Myriophyllum heterophyllum, Variable Watermilfoil

Gardens & Landscapes, Invasive Species, Extension Master Gardener January 28, 2013 Print Friendly and PDF

Invasive Species: Myriophyllum heterophyllum, Variable Watermilfoil

Variable watermilfoil is an invasive perennial aquatic herb that has leaves of two noticeably different forms. The submerged leaves are finely dissected, whorled, reddish/greenish-brown, and 1/2 to 2 1/2 in. (1.3 to 6.4 cm) long. The emergent leaves are small, oval, bright green, whorled, and up to 1/4 in. (0.6 cm) wide. Emergent leaves stand 6 to 8 in. (15.2 to 20.3 cm) out of the water and may not be apparent until late summer. Flowering occurs from June to September. Flowers are emergent on 2 to 12 in. (5.1 to 30.5 cm) green to reddish stalks. Petals are less than 0.1 in. (3 mm) in length and are subtended by downward curved bracts. Fruits are small, nearly round, and have a rough surface. Variable watermilfoil is native to the southern United States but is considered invasive in much of the Northeast. This species is listed as endangered or threatened in three states (Kentucky, Ohio, and Pennsylvania).

What are invasive species and why should we be concerned about them? 


Taxonomy: Scientific and Common Names for This Species

Haloragales > Haloragaceae > Myriophyllum heterophyllum Michx.

Synonym(s): twoleaf watermilfoil, broadleaf water-milfoil

Myriophyllum heterophyllum - USDA PLANTS Profile

Distribution Maps

Variable watermilfoil - The reported distribution of this invasive species across the United States. (Source: Invasive Plant Atlas of the United States)

Up-to-the-minute distribution maps and why they are important

Reporting This Invasive Species

What is the best way and place to report the occurrence of an invasive species? 

 How to report an invasive species sighting to EDDMapS - Early Detection & Distribution Mapping System

EDDMapS - Report an invasive species to EDDMapS

Cooperative Extension Offices - Find your local Cooperative Extension office on this map provided by USDA

How to Identify

This invasive species can be identified by looking for the characteristics described in the paragraphs that follow.

Plant

An invasive perennial aquatic herb that has leaves of two noticeably different forms.

Graves Lovell, Alabama Department of Conservation and Natural Resources, bugwood.org Leslie J. Mehrhoff, University of Connecticut, bugwood.org

Foliage

The submerged leaves are finely dissected, whorled, reddish/greenish-brown, and 1/2 to 2 1/2 in. (1.3 to 6.4 cm) long. The emergent leaves are small, oval, bright green, whorled, and up to 1/4 in. (0.6 cm) wide. Emergent leaves stand 6 to 8 in. (15.2 to 20.3 cm) out of the water and may not be apparent until late summer.

Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Leslie J. Mehrhoff, University of Connecticut bugwood.org Graves Lovell, Alabama Department of Conservation and Natural Resources, bugwood.org

Flower

Flowering occurs from June to September. Flowers are emergent on 2 to 12 in. (5.1 to 30.5 cm) green to reddish stalks. Petals are less than 0.1 in. (3 mm) in length and are subtended by downward curved bracts.

Leslie J. Mehrhoff, University of Connecticut, bugwood.org Leslie J. Mehrhoff, University of Connecticut, bugwood.org

Fruit

Fruits are small, nearly round, and have a rough surface.

   
bugwood.org bugwood.org

Native Species That Resemble Variable Watermilfoil

 

- Images at invasive.org

   
bugwood.org bugwood.org

 

- Images at invasive.org

   
bugwood.org bugwood.org

 

Additional Images for Variable Watermilfoil

Variable watermilfoil - Images at Invasive.org

 

Learning Resources for Variable Watermilfoil

Myriophyllum heterophyllum Identification Card - U.S. Fish & Wildlife Service

Myriophyllum heterophyllum Fact Sheet - U.S. Fish & Wildlife Service

 

Additional Information, Biology, Control and Management Resources

Control and management recommendations vary according to individual circumstances. Location, habitat, weather, and a variety of other conditions are factors that help determine the best treatment choice. To find the safest and most effective treatment for your situation, consult your state's land-grant institution. If you will use chemicals as part of the control process, always refer to the product label.

United States Land-Grant University System - Find your land-grant university's College of Agriculture, Cooperative Extension Service, or other related partner on this map provided by USDA.

Invasive Plant Atlas of New England - EDDMapS

Center for Aquatic and Invasive Plants - University of Florida, IFAS

Environmental Fact Sheet - NHDES

Plant Fact Sheet - Washington State Noxious Weed Control Board

Invasive Species Compendium - CABI

Global Invasive Species Database - Invasive Species Specialist Group


 

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.