COMMON NAME

Variable-leaf watermilfoil
two-leaf watermilfoil

FULL SCIENTIFIC NAME

Myriophyllum heterophyllum Michx.

FAMILY NAME COMMON

Watermilfoil family

FAMILY SCIENTIFIC NAME

Myriophyllum heterophyllum

IMAGES

Incursion/Habitat

Emergent Stem

NOMENCLATURE/SYNONYMS

Synonyms: None

DESCRIPTION

Botanical Glossary

Myriophyllum heterophyllum is an herbaceous aquatic plant that has stout stems up to 3 mm (0.1 in.) in diameter. The leaves are green and in whorls of 4-6. There are two types of leaves. The highly variable emergent leaves are bracts and can reach 10-15 cm (4-6 in.) above the water. These leaves are stiff and can be serrated to lobed along the margins, lanceolate or lance-spatulate to elliptic. These emergent leaves are 0.4-3 cm (0.2-1.2 in.) long and 1.5-5 mm (0.06-0.2 in.) wide. The submerged leaves are feather-like, pinnate, 2-5 cm (0.8-2 in.) long and 2-4 cm (0.75-1.5 in.) wide. They have 4-10 paired pinnae. Mudflat forms have 4-5 paired pinnae. The flowers are small, and the reddish, oval shaped petals are 1.5-3 mm (0.0625-0.125 in.) long. The fruit are subglobose in shape, have a scabrous texture and are 1-1.5 mm (0.04-0.05 in.) long and wide. Each of the mericarps is 2-ridged on the back but rounded on the sides, and is prominently beaked. The fruits and flowers appear from June-September. Page References Crow & Hellquist 194, Fernald 1074, Gleason & Cronquist 309, Holmgren290, Magee & Ahles 772. See reference section below for full citations.

SIMILAR SPECIES

Myriophyllum spicatum L. (Eurasian watermilfoil)
M. verticillatum L. (whorled watermilfoil)
M pinnatum (Walt.) B.S.P. (cutleaf watermilfoil)

REPRODUCTIVE/DISPERSAL MECHANISMS

The main method of dispersal appears to be by vegetative parts being moved around by people, animals and water currents. Waterfowl also eat the seeds and the vegetative parts to some extent.

DISTRIBUTION

Myriophyllum heterophyllum is native to the southern part of the United States from Florida to central and north Texas. It can now be found as far north and west as North Dakota and as far north and east as Maine and Quebec. It is present in all the states of New England except for Vermont.

HISTORY OF INTRODUCTION IN NEW ENGLAND

This plant is native to the United States, but early maps show that it was not historically in New England (though it was in nearby Long Island). It appears that it made its way up to New England via waterways. The earliest record in New England is from 1932 in Bridgeport, CT. It seems that this specimen resulted from intentional introduction. It is likely that other New England populations of this plant are a result of such intentional introductions.

HABITATS IN NEW ENGLAND

This plant is found in a variety of different aquatic habitats, such as lakes, ponds, swamps, rivers and mudflats. Myriophyllum heterophyllum can be found in water up to 1.8 m (6 ft.) deep.

THREATS

Myriophyllum heterophyllum has the ability to completely congest waterways and crowd out other species of aquatic plants. It also hinders recreational activities such as boating, swimming and fishing. The dense growth that forms also provides breeding areas for mosquitoes and degrades the quality of the water for fish and other aquatic wildlife. Many of the New England specimens are hybrids between Myriophyllum heterophyllum and M. pinnatum. The invasive ability of the hybrid is still unclear.

MANAGEMENT LINKS

Texas Agricultural Extension Service- Aquaplant

DOCUMENTATION NEEDS

Documentation required: Herbarium specimen or mounted snippet of the branch.
Best time for documentation: Summer, fall.

ADDITIONAL INFORMATION

Integrated Taxonomic Information System
Has general taxonomic information about the species.

The PLANTS database
General information and a map

North Carolina State University, Crop Science Department
Photographs and very general information

New Hampshire Department of Environmental Services
General information including control methods

Crow G.E. and C.B. Hellquist.  2000.  Aquatic and Wetland Plants of Northeastern North America. Vol 1.  University of Wisconsin Press, Madison.

Fernald, M.L.  1950.  Gray's Manual of Botany 8th edition.  American Book Company, New York.

Gleason H.A. and A.C. Cronquist.  1991.  Manual of Vascular Plants of the Northeastern United States and Adjacent Canada. 2nd ed.  New York Botanical Garden, Bronx, New York.

Holm,L.G., Pancho, J.V., Herberger, J.P., and Plucknett, D.L. 1979. A Geographical Atlas of World Weeds. John Wiley and Sons, New York.

Holmgren N.H. 1998. Illustrated Companion to Gleason and Cronquist's Manual.  New York Botanical Garden, Bronx, New York.

Les, D.H. and L.J. Mehrhoff.  1999.  Introduction of nonindigenous aquatic vascular plants in southern New England: a historical perspective.  Biological Invasions 1:281-300.

Magee D.W and H.E. Ahles. 1999.  Flora of the Northeast. University of Massachusetts Press, Amherst.

Moody, M. and D. Les.  2002.  Evidence of hybridity in invasive watermilfoil populations.  PNAS 99:14867-14871.

USDA, NRCS.  2001.  The PLANTS Database, Version 3.1. (http://plants.usda.gov).  National Plant Data Center, Baton Rouge, LA 70874-4490 USA.