Variable-leaved watermilfoil

Myriophyllum heterophyllum

Images of this species:

Common look-alikes:

Identification

Many watermilfoils, native or not, can be difficult to distinguish from one another, especially in the absence of fruits or flowers. In addition to the non-native watermilfoils, variable-leaved and Eurasian (Myriophyllum spicatum), six native watermilfoils exist in Vermont. To identify variable-leaved watermilfoil, look for:

  • Densely packed whorls of four to six underwater leaves around the stem
  • Underwater leaves with 7 to 11 pairs of leaf segments per leaf
  • On mature plants, blade-like leaves with serrated edges appearing above the water surface. Flowers develop at the base of these above-water leaves, forming an erect, stiff spike
  • Thick, robust, often reddish stems

 

Biology

Origin

Native to much of the southeastern and midwestern US, but not indigenous to New England, where it has now been confirmed in all states.

Habitat

Variable-leaved watermilfoil is an extremely well-adapted plant, and can thrive in a variety of environmental conditions. Most often, it is found in relatively shallow bays and coves, but can also inhabit deeper areas and those areas with moderate current. It can grow rooted in water depths up to 20 feet, and potentially deeper under clear water and well-lighted conditions. This plant can colonize various substrates, including organic muck, silt, sand, and gravel. Plants can form terrestrial morphs when stranded out of water, and are able to remain in this land-adapted form until submerged, at which time they revert back into their typical aquatic form.

LifeCycle

Variable-leaved watermilfoil is a hardy perennial, and can propogate through root division, fragmentation, turions, and seeds. Often, but not always, colonies produce spikes that emerge from the water and flower in mid to late summer. Some plants may overwinter intact, while others will break apart and die back to their rootstalks. Plants will begin to grow rapidly from sprouts, turions, and seeds in the spring, and will usually outcompete other plants for light and space.

Management Options

As with most aquatic invasive plants, control of a population once it has become established is difficult and often infeasible. Because of the variety of ways this plant can propogate and spread, eradication of an established population is highly unlikely. As with Eurasian watermilfoil, there are physical, mechanical, and chemical techniques that can be used to manage the impacts of the species, but these are maintenance techniques that do not accomplish long-term eradication.

If found early, a rapid response effort involving the removal of all plants and plant material and subsequent checks to remove any new growth, an infestation may be avoided.

Vermont Distribution

Map of national distribution of variable-leaved watermilfoil. Regions where the plant is believed to be native are indicated in orange; introduced populations are indicated in red.

Map of nationwide distribution of variable-leaved watermilfoil

Documented populations of variable-leaved watermilfoil in Vermont. Note: rapid response efforts to the finding of plants in Hall's Lake appear to be successful, as no new plants have been found in recent years.

Documented populations of variable-leaved watermilfoil in Vermont

 

How You Can Help

For most aquatic invasive species, humans are the primary vector of transport from one waterbody to another. Many of these nuisance plants and animals can be unknowingly carried on fishing gear, boating equipment, or in very small amounts of water in a watercraft. The easiest and most effective means to ensure that you are not moving aquatic invasives is to make sure that your vessel, as well as all your gear, is drained, clean, and dry.

BEFORE MOVING BOATS BETWEEN WATERBODIES:

  • CLEAN off any mud, plants, and animals from boat, trailer, motor and other equipment. Discard removed material in a trash receptacle or on high, dry ground where there is no danger of them washing into any water body.

  • DRAIN all water from boat, boat engine, and other equipment away from the water.

  • DRY anything that comes into contact with the water.  Drying boat, trailer, and equipment in the sun for at least five days is recommended. If this is not possible, then rinse your boat, trailer parts, and other equipment with hot, high-pressure water.

Interested in monitoring for aquatic invasives?

Citations

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.

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

V. Howard. 2016. Myriophyllum heterophyllum. USGS Nonindigenous Aquatic Species Database, Gainesville, FL. Revision Date: 10/31/2008 

Maine Volunteer Lake Monitoring Program, Variable Water Milfoil