A few submerged plants are invasive


Characteristics of invasive plants

  Why do they become invasive?  In lakes, these plant species become invasive because they can quickly spread. They have been known to crowd out native plants, and create dense mats that interfere with recreational activity. Invasive plants reproduce by different mechanisms:

-Seeds are produced, many of which lie dormant for a long time and germinate sporadically, leading to the conclusion that this is sexual reproduction

-Established plants can also spread by producing stolons that come from the root crowns and can produce a new plant up to 1 m away

-The method of spread is asexually from stem fragments that become brittle and break apart in the fall. These fragments can float long distances to new areas, and settle to become a new plant. Recreational activities (such as boating), and other disturbances, can carry stem fragments to other areas or bodies of water to create new plants.


The following 3 types of invasive plants are found in lac Mercier:  

*** Eurasian water milfoil   (Myriophyllum Spicatum) Eurasian water milfoil was introduced to North America nearly 50 years ago, and it has become an undesirable invasive species in many lakes.

Eurasian water milfoil reproduces from seeds. It also spreads by stolons from the roots.

The primary method of spread is by regrowth of plant fragments, which increases the rate in which the plant can spread and grow.

This is a submerged plant that grows quickly, and once the stems hit the water surface,  they branch in all different directions to produce a dense mat of vegetation that blocks sunlight for plants below the surface.

These plants can easily cover a lake surface, from shore to shore.


Photo: laclabelle.org 


Because of its ability to out-compete native plant species, a monoculture is created, and aquatic plant diversity can decrease.

These thick mats of vegetation can add more phosphorous and nitrogen to the water column, and alter water quality by raising pH and temperature and decreasing dissolved oxygen, making for a poor habitat for fish, waterfowl and other species.

                           Eurasian milfoil

Eurasian water milfoil can interfere with boating, fishing, and swimming, when one encounters the dense mats of vegetation. The vegetation can become entangled in boat propellers and boating equipment. It is found in sand, rocky and in soft sediments, in depths from a few centimetres up to several meters.

They root in sediment, sand, gravel, and vegetation debris, at depths varying from a few centimetres, to several meters of water.

The stems commonly grow to lengths of six to nine feet. The very small soft leaves, about two inches long, are deeply divided, resembling feathers.

It grows to the surface, and branches out to several secondary stems creating a dense mass. It’s very small white or reddish flowers, and it’s dark brown fruits form spikes which are held above the water.


  ***  Elodea nuttallii     (western waterweed)

These underwater perennial plants grows less than 1 m long, with slender, generally branched trailing stems.  



Elodea nuttallii is found in sandy and in soft sediments, in depths of up to several meters.

Leaves are pale green, folded along the midrib, and somewhat recurved with their tip tapered to a slender point. Small white flowers occur at the ends of long, thread-like stalks, blooming from July to September. Male flowers detach and become free-floating.

The shoots grow rapidly towards the surface without splitting, where they form a densely branched canopy. When water temperatures become warmer, the plant start to regenerate new lateral shoots. They propagate by stem fragments, over wintering buds, and rarely by seeds.  


*** Broad-leaved Pondweed   (Potamogedon amplifolius)

This native plant is one of the most invasive species in our lakes.

                                                                     Regional AIS Education Specialist, Golden Sands RC&D Council, Inc., Stevens Point, WI

Broad-leaved pondweed grows amongst the submerged plant community.

It is found in soft sediment soil, at depths  from two to four meters. The stalks of the floating oval large leaves are generally quite long, arched and folded.

The usually wavy-edged underwater leaves are curved into a banana shape, and oval, leathery floating leaves grow on or near the water surface.

The small green-brown flowers, followed by fruit, occur among the floating leaves, and are densely arranged on an emergent dense spike.  


*** Ribbon-Weaved Pondweed   (Potamogedon epihydrus)

Courtesy of Edward G. Voss. USDA NRCS. 1995. Northeast wetland flora:

Field office guide to plant species. Northeast National Technical Center, Chester