Key to Non-native and invasive aquatic plants in Rhode Island
(Adapted from C. Barre Hellquist, Massachusetts College of Liberal Arts)
1. Leaves arising from base of plant in flowering plants
2. Flowers white; leaves arrow-shaped or lance-like, lacking a marginal vein………...………..1. Sagittaria
(arrowhead). The genus Sagittaria contains various species that are found statewide and occasionally
may be a source of problems.
(Native)
(Native)
2. Flowers green (spathe & spadix); leaves arrow-shaped with a distinct marginal vein ……………………..
……………………………………………………………………………….……....2. Peltandra virginica
(arrow arum). Arrow arum is not considered weedy, but is included due to its similarity to arrowhead
and pickerel weed.
Peltandra virginica (Native)
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1. Leaves arising from stem in flowering plants
3. Plant upright with showy blue or purplish flowers…………………………...….3. Pontederia cordata
(pickerel weed). Common throughout Rhode Island, may become weedy along the shoreline.
Pontederia cordata (Native)
3. Plant submersed and/or floating, flowers white, yellow or green
4. Main portion of leaves are floating
a) Triangular leaves arranged in a rosette with white flowers are white, fruit are spiny, 4-spiked,
barbed nuts...........................................................................4a. Trapa natans (water chestnut).
This is an extremely troublesome plant in various waterbodies in Rhode Island. But since it is an
annual plant, it MAY be more manageable than many through hand pulling prior to seed
production.
Trapa natans (Non-Native)
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b) Round to heart-shaped leaves with wavy edges, multiple leaves per stem, and emergent
yellow flowers with fringed petals ...................... Nymphoides peltata (yellow floating heart)
c) Large (up to 2 feet in diameter) round leaves without a notch, large pale yellow or white
flowers with yellow seed pod in the center, seed pods dry well, and are often used as an
ornamental item ….………………………………………Nelumbo lutea (American lotus)
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4. Main portion of plant submersed, forming various fruits
5. Plants with finely dissected leaves
6. Submersed leaves opposite, appearing fan-shaped…………...….......5. Cabomba caroliniana
(fanwort). Extremely aggressive plant found throughout the state.
Cabomba caroliniana (Non-Native)
6. Submersed leaves whorled, not appearing fan-shaped.
7. Leaves forking, plants lacking roots……...………………....6. Ceratophyllum demersum
(coontail). Common native aquatic that may be confused with fanwort and watermilfoil.
Ceratophyllum demersum (Native)
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7. Leaves with leaflets off the central portion, plants with roots.
8. Plants with conspicuous emersed feathery leaves ………7. Myriophyllum aquaticum
(parrot-feather). Limited presence, but has been recently found in the state.
Myriophyllum aquaticum (Non-Native)
8. Plants lacking conspicuous feathery leaves
9. Emergent bracts longer than flowers and fruits, leaf tips rounded in
outline………………………………...…….........8. Myriophyllum heterophyllum
Common weedy species throughout the state. All species of watermilfoil can
become weedy, so no movement of the various species should occur. Other
confusing species are illustrated below and in the next entry.
Emergent portion
more robust than
with the native
species
Myriophyllum heterophyllum (Non-Native)
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Myriophyllum humile (Native)
9. Emergent bracts short, about the same size as the flowers and fruits, leaf tips flat
in outline…………………………………………...…..9. Myriophyllum spicatum
(Eurasian watermilfoil). Limited distribution in RI. All species of watermilfoil
can become weedy, so no movement of the various species should occur.
Often confused with other non-native and native species of watermilfoil.
Myriophyllum spicatum (Non-Native)
5. Plants with linear leaves
10. Leaves alternate along stem, fruits small drupelets………………..…….10. Potamogeton crispus
(Crisped pondweed). The pondweeds, Potamogeton, are an extremely diverse genus of aquatic
plants. Many species are common in Rhode Island and several species may become weedy. Four
of the common species are illustrated below.
Potamogeton crispus (NN) P. amplifolius (Native)
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P. perfoliatus (Native) P. pusillus var. pusillus (Native)
10. Leaves whorled or sub-opposite, appearing whorled
11. Fruit and flowers borne in leaf axils, green, submersed
12. Leaf bases tapered, leaf serrations small (difficult to see with unaided eye)………...……
……………………………………………..………………...…..11. Najas guadalupensis
(naiad). This species is not common in Rhode Island, but may become aggressive when
established. This species is often confused with the native Najas flexilis.
Najas guadalupensis (Native)
Najas flexilis (Native)
12. Leaf bases lobed, leaf serrations conspicuous…………………….………12. Najas minor
(European naiad). This Eurasian species has limited distribution in Rhode Island, and
typically found in very alkaline lakes and ponds..
Najas minor (Non-Native)
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11. Fruit and flowers terminal or in leaf axils and/or terminal, often floating; flowers white; leaf
bases linear.
13. Leaves mostly in whorls of 3’s.
14. Leaves generally 0.3-1.5 mm wide, pointed at tip…………….…..13. Elodea nuttallii
Common species in acidic waters.
Elodea nuttallii (Native)
14. Leaves generally 1.0-5.0 mm wide, blunt at tip…………....….14. Elodea Canadensis
(Elodea). Common species in neutral to alkaline lakes.
Elodea canadensis (Native)
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13. Leaves mostly in whorls of 4-8.
15. Leaf margins appearing entire to the unaided eye (with minute teeth under
magnification)……………………………………………….……….15. Egeria densa
(South American waterweed). A common aquarium plant, currently limited in Rhode
Island waters.
Egeria densa (Non-Native)
15. Leaf margins distinctly toothed, even to the unaided eye…….16. Hydrilla verticillata
This species is a significant problem in the southeast. Not known to be in Rhode
Island, it is established in southeastern Connecticut and on Cape Cod. Be on a
constant lookout for this potentially aggressive plant.
Hydrilla verticillata (Non-Native)
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Most illustrations are from the Aquatic Vascular Plants of New England series with the exception of
those that are noted below:
Center for Aquatic and Invasive Plants website, University of Florida (http://plants.ifas.ufl.edu/),
accessed 8 July 2011 (Nymphoides peltata)
Center for Aquatic and Invasive Plants website, University of Florida (http://plants.ifas.ufl.edu/),
accessed 8 July 2011 (Nelumbo lutea)
Crow, G.E. & C.B. Hellquist. 1982, 1983. Aquatic vascular plants of New England, New Hampshire
Agr. Exp. Stat. Bull. 520, 524. Durham, NH.
Crow, G.E. & C.B. Hellquist, In press. Aquatic and wetland plants of northeastern North America. Univ.
Wisconsin Press, Madison. (Myriophyllum aquatica)
Godfrey, R.K. & J.W. Wooten. 1979. Aquatic & wetland plants of southeastern United States,
Monocotyledons, U. Georgia Press, Athens 712 pp. (Hydrilla)
Hellquist, C.B. & G.E. Crow. 1981, 1982, 1984, Aquatic vascular plants of New England, New
Hampshire Agr. Exp. Stat. Bull 515, 518, 523, 527. Durham, NH.
Reed, C. 1970. Selected weeds of the United States. Agr. Handb. No. 366, Agr. Res. Serv., U.S. Dept.
Agr. (Ceratophyllum demersum, Najas guadalupensis, Najas flexilis, Potamogeton crispus,
Potamogeton pusillus)
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