Nymphoides peltata

Nymphoides peltata (syn. "Villarsia nymphaeoides", "Limnanthemum peltatum" S.G. Gmel., "Nymphoides nymphaeoides" (L.) Britton, fringed water lily, yellow floating heart, floating heart, water fringe, entire marshwort) is perennial, rooted aquatic plant with floating leaves of the family Menyanthaceae. 
"N. peltata" is native to East Asia and the Mediterranean. It is the only species in the genus Nymphoides that can be found in temperate regions. "N. peltata" has spread outside its native range and now occurs in North America Ireland, Sweden, and New Zealand. In many of its introduced regions, "N. peltata" is considered a noxious weed because it can form dense mats that shade out native aquatic plant species, thereby reducing biodiversity, and also inhibit recreation.
Description.
"N. peltata" is an aquatic bottom-rooted perennial species with underwater creeping stolons that extend up to 2 meters. Each node on a stolon can produce a new shoot and roots. "N. peltata" has cordate floating leaves that are 3-15 cm in diameter, green to yellow-green in color, have purple-tinted undersides, and are attach to submerged rhizomes. The leaves have slightly wavy margins and support a lax, or loose, inflorescence of two to five yellow, five-petal flowers (2-4 cm in diameter) with fringed petal margins. "N. peltata" peduncles can rise several inches out of the water. Each flower produces a 1.5-2.5 cm beaked capsule which hold many flattened seeds with stiff marginal hairs.
"N. peltata" is similar in appearance to "Nuphar variegata" and species in the genus Nymphaea. "N. variegata" can be distinguished from "N. peltata" most easily by its larger leaves, which can measure up to 30 cm, and its cup-shaped flower. "Nymphaea" have angled leaf bases as opposed to the rounded leaf bases of "Nymphoides", and "Nymphoides" have significantly longer peduncles that support smaller flowers. In order to identify different species within "Nymphoides", flowers are usually required.
Ecology.
"N. peltata" is a freshwater species and does not occur in waters with an average chlorinity exceeding about 300 mg/l. "N. peltata" is most commonly found in slow-moving rivers, lakes, ponds, and reservoirs, but it can also establish in swamps and wetlands. It is also known to occur in ditches, canals, break-through pools of dikes, and backwaters, especially those subject to winter flooding. The presence of N. peltata can signal an environment with water movement.
"N. peltata" spreads the most aggressively in eutrophic lakes with neutral to alkaline water, but the species has also been found in oligotrophic lakes and acidic ponds. It most frequently occurs in water 1 to 1.5 m deep, but it can survive in water between 0.3 and 3.0 meters deep. The ideal substrate for "N. peltata" consists of clay or clay covered with a thin layer of sapropel. "N. peltata" occurs in temperate environments and has even been found eastern Ontario and western Russia. The northernmost limit of "N. peltata" is approximately the 16°C July isotherm.
Depending on water temperature, "N. peltata" flowers between May and October in the Northern Hemisphere and between October and April in the Southern Hemisphere. Each flower survives a single day, while leaves can persist from 23-43 days. Seeds are released 32-60 days following the end of the flowering period and can germinate under hypoxic conditions. In fall, the aboveground biomass of "N. peltata" dies, sinks to the substrate and decomposes, and the plant overwinters as dormant rhizomes. These rhizomes can survive freezing temperatures up to -30°C. During the winter, stolons and stems either on or buried beneath the substrate can remain dormant until spring, and some small submerged leaves measuring 1-2 cm sometimes grow on these stems. After winter, the species requires light and oxygen to produce new growth and floating leaves begin to appear in spring.
"N. peltata" can reproduce vegetatively or sexually. Fragments of one plant, including stolons, rhizomes, and leaves attached to part of a stem, can also develop into a new plant. Seeds are produced either by cross or self-pollination, though self-pollination usually produces fewer and less viable seeds than cross-pollination. Seed dispersal is facilitated by the semi-hydrophobia of seeds, which causes them to float on the water's surface until disturbed. Seeds are suited to adhering to surfaces such as the flanks, folds of skin on digit webs, and the short feathers on the heads of waterfowl, which are hydrophobic. While in flight, the marginal trichomes surround the seeds keep the seeds attached to the bird, but once again in water, the seeds detach and sink to the substrate, where germination can commence. Adherence to amphibious animals and boats are two other possible dispersal mechanisms. Conversely, seeds that are eaten by waterfowl or fish are completely digested and no longer viable.
As an invasive species.
"N. peltata" are commonly sold for use in ornamental water gardens. The species is intentionally or accidentally transferred to lakes and rivers outside of its native range. In the United States, the first recorded occurrence of the plant was in 1882 in Massachusetts, and the plant has been sold domestically since the 1930s. "N. peltata" has been recorded in 29 U.S. states and Washington, D.C., and it has naturalized in slow-moving waters. In Canada, "N. peltata" has been found in Newfoundland, Nova Scotia, Quebec, Ontario, and British Columbia, but has not yet been naturalized. "N. peltata" was intentionally introduced to Sweden in the early 19th century and was first recorded as a non-indigenous species in 1870.[2] Native to central and eastern Britain, N. peltata was transported around Britain and Ireland as an ornamental plant and first documented in Ireland in before 1866. In 1988, "N. peltata" was discovered in New Zealand, where it is considered a noxious weed. It is also designated an invasive species in Sweden, Ireland, and parts of North America. 
However, while "N. peltata" is an invasive species outside its native range, in its indigenous to Japan where it is now classified as a vulnerable species. In Spain, Belarus, and the Czech Republic, "N. peltata" is critically endangered, and it is endangered in Lithuania, and vulnerable in Germany and Switzerland.
"N. peltata" can have many negative ecological and social impacts on its introduced regions. "N. peltata" can rapidly grow and spread to form dense patches of vegetation on the water’s surface that restrict the amount of light penetrating the surface. As a result, native submerged macrophytes are excluded and biodiversity is reduced. Especially if algae are shaded out, entire food webs can be disrupted by dense populations of "N. peltata". Dense "N. peltata" mats can also reduce the amount of oxygen in water and create stagnant water areas. Fish and other aquatic wildlife can also be forced to relocate once "N. peltata" mats form. "N. peltata" also affects internal fertilization patterns as it moves nitrogen and phosphorus up from the sediment to its floating biomass as it grows and releases these nutrients back into the ecosystem during its winter decomposition.
Socially, "N. peltata" can negatively impact recreational activities such as boating, fishing, and swimming. The presence of mats of "N. peltata" can also decrease the aesthetic value of waterbodies which can, in turn, depreciate property and tourism values.
Control strategies.
As "N. peltata" can propagate through fragmentation, mechanical control is a challenging strategy because it can often aid in dispersal. One approach is to cut leaf petioles one to two times each spring and summer. Cutting, harvesting, and covering plants with barrier materials can sometimes result in a successful control effort. Hand raking can be a viable strategy in very localized areas. However, dredging is not an effective method as roots and rhizomes can survive mechanical dredging.
There are no known effective biological control agents for "N. peltata". While grass carp may feed on parts of "N. peltata", the fish usually first consumes submerged macrophytes and has not been shown to feed on "N. peltata".
Glyphosate applied to floating leaves after emergence during late spring or summer has been used as a chemical control for "N. peltata". Nonetheless, repeated applications are necessary, and spraying floating leaves shows 40-50% control for a single season. Dichlobenil is more effective than glyphosate in controlling "N. peltata". The application of the chemical in the spring during emergence coupled with mechanical removal of floating leaves and repeated dichlobenil applications has been successful. However, the producers of dichlobenil advise not to treat more than 20% of a waterbody and to not treat areas where flow exceeds 90m/hour. In Ireland, dichlobenil is no longer legally allowed to be used near or in waterbodies as it can potentially harm other aquatic organisms such as fish.
Other.
"N. peltata" is an edible plant. Leaves and flowers are cooked as a potherb, and so are the stems, but only the interior of the stem is eaten. Seeds are ground and used for other culinary purposes. The fresh leaves are commonly used to treat headaches.