common frogbit, European frog's-bit

The growth form is stoloniferous, the stolons having seasonally dimorphic terminal buds with one root. The leaves are petiolate and floating or, in dense vegetation, emergent.  The leaf blade is ovate to orbicular in shape, typically measuring 1.2-6 x 1.3-6.3 cm, with a chordate to reniform base, and entire margin.  Veination is palmate with cross-veins. The primary veins form a 75--90° angle with the midvein and are broadly curving.  Aerenchyma are confined to the midvein region (not margin to margin as in Limnobium).  Individual aerenchyma space, located approximately 1 mm from either side of midvein, typically measures 0.1-0.5 mm across its longest axis (eFloras 2008; Gleason and Cronquist 1991).

The species is dieocious (male and female flowers found on separate plants). Both sexes of flowers have an outer whorl of three greenish-red sepals, and a whorl of three membranous white petals.  Staminate (male) flowers occur in cymose inflorescences of 2 to 5 flowers on pedicels up to 4 cm long.  The inflorescence is enclosed by a spathe of one or two scale leaves which subtend the first two flowers.  The stamens number 9 to 12 and are arranged in 3 trimerous whorls, with an innermost whorl of staminodes.  The first and third whorls of stamens are partially united along their filaments, and the second whorl of stamens is fused to the staminodes.  The anthers are basifixed and consist of four micro-sporangia, with pollen grains dehiscing through lateral slits.  In the center, there is a large pistil-like structure (Scribailo and Polsluzny 1985).

Pistillate (female) flowers are solitary and enveloped in a tubular hypanthium, with pedicels up to 9 cm long. The ovary is inferior, with six dorsiventral styles.  Each style is bifurcated at the end into two flat, papillose stigmas. There is a whorl of nectaries that occur as appendages on the three antipetalous styles, and a whorl of filament-like staminodes. The fruit is a berry that dehisces longitudinally, releasing seeds 1 to 1.3 mm in length (Scribailo and Polsluzny 1985).

European frog-bit was first recorded in the United States in 1974 from the Oswegatchie River, a tributary of the St. Lawrence River, in northern New York. By the early 1980s, occurrence in that state increased to inland sites south of the St. Lawrence River, and by the early 1990s, to bays and marshes along Lake Ontario (Catling and Dore 1982; Invasive Plants of Canada Database 1995). By 2000, it had spread to eastern New York, in the southern reaches of Lake Champlain, at Mill Bay (pers. comm. H. Crosson, Vt. Dept. Env. Cons. 2001).

European frog-bit was first discovered in Lake Champlain during 1993 at its northern reaches, near Grande Isle, Vermont. It appeared confined there until 1999 when plants were first found along the southern shore of Lake Champlain at Benson, Orwell and West Haven, VT (pers. comm. H. Crosson, Vt. Dept. Env. Cons. 2001).

European frog-bit was identified in 2000 as new to the state of Michigan, where populations are known from two sites: 1) Lake St. Clair marshes and 2) Detroit River marshes.  A few unidentified plants were first observed in a dredged slough at Lake St. Clair in 1996. Within two years plants had become abundant throughout the marsh and formed dense mats in cut ponds. The Detroit River marshes lie downstream of Lake St. Clair. Both localities drain the Detroit River which flows into Lake Erie (J. Daniels 2000; pers. comm. J. Champion, Huron Clinton Metroparks, and A.A. Reznicek, Univ. of Mich., 2001).

Canada: Southeastern Ontario and adjacent western Quebec, including the Ottawa, Rideau and St. Lawrence Rivers and the shorelines of Lake Ontario and Lake Erie.

Hydrocharis morsus-ranae is a dioecious plant, meaning that the male and female flowers occur on different plants (IL DNR 2009). Populations are typically only contain one sex, therefore sexual reproduction and seed production is quite low (Campbell et al 2010, IL DNR 2009). Instead, common frog-bit expands by produces stolons, which can produce juvenile plants (O’Neill Jr. 2007). In the autumn, the ends of the stolons produce turions (vegetative buds that survive through the winter), which break off the main plant, sink to the bottom of the waterbody, and go dormant (Cutter 1964, O’Neill Jr. 2007). In the spring, the turions float to the surface and begin growing (IL DNR 2009). About half of H. morsus-ranae propagules are still able to float after being in the water 6 months; indicating that dispersal via waterways is probably essential to the expansion of this species (Sarneel 2012). A single H. morsus-ranae plant can create 100-150 turions in a single growing season (IL DNR 2009, O’Neill Jr. 2007).

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic

As colonies H. morsus-ranae displace native plants; other native aquatic life experience a reduction in food and habitat (Environment Canada 2003, WI DNR 2012). Dense mats can inhibit the movement of waterfowl or larger fish; which could alter predator/prey cycles as the waterfowl and fish move to other locations to find food (O’Neill Jr. 2007, University of Minnesota, Wisconsin Sea Grant Institute 2012).  Catling et al. (1988) surveyed live H. morsus-ranae mats and found a decline in snails, crustacean, and insect larvae (in Mudrzynski et al. 2011).

Dense stands can alter the water flow or currents (Mikulyuk and Nault 2011). In the autumn, H. morsus-ranae dies and sinks to the bottom of the water body, where it decomposes (IL DNR 2009).  Altered hydrology and/or increased decomposition can reduce oxygen concentration in nearby waters and could potentially lead to the death of nearby plants, insects or fish and insects (Catling et al. 2003, IL DNR 2009). However, Thomas and Daldorp (1991) found that the addition of the H. morsus-ranae had no effect on the macrophyte community or on the dissolved oxygen profiles.

Hydrocharis morsus-ranae has a moderate socio-economic impact in the Great Lakes.
Realized:
Large infestations of H. morsus-ranae have reduced water currents in canals and irrigation systems (Catling et al. 2003). Dense layers of tangled stems and roots can wrap about boat propellers and impede water traffic (Lui et al. 2010, University of Minnesota Wisconsin Sea Grant Institute 2012). Large populations of common frogbit also limit recreational activities such as swimming, fishing, and waterfowl hunting (Grant 2013, Lui et al. 2010, University of Minnesota Wisconsin Sea Grant Institute 2003).

Potential:
The decreased recreational and aesthetic value linked to large populations of H. morsus-ranae can lead to a reduction in property value along the affected waterfront and even cause declines in tourism and associated revenue (Mikulyuk and Nault 2011).

There is little or no evidence to support that Hydrocharis morsus-ranae has significant beneficial effects impacts in the Great Lakes.
Realized:
Some species of water birds, fish, and insects feed on H. morsus-ranae (O’Neill Jr. 2007).

Some property owners consider it to have aesthetic appeal and it is used in water gardens (New York Invasive 2012).

Potential:
Spermidine, an anti-aging compound, can be found in dormant turions of H. morsus-ranae (Villanueva et al. 1985).

Prohibited in Illinois, Michigan, Minnesota, and Wisconsin (GLPANS 2008). In Minnesota, possession, import, purchase, transport, or introduction of H. morsus-ranae will result in a misdemeanor (MN DNR) 2013).

The New York Invasive Species Council ranks this species as posing a very high ecological threat and recommends that it be regulated (New York Invasive 2010).

The Great Lakes Life & Wildlife Commission have no found H. morsus-ranae in their ceded territories, but recommend immediate control upon detection (Falck and Garske 2003).

Hydrocharis morsus-ranae is restricted in Ontario (regulatory amendments to Ontario Regulation 354/16 under the Invasive Species Act, 2015).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control
Biological
Grass carp, Ctenopharyngodon idella, feeds on H. morsus-ranae. However, this introduction of this species may also have a negative effect on native vegetation, which might outweigh the benefits of H. morsus-ranae control (Mikulyuk and Nault 2011).

Physical
Mowing does not control H. morsus-ranae population (Sager and Clerc 2006). Removing manual harvesting may provide temporary control (IL DNR 2009, WI DNR 2012). To improve efficacy of this method, harvesting should occur in the spring; after the turions have begun growing, but before dense mats form (Catling et al. 2003).

Another possible method for small water bodies would be to have a water draw-down after turions have germinated, but before extensive growth occurs (Catling et al. 2003).

Chemical
Diquat, imazapyr, penoxsulam, and imazamox offer excellent control of H. morsus-ranae (AERF 2013).

Application of flumioxacin by backpack sprayer is proving effective (European Frogbit Collaborative 2021, Personal Communication).

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.