Family: Araceae
  L. (water lettuce)

Field Description: Free floating perennial herb consisting of hefty, velvety rosettes suspended above fibrous underwater roots.  Leaves can reach 15 cm in length and are light yellow to gray green and densely pubescent with grooved, parallel veins.  Offshoots develop during the growing season at stolon tips. White to green spathes hidden in the central axis of the leaf bases form flattened, oval fruits.
Habitat: Low velocity water bodies, canals, ponds and swampy backwaters.
Native Range: Pantropical.
Nonindigenous Range: Established populations limited to the most southern portions of the lower United States, including Hawaii. Abundant where established along the eastern coastal plain, north to the Waccamaw River drainage of South Carolina.  Disjunct occurrences have been recorded in more northern ranges, including Illinois and Ohio and in the Mid-Atlantic States from New York, Delaware and Rhode Island.  It is not known if these colonies originate each year from new escapes or if the plants reoccur each summer from a seed source in the substrate.

MD  DE  NJ  NY  RI 2003* 1993 2003* 2000 2001

Comments:  Pistia vegetation is intolerant to frost.  Sexual reproduction has been found to be important in the dynamics of some populations in the Netherlands, Australia, and Florida.  Seeds survive prolonged experimental periods in water at 4°C and several weeks in ice at –5°C after which germination occurred between 20°C and 25°C. With appropriate springtime germination conditions (i.e. silty/muddy substrates, clear shallow water and warm temperatures), overwintering by seed could account for population reoccurrence in temperate regions of the U.S.  Seedlings are light green, pubescent and about the size of Spirodela polyrhiza.  Though a weedy, prolific grower, Pistia remains a popular ornamental water garden plant.

Family: Hydrocharitaceae
  L. (European frogbit)

Field Description: Floating (sometimes emergent) perennial herb producing floating leaves (1.2 – 6 cm x 1.2 – 6.4 cm) on long petioles that arise from a central whorl.  Roots rarely anchor plants. Leaves are leathery and cordate with arching veins that mimic the leaf outline. Flowers white with 3 showy petals that exceed the sepals. Fruit a globose, many-seeded berry with seeds measuring nearly 1 mm. 
Habitat: Quiet, shallow edges of rivers, lakes and open marshes.
Native Range: Europe.
Nonindigenous Range: Few sites exist in the United States where most occurrences, especially that in Washington, are rather recent. Established in the Mid-Atlantic States in New York and Vermont. The earliest U.S. record was from the Oswegatchie River, east of the St. Lawrence River, New York.  By the early 1980s plants had spread to inland sites south of the St. Lawrence. By the early 1990s, bays and marshes along Lake Ontario had become affected and plants first appeared in Vermont at Lake Champlain.

NY  VT 1974 1993

Comments: An upcoming invasive in Region 5.  Forms dense floating mats when roots become entangled in other aquatic vegetation.  Resembles the closely related, native American frog-bit (Limnobium spongia), the latter distinguished by a convex layer of spongy, gelatinous, red tinged tissue beneath the leaf.  Primarily reproducing vegetatively, through stolons and turions. First entered Canada in the 1930s as an escapee from ornamental cultivation, later spread encouraged by motorboats.

Family: Lemnaceae
  (G. Mey.) Les & D.J. Crawford  (dotted duckweed)

Field Description: Tiny aquatic plants composed of free-floating fronds with fine roots.  Fronds are longer than broad, measuring 1.5 to 6 mm long and 1-3 mm broad.  Fronds are narrowly egg-shaped to slightly kidney-shaped and intensely green in color. A waxy cuticle induces plants to sparkle in the sunlight. Roots generally number from 2-4, but can range up to 7.  Nerves commonly seen as 3-5, or only as a median ridge.  Flowers, nearly microscopic, are enclosed in a pouch under the frond.
Habitat: Quiet, slow-moving, nutrient-rich water - ponds, ditches and swamps.
Native Range: Australia and Southeast Asia.
Nonindigenous Range: Well established in the United States, more frequent in the Southeast, occasionally found along the West Coast and in Hawaii.  Rather recently established in the Mid-Atlantic States, not only along the milder coastal plain of DelMarVa, but extending west to the colder climates of western Pennsylvania and Massachusetts.

VA MD DE PA MA 1966 1970 1991 1956 1986*

Comments: Colonizes quickly, commonly to form extensive, pure stands. Landoltia punctata can readily be distinguished from native duckweeds, especially with a 10X lens. Lemna species always have one root per frond, while Spirodela polyrhiza has as many as 5-21 roots.   Landoltia fronds are sensitive to severe frost. In milder temperate climates plants generally perennate from starch filled resting fronds that develop in late summer. However, resting fronds do not overwinter in zones with severe winters.  Additionally, Landoltia cannot produce turions and its seeds are sensitive to cold, thereby spread into the coldest regions of the Mid-Atlantic should be limited.

Family: Menyanthaceae
  (Gmel.) Kuntze (yellow floatingheart)

Field Description: Attractive perennial herb, with branching stems arising from rooted rhizomes and leathery floating leaves. Leaves cordate, upper surfaces smooth, lower surfaces rough in texture.  Flowers, 2-3 cm across, carried above the water on slender stalks, petals yellow with margins sometimes fringed. Elongated fruits contain flat seeds.
Habitat: Permanent, quiet waters; river shorelines, ponds, reservoirs and ditches.
Native Range: Europe, India, China and Japan.
Nonindigenous Range: Established at disjunct locales around the United States, a bit more commonly, and for many years, in the Mid-Atlantic States. Locally frequent along the Hudson-Hoosic River drainage in New York. Found in state game lands in the Upper Susquehanna drainage in Pennsylvania in the early 1980s. Known from Newport County, Rhode Island, in the Narragansett drainage.

MD  PA  NJ  NY  CT  RI MA  VT 1957 1915 1940 1929 1939 1977 1882 1863

Comments: Naturally dispersed by seed. Seeds float when wet, are picked up and transported by waterfowl.  When the bird returns to water, the seeds detach and sink to later germinate (Cook, 1990). Introduced for water gardening and still sold today.

Family: Pontederiaceae
  (Mart.) Solms  (water hyacinth)

Field Description: Inflated petioles, submersed stolons, and hanging roots produce an extremely buoyant free-floating aquatic herb. Thick, glossy leaves, nearly round to kidney shaped arise from a central cluster. Flowers are arranged in a terminal spike with 5-20 showy blue to purple, sometimes white flowers with a yellow spot surrounded by a darker purple hue. Fruits a many-seeded dehiscent capsule. 
Habitat: Ponds, lakes and sluggish streams.
Native Range: Upper Amazon Basin, Brazil.
Nonindigenous Range: Established in the more southern United States, including Hawaii. Recorded from Missouri, Illinois, Kentucky and Tennessee where plants apparently escape summertime cultivation to grow as annuals, but do not persist vegetatively through the winter. Verified as perennating in the Mid-Atlantic States only in southeastern Virginia. Collected in New York and Maryland.  Found at the Great Cypress Swamp, Delaware, and at isolated, natural sites along the Delaware River in New Jersey.  Seed could play a role in the recurring populations of these temperate states.

VA  MD  DE  NJ  NY 1988* 1998 1993 2002 1994*

Comments: A popular plant in the water gardening industry, water hyacinth proliferates clonally and also by seed.

Family: Trapaceae
  L. (water chestnut)

Field Description: Floating terminal rosettes of this large annual plant are usually rooted on long stems in muddy substrates. Leaves alternate above, and sharply toothed.  Submersed leaves filiform.  Flowers small, white, attached to the central axis on short stalks.  Fruit a sturdy sharp-spined nut produced underwater.
Habitat: Lakes, ponds, and streams, often under eutrophic conditions, and in freshwater intertidal zones.
Native Range: Eurasia.
Nonindigenous Range: Known in the United States only from the Mid-Atlantic States where populations are long time established yet still advancing into new territory.  Reported from the Great Meadows National Wildlife Refuge, Massachusetts. Recently introduced to New Hampshire, in the Nashua River, and to Connecticut in the Connecticut River. A repeat offender to tributaries of the Chesapeake Bay, where plants reappeared in the late 1990s on the Bird and Sassafras Rivers, tributaries of the Chesapeake Bay, Maryland.  Well known from Lake Champlain and the Lake George drainage, Vermont.   Problematic in lakes and ponds in Pennsylvania.

VA MD  PA  NJ  NY  CT MA  VT  NH 1923 1923 1977 1956* 1884 1999* 1875 1942 1998

Comments: Invasive, aggressive species with spiny fruits inflicting wounds to feet and paws.  The species may be difficult to extirpate, as fruits may remain viable for up to 12 years in the substrate. Each nut can produce 10-15 rosettes and each rosette produces approximately 20 seeds (IPANE, 2001). Reproduction by seed and fragmentation, fragments often spread by water, waterfowl and motorboats.