Invasive Exotic Plants of Canada Fact Sheet No. 3

European Frog-bit - frog's-bit, frogbit; petit nénuphar, morène aquatique
Hydrocharis morsus-ranae L.
Frog-bit Family - Hydrocharitaceae

Field Recognition
European frog-bit is a small, free-floating, aquatic herb, remeniscent of a tiny water lily. Its round, smooth, leathery, heart-shaped leaf blades reach a diameter of about 5 cm. The lower surface of the blades is usually dark purplish-red in colour. The floating blades have elongate stalks, 4-6 cm long, and form a rosette from the short submerged stem. Horizontal runners form at the base of the plants and give rise to new plantlets at their tips. Long unbranched dangling roots have the appearance of elongate narrow bottle brushes due to the dense cover of green root hairs. By mid-summer dense mats of interlocking plants form over the surface of the water. A single white flower, about 1.5 cm in diameter, having a small yellow area at the base of each of the three petals, is borne at the top of the 8-10 cm long flower stalk.

Habitat
Its preferred habitats are quiet, open water in marshes, ditches and swamps. When present along river shorelines or streams, plants are generally found in quiet backwaters.


Roadside marsh with dense mats of European frog-bit
in areas of open water [southeastern Ontario].

History of Introduction and Spread
On May 11, 1932, plants of European frog-bit, obtained from Zurich, Switzerland, were introduced into a trench within the Central Experimental Farm Arboretum, Ottawa. By 1939, plants had become established in the adjacent Rideau Canal. In the early 1950s, plants were found in the Ottawa River near the Rideau Canal and at Montreal Island. This last site may have also been the result of material escaped from the Montreal Botanic Garden. European frog-bit spread rapidly and became a dominant in marshes throughout southeastern Ontario. By 1980, plants had spread down the St. Lawrence River to about 200 km northeast of Montreal and into the Bay of Quinte area in eastern Lake Ontario. It had also made its way to a disjunct locality at Rondeau Provincial Park on Lake Erie.


Distribution of European frog-bit in North America (updated March 2001). The hatched area represents
the Mixedwood Plains Ecozone of Canada.

In Canada, European frog-bit is found exclusively within the Mixedwood Plains Ecozone, a biogeographic region that encompasses parts of southern Ontario and the St. Lawrence Lowlands within Quebec. In Ontario, European frog-bit is found most abundantly throughout southeastern Ontario and the lower Trent-Severn waterway. It is also found in scattered marshes westward along the north shore of Lake Ontario and Lake Erie to Point Pelee National Park. European frog-bit is now also found as far downstream on the St. Lawrence River as Quebec City and is present in the Eastern Townships of Quebec. In recent years, it has spread to adjacent northern New York State at scattered points along the southeastern shore of Lake Ontario and at various inland sites just south of the St. Lawrence River. It was found in Lake Champlain in 1993, near Grande Isle, Vermont, and by 1999 had extended southward along Lake Champlain to Benson, Orwell and West Haven, Vermont. European frog-bit was first identified from Lake St. Clair in 1996. It is now found throughout the marshes of Lake St. Clair and the Detroit River marshes, Michigan (see USGS/NAS and Sea Grant)

Biology
European frog-bit is an annual, aquatic herb that reproduces both sexually and asexually. Plants have unisexual, male or female flowers. Although these attract a variety of insect pollinators, sexual reproduction by seed may be limited or non-existent in some populations. Where only male or female plants are present, no seed production is possible. Such local populations likely arise through vegetative growth from a single plant or overwintering bud (turion). Low seed production may also occur in mixed sex populations because there tends to be a high male sex ratio. Sexual reproduction is only of minor importance considering the rapid vegetative growth and production of new plantlets from the ends of the long cord-like stolons produced by each floating plant. By mid summer, extensive vegetative growth results in the formation of dense floating mats of intertwined plants.

European frog-bit overwinters primarily by forming small vegetative buds called turions that are formed at the base of the plants and at the ends of the stolons. These turions sink to the bottom as the plants deteriorate late in the year and then float to the surface in the spring and develop into a new crop of plants. These buds apparently only grow if they have overwintered at low temperatures.

Environmental and Economic Impacts
The dense floating or stranded mats are so well developed in some areas that they limit boat traffic and inhibit recreational activities. These mats likely also have an impact on aquatic life. The dense layer of leaves and entangled stolons and roots limit light penetration and light availability for other aquatic plants. The mats may also completely fill the water column in the shallows and hinder the movement of aquatic animals or at least make the sites less attractive or physically unsuitable for fish or other wildlife.

Control Measures
No control measures have yet been reported for European frog-bit. However, mechanical removal of mats where they have become a nuisance to boaters may provide a temporary solution.

Interesting Facts
The generic name is derived from the Greek words hydor, water and charis, grace. The descriptive adjective comes from the Latin morsus, bite and rana, frog. The common name has been said to be based on the belief that frogs like to search for small aquatic prey amongst the plants.

Plants are easily grown in aquaria and can be maintained for use in classroom studies to demonstrate the phenomenon of cytoplasmic streaming which is readily evident in the long root hairs. A study of the growth response of European frog-bit roots to various relatively high concentrations of hydrogen ions has shown that they do not seem to suffer adverse effects. Interestingly, European frog-bit is presently found almost exclusively in sites to the south of the Precambran Shield. The Shield is a region of ancient bedrock on which soils and lakes tend to be acidic in nature. It is likely that European frog-bit requires the nutrient rich waters found south of the Shield in the Mixedwood Plains Ecozone to support the prolific vegetative growth produced in the course of a summer. In the British Isles, European frog-bit is reported as occurring in calcareous regions where they are locally common.

Selected References

Catling, P.M., and K.W. Spicer and L.P. Lefkovitch. 1988. Effects of the introduced floating vascular aquatic, Hydrocharis morsus-ranae (Hydrocharitaceae), on some North American aquatic macrophytes. Naturaliste canadien 115: 131-137.

Catling, P.M. and Z. S. Porebski. 1995. The spread and current distribution of European frogbit, Hydrocharis morsus-ranae L., in North America. Canadian Field-Naturalist 109: 236-241.

Clapham, A.R., T.G. Tutin and E. F. Warburg. 1962. Flora of the British Isles. Cambridge University Press. 1269 pp.

Cook, C.D.K. and R. Luond. 1982. A revision of the genus Hydrocharis (Hydrocharitaceae). Aquatic Botany 14: 177-204.

Dore, W.G. 1954. Frog-bit (Hydrocharis morsus-ranae L.) in Ottawa River. Canadian Field-Naturalist 68:180-181.

Lumsden, H.G. and D.J. McLachlin. 1988. European frog-bit, Hydrocharis morsus-ranae, in Lake Ontario marshes. Canadian Field-Naturalist 102: 261-263.

Minshall, W.H. 1940. Frogbit Hydrocharis morsus-ranae L. at Ottawa. Canadian Field-Naturalist 54: 44-45.

Minshall, W. H. and G.W. Scarth. 1952. Effect of growth in acid media on the morphology, hydrogen-ion concentration, viscosity, and permeability of water hyacinth and frogbit root cells. Canadian Journal of Botany 30: 188-208.

Niklitschek, A. 1932. Water lilies and water plants. Chatto & Windus, New Flora & Silva, London. 136 pp.

Scribailo, R.W., K. Carey and U. Posluszny. 1984. Isozyme variation and the reproductive biology of Hydrocharis morsus-ranae L. (Hydrocharitaceae). Journal of the Linnaean Society 89: 305-312.

Scribailo, R.W. and U. Posluszny. 1984. The reproductive biology of Hydrocharis morsus-ranae. I. Floral biology. Canadian Journal of Botany 62: 2779-2787.

White, D.J., E. Haber and C. Keddy. 1993. Invasive plants of natural habitats in Canada: An integrated review of wetland and upland species and legislation governing their control. Report prepared for the Canadian Wildlife Service, Environment Canada. 121 pp. Cat. No. CW66-127/1993E.

 

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Prepared by: Erich Haber
National Botanical Services, Ottawa, ON, Canada
ehaber@magi.com
June, 1996 [range updated, March, 2001]