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| NAS - Nonindigenous Aquatic Species |
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Common Name: variable leaf water-milfoil, twoleaf water-milfoil
Synonyms and Other Names: None
Taxonomy: available through 
Identification: A perennial herbaceous submersed aquatic plant that produces a short, emergent floral spike above the waterline. Submersed leaves (2-4.5 cm long) are pinnately divided into 4-10 leaflet pairs, giving them a feather-like appearance and are arranged in whorls of 4-6 around a stout stem (3 mm in diameter).
Native Range:
Native to United States, but historically known only in the Southeast and Midwest.
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Interactive maps: Continental US, Alaska, Hawaii, Caribbean
Nonindigenous Occurrences:
While native to portions of the U.S., there is no historic evidence of M. heterophyllum being native to New England, where it has now been confirmed in all states. The first New England record is from 1932 in Bridgeport, Connecticut, where it is thought to have escaped cultivation and established in Burrell's Pond and, by 1974, it was widespread in New York (Les & Mehrhoff 1999). Found throughout much of Massachusetts, Connecticut, northern Rhode Island, southwestern Maine. The distribution in New Hampshire was originally concentrated around Lake Winnipesaukee, but has spread considerably in the past 10-15 years and is now found in numerous lakes and bays (Thum & Lennon 2006). In October 2008, DNA analysis confirmed the first instance of this species in a Vermont lake; rapid response initatives were quickly inacted hoping to reduce the chance of spread to further lakes (A. Bove, pers. comm).
Populations of M. heterophyllum have been recently been identified in five lakes in western Washington (Pierce and Thurston counties) (J. Parsons, pers comm). Samples from local aquarium suppliers have confirmed this species is being sold, typically under the common name of "Red Foxtail," in the Pacific Northwest. Siltcoos Lake is home to the only confirmed infestation in Oregon; this population has probably been present since 2000 (M. Moody, pers comm).
Means of Introduction: Released aquarium or pond plants. Secondary spread possible through hitchhiking on boats/trailers or waterfowl.
Status:
Impact of Introduction:
Remarks: Molecular data supports the fact that some aggressively growing populations of variable-leaf milfoil are actually hybrids between M. heterophyllum and the native M. pinnatum; these hybrids, found in Connecticut, propagate primarily vegetatively (Moody & Les 2002). Populations sampled from 25 lakes in New Hampshire, however, suggest that the pure M. heterophyllum is also capable of forming aggressively-spreading, monotypic stands (Thum & Lennon 2006).
References
Other Resources:
Author: V. Howard
Revision Date: 10/31/2008 Citation for this information:
V. Howard. 2009. Myriophyllum heterophyllum. USGS Nonindigenous Aquatic Species Database, Gainesville, FL.
<http://nas.er.usgs.gov/queries/FactSheet.asp?speciesID=236> Revision Date: 10/31/2008
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