biodiversity by crowding out native species and shading underwater plant growth needed by fish. Its rapid growth also can clog waterways and hinder recreational activities such as boating, canoeing and fishing.

Frogbit resembles a miniature waterlily, but its small, pistilate (female) flowers have only 3 petals around a yellow center, unlike the showy native waterlilies with their numerous petals in concentric rows. The frogbit flower resembles those of arrowhead plants, which is not too surprising because both are monocots in the Order Alismatales (although they are in different families). The plant could survive very well without flowers, because its seeds contribute very little to its spread. Instead, the plant can hitch-hike on boats, while its winter buds (called turions) sink to the bottom and lie dormant until spring, when they rise and travel with water currents. The buds grow into complete plants which then enlarge by sending out runners (called stolons) which sprout additional plants in an expanding network that can interlock with adjacent colonies.
Frogbit's round, floating leaves (generally two inches or less in diameter), are often notched like water-lilies and grow in rosettes (i.e., radiating from a center point). Dangling roots (to 12 inches long) seldom anchor in the bottom mud, allowing mobility on surface currents, even blowing upstream.

European Frogbit can be distinguished from its native counterpart (Limnobium spongia) by checking the leaf stems. American Frogbit has a midline groove.

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1 comment:

Cindy said...

Sigh..yet another invader. Thanks for alerting me!