Milfoil Control

Eurasian watermilfoil is a non-native aquatic plant that is present in most U.S. states and much of Canada. This plant is known for its rapid growth and ability to spread, which can lead to significant problems within a lake. Commonly found in shallow bays and along the shoreline, milfoil forms dense beds that can seriously impair the recreational use of a lake, reduce the availability of fish spawning grounds, outcompete beneficial native plants, and otherwise alter a lake’s natural environment.

The growth and spread of Eurasian watermilfoil is a threat to all our lakes and ponds. Once Eurasian watermilfoil has infested a lake there is no known way to eradicate it. Lake managers can only seek to control it by integrating the most effective, economically feasible, and environmentally sound methods available.

Ecology

watermilfoilEurasian watermilfoil is not native to North America but originates from Europe, Asia and northern Africa. As an "introduced" species to this continent, Eurasian watermilfoil has no natural controls (insects, bacteria, fungi) to keep its growth in check. In North America it has the potential to completely infest lakes once introduced. Native types of watermilfoil rarely attain such extensive growth.

Eurasian watermilfoil stems can reach the surface in up to 20 feet of water, growing up from the lake bottom each year from a fibrous root system. Milfoil grows and spreads extremely quickly, forming dense surface mats. Unlike most native aquatic plants, which are usually associated with particular water qualities, Eurasian watermilfoil will grow readily in many types of lakes, as well as on almost any lake bottom type: silty, sandy, or rocky.

The presence of Eurasian watermilfoil often brings a change in the natural lake environment. Over time, it may outcompete or eliminate the more beneficial native aquatic plants, severely reducing natural plant diversity within a lake. Since its growth is typically dense, milfoil weed beds are poor spawning areas for fish and may lead to populations of stunted fish. Although many aquatic plants serve as valuable food sources for wildlife, waterfowl, fish, and insects, Eurasian watermilfoil is rarely used for food. Commonly found in shallow bays and in bands along the shoreline, dense surface mats of milfoil can also make fishing, boating and swimming virtually impossible.

Eurasian watermilfoil reproduces almost exclusively by the breaking off of fragments which can drift away, sink, develop roots, and grow into plants. A fragment just a few inches long is capable of starting a new plant. This fragmentation occurs both naturally and as a result of human activity. Within a lake, wind and waves may break plants loose, allowing them to drift into new locations and root. Boating activity through dense milfoil beds also contributes to the fragmenting and spread of milfoil plants.

How is Milfoil Spread?

Human recreational activities usually account for the spread of non-native aquatic plants and animals between lakes.  Fragments of aquatic plants cling to the propellers of boat motors or to boat trailers and, if not removed, can start new populations when the boat is launched into another waterbody. Unfortunately, once Eurasion watermilfoil has been introduced into a lake, there is no way to completely eradicate it.

To stop the further spread of non-native aquatic species, it is imperative that all plant fragments are removed from boats before putting in or leaving a lake's access area.  Removed plant material should be properly disposed of in a trash receptacle or on high, dry ground where there is no dnager of them washing into any waterbody.

It is illegal to transport Eurasion watermilfoil, water chestnut, zebra mussels or quagga mussels to or from any Vermont surface water. Any person found transporting these species to or from a Vermont lake or pond will be in violation of this law. Violators are subject to a penalty of $150 per violation (pursuant to 10 V.S.A.§1266).

A state noxious weeds quarantine rule (106K PDF) also prohibits the transport of many aquatic weeds, as well as their sale, distribution, possession, or cultivation.

Control

Since there is no way to completely eradicate Eurasion watermilfoil from a lake once it has been introduced, control efforts must instead focus on: controlling newly introduced infestations, preventing further spread of the plant, or reducing the nuisance level of the problem.  Some methods are more appropriate for well-established populations, whlie others are better suited for those that are recent introductions.

The State of Vermont is concerned about the impacts certain milfoil control methods could have on the environmnet. Bottom barriers, all mechanically powered devices (harvesters, hydrorakes), herbicides, and biological controls require permits from the Department of Envrionmental Conservation before they can be used to control nuisance aqautic plant growth.  Contact the Department at (802) 241-3777 to determine if your proposed control method requires a permit and to obtain permit applications.

Bottom BarrierBottom Barriers, specially made sheets of materials such as fiberglass, polypropylene, or polyvinyl chloride (PVC), anchored to a lake bottom will prevent plant growth by blocking sunlight.  Bottom barriers are most appropriate to control growth in localized areas such as in swimming areas, around docs or to create boat lanes out to deeper water.

With diver operated suction harvesting, scuba divers use suction hoses powered by a surface compressor to selectively remove milfoil from the lake bottom.  Although too labor intensive on a large scale, this method has proven to be highly successful at combating newly established infestations in Vermont lakes.

hydrorake removes plant roots and shoots by raking the lake bottom.  Any removed material must then be depostied on shore.  Hydroraking has ahad limited use in the state but is most practical ofor providing short-term relief from dense milfoil infestations.

When done properly, pulling milfoil plants by hand is highly effective for controlling small, newly introduced milfoil populations.

Mechanical HarvesterMechanical harvesters cut off the milfoil (and any other plants) below the water surface, gathering the cut material as they move through the plant bed. Milfoil roots are not removed in the this process. Mechanical harvesting, like lawn mowing, merely reduces the height of plant growth temporarily in order to make the lake more usable. Removing the plant material from the lake does however, prevent the plants from contributing to the sediment which rapidly accumulates under dense aquatic plant beds. Mechanical weed harvesting has been used on several heavily infested Vermont milfoil lakes.

Other potential milfoil control methods include rotavating, chemical herbicides and biological controls such as grass carp.

Rotavating involves a machine that "tills" the lake bottom, dislodging both the roots and stems of the plant. Plants are either collected by a mechanical harvester or, if conducted in the late fall, are allowed to wash ashore and dry over the winter. This method has yet to be used in Vermont.

There are a number of federally registered aquatic herbicides that control Eurasion watermilfoil.  Consderations for use include cost, the potential need for repeated applications, and product label restrictions that prevent their application in lakes used as water supplies.

WeevilBiological controls such as insects, bacteria or fungi that will impact milfoil are in the experimental stages only. Their use as a milfoil control method may prove to be the control of the future. The use of plant-eating fish such as the grass carp, a native to China, is currently illegal in Vermont.

The VTDEC has been working with the watermilfoil weevil Euhrychiopsus lecontei since 1989. The weevil was responsible for at least one Eurasion watermilfoil decline in Vermont, in Brownington Pond in Brownington. It has been found naturally occurring in many other milfoil-infested lakes in Vermont. The weevil, a native aquatic insect, has shown promise as a potential biological control agent for Eurasian watermilfoil, and is currently the subject of ongoing research.

Resources