Egeria densa Planch., Hydrocharitaceae

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Present on Pacific Islands?  yes

Primarily a threat at high elevations?  no

Risk assessment results: 

Australia:  Reject, score: 22 (Go to the risk assessment)
Risk assessment from the Government of Queensland, Australia (PDF format).

Common name(s): [more details]

Chinese: shui yun cao

English: Brazilian elodea, Brazilian waterweed, dense waterweed, egeria, leafy elodea

Habit:  aquatic herb

Description:  "Submerged, rooted or drifting aquatics of fresh water.  Stems cylindric, simple or branched; lower leaves opposite or in whorls of 3, upper leaves in whorls of 4-8, linear to ovate serrulate; flowers dioecious; staminate flowers 2-4 together within spathe; petals 3, white; stamens 9, pistillate flowers on slender hypanthium-base; sepals 3; petals 3, stigma trifid; ovary 1-celled; ovules several on 3 linear placentae; fruit a transparent capsule" (Stone, 1970; p. 101).

Habitat/ecology:  Shallow, still water of lakes or streams. Forms dense mats displacing native species, depleting oxygen and otherwise changing the character of streams and lakes. In Hawai‘i, "sparingly naturalized in ponds and perhaps also slow-running ditches" (Wagner et al., 1999; p. 1443).

Propagation:  Pieces and cuttings, often spread by dumping of aquarium plants.

Native range:  South America

Presence:

Pacific
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Cook Islands Southern Cook Islands	Rarotonga Island		McCormack, Gerald (2011)
French Polynesia Society Islands	Tahiti Island	introduced invasive cultivated	Welsh, S. L. (1998) (p. 349) Vouchers cited: Fosberg 62976, Florence 4622, Florence 3165
French Polynesia Society Islands	Tahiti Island	introduced invasive	Florence, J./Chevillotte, H./Ollier, C./Meyer, J.-Y. (2011) Vouchers cited: J. Florence 3165 (PAP), J. Florence 4622 (PAP), W.A. Whistler & M. Guérin 1 (PAP) (as Hydrilla verticillata (L.) Presl) Naturalisé. Menace pour la biodiversité.
French Polynesia Society Islands	Tahiti Island		Bishop Museum (Honolulu) (1983) (voucher ID: BISH 554435) Taxon name on voucher: Egeria densa Planch.
French Polynesia Society Islands	Tahiti Island		Bishop Museum (Honolulu) (1982) (voucher ID: BISH 699484) Taxon name on voucher: Egeria densa Planch.
French Polynesia Society Islands	Tahiti Island		National Tropical Botanical Garden (U.S.A. Hawaii. Kalaheo.) (1981) (voucher ID: PTBG 37268) Taxon name on voucher: Egeria densa
State of Hawaii Hawaiian Islands	Hawai‘i (Big) Island	introduced invasive cultivated	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (p. 1443)
State of Hawaii Hawaiian Islands	Kaua‘i Island	introduced invasive cultivated	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (p. 1443)
State of Hawaii Hawaiian Islands	Maui Island	introduced invasive	Oppenheimer, Hank L. (2003) (p. 14) West Maui. Voucher cited: Oppenheimer, L. Nelson & F. Duvall H110002 (BISH, PTBG)
State of Hawaii Hawaiian Islands	Moloka‘i Island	introduced invasive cultivated	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (p. 1443)
State of Hawaii Hawaiian Islands	O‘ahu Island	introduced invasive cultivated	Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. (1999) (p. 1443)

Pacific Rim
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Australia Australia (continental)	Australia (continental)	introduced	U.S. Dept. Agr., Agr. Res. Serv. (2011)
Australia Australia (continental)	Queensland	introduced invasive	Queensland Herbarium (2002) (p. 3)
China China	China (People's Republic of)	introduced cultivated	Zhengyi, Wu/Raven, Peter H./Deyuan, Hong (2011) Known from a cultivated fish pond, Guangdong.
Japan Japan	Japan (country)	introduced	Mito, Toshikazu/Uesugi, Tetsuro (2004) (p. 190)
New Zealand New Zealand	New Zealand (country)	introduced invasive	Healy, A. J./Edgar, E. (1980) (p. 26) Voucher cited: B. E. G. Molesworth (AK 140634)
New Zealand New Zealand	New Zealand (country)	introduced invasive	Owen, S. J. (1997)

Indian Ocean
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Australia (Indian Ocean offshore islands) Christmas Island Group	Christmas Island	introduced cultivated	Swarbrick, J. T. (1997) (p. 112)

Comments:  Recently discovered in French Polynesia (Jean-Yves Meyer, personal communication).

Control:  See "Biology and control of aquatic plants: A best management practices handbook"  for control information (large file, PDF format).

Chemical: Diquat (Reglone) or acrolein.

Additional information:
Information on the Environment Bay of Plenty, New Zealand, web site.
Fact sheet from the Bay of Plenty Regional Council, New Zealand.
Photo at University of Florida, Center for Aquatic and Invasive Plants.
Information from the Global Invasive Species Database.
Fact sheet from the Government of Queensland, Australia. (PDF format).
Information sheet from Weedbusters New Zealand.
See information in the book "Biology and control of aquatic plants: A best management practices handbook"  (large file, PDF format).

Additional online information about Egeria densa is available from the Hawaiian Ecosystems at Risk project (HEAR).

Information about Egeria densa as a weed (worldwide references) may be available from the Global Compendium of Weeds (GCW).

Taxonomic information about Egeria densa may be available from the Germplasm Resources Information Network (GRIN).

References:

Bishop Museum (Honolulu). 1982. Voucher specimen #BISH 699484 (Florence, J. 3165).

Bishop Museum (Honolulu). 1983. Voucher specimen #BISH 554435 (Florence, J. 4622).

Florence, J./Chevillotte, H./Ollier, C./Meyer, J.-Y. 2011. Base de données botaniques Nadeaud de l'Herbier de la Polynésie Française (PAP). (online resource).

Gettys, Lyn A./Haller, William T./Bellaud, Marc; eds. 2009. Biology and control of aquatic plants: A best management practices handbook. Aquatic Ecosystem Restoration Foundation, Marietta, Georgia. ISBN 978-0-615-32646-7. 200 pp.

Healy, A. J./Edgar, E. 1980. Flora of New Zealand, Volume III: Adventive cyperaceous, petalous & spathaceous monocotyledons. Government Printer, Wellington, New Zealand. 220 pp.

McCormack, Gerald. 2011. Cook Islands Biodiversity Database, Version 2007.2. Cook Islands Natural Heritage Trust, Rarotonga.

Mito, Toshikazu/Uesugi, Tetsuro. 2004. Invasive alien species in Japan: the status quo and the new regulation for prevention of their adverse effects. Global Environmental Research 8(2)/2004: 171-191.

Oppenheimer, Hank L. 2003. New plant records from Maui and Hawai‘i Counties. In: Evenhuis, Neal L. and Eldredge, Lucius G., eds. Records of the Hawaii Biological Survey for 2001-2002. Part 1: Articles. Bishop Museum Occasional Papers. 73:3-30.

Owen, S. J. 1997. Ecological weeds on conservation land in New Zealand: A database. Working draft. Wellington, New Zealand. Department of Conservation.

Queensland Herbarium. 2002. Invasive Naturalised Plants in Southeast Queensland, alphabetical by genus. Modified from: Batianoff, George N. and Butler, Don W. (2002). Assessment of Invasive naturalized plants in south-east Queensland. Appendix. Plant Protection Quarterly 17, 27-34. 11 pp.

Stone, Benjamin C. 1970. The flora of Guam. Micronesica 6:1-659.

Swarbrick, J. T. 1997. Environmental weeds and exotic plants on Christmas Island, Indian Ocean: a report to Parks Australia. 101 pp. plus appendix.

U.S. Dept. Agr., Agr. Res. Serv. 2011. National Genetic Resources Program. Germplasm Resources Information Network (GRIN). Online searchable database.

Wagner, Warren L./Herbst, Derral R./Sohmer, S. H. 1999. Manual of the flowering plants of Hawaii. Revised edition. Bernice P. Bishop Museum special publication. University of Hawai‘i Press/Bishop Museum Press, Honolulu. 1919 pp. (two volumes).

Welsh, S. L. 1998. Flora Societensis: A summary revision of the flowering plants of the Society Islands. E.P.S. Inc., Orem, Utah. 420 pp.

Zhengyi, Wu/Raven, Peter H./Deyuan, Hong. 2011. Flora of China (online resource).