How to Control Eurasian Milfoil and other Aquatic Plants

Eurasian Milfoil Control



How to Control Eurasian Milfoil and other Aquatic Plants

There are several forms of milfoil with some being considered exotic forms of milfoil such as Eurasian milfoil originating in Europe and Asia were brought to the Americas in the early 1900's likely by water craft of some sort.  These types of "invasive" milfoil tend to spread very quickly and readily.

Prevention of Milfoil and other water weeds is the first defense in exotic species control.  However, once an exotic weed, especially a fast moving growth such as Milfoil,  has colonized a lake, early detection and a rapid response approach is the most effective method to control the spread of the Milfoil.  What approach works best in a given situation is dependent on several variables. 

Milfoil and most other exotic plants are often transported on boats and boat trailers.  If you trailer your boat from lake to lake, wash before re-launching and visually check the boat, trailer and motor.

Monitoring is key to early detection.  Plant surveys are often conducted with a global positioning system that allows the specific location of aquatic plants to be documented.  Below is the difference between Eurasian Milfoil and the less invasive Norther Milfoil.  (Both are best controlled with Aquacide Pellets)


Eurasian Northern Watermilfoil

University of Minnesota.

Herbicide Treatments:

This is commonly used to control invasive exotic plants (including Eurasian Milfoil).  There are currently more than 300 herbicides registered with the EPA.  Of those only a fraction are approved for use in and around the aquatic environment.  There are 2 basic types of herbicides: systemic and contact. 

Systemic herbicides (Aquacide Pellets, Fluridone Liquid and Shore-Klear Liquid) are taken up by the plant and translocate to the root system to provide season-long control.  It generally takes several weeks for impact to become apparent but they will kill the roots and in the case of Aquacide Pellets, they are easy to apply and start being absorbed by the Milfoil and other broadleaf weeds almost immediately.

Contact herbicides (Aquathol Super K Granular, Hydrothol Granular and Harvester Liquid) only affect the portion of the plant that comes into contact with the herbicide.  Plants usually die-back within a week, but some plants may grow back later in the season since roots stay intact.  In general, herbicide treatments should target nuisance exotic species such as Eurasian Milfoil and have minimal impacts on most native plant species.

Mechanical Harvesting:

This involves cutting and removing vegetation.  Mechanical harvesting (Water Weed Rake & Weed Razor) has the advantage of removing biomass and help slow the rate at which plant material accumulates.  Harvesters have limited operational flexibility and can agitate bottom sediment and temporarily increase turbidity.  Attempts to control certain plant types by harvesting may not prove entirely effective.  This is especially true with Eurasian Milfoil due to the fact that this plant may proliferate and spread via vegetation propagation.

Biological Control:

Milfoil weevil is an aquatic insect that is native to North America and appears to be common in the Midwest.  The weevil has been found to feed almost exclusively on milfoil species.  The milfoil weevil can be effective if adequate densities can persist through the summer and among years.  However, many of the sites investigated have failed to sustain sufficient herbivore (weevil) density to affect control.

Integrated Control:

Integrated control involves using a combination of control measures.  For example, many lakes use a combination of herbicide and mechanical harvesting.  Herbicide treatments are performed early to control Eurasian Milfoil, and harvesting is conducted later to control other nuisance plants.

Bottom Line:

Aquatic plant control is an ongoing challenge.  The approach or combination of approaches that work best in a particular lake depend on local conditions.  Once an exotic plant has been introduced in a lake, a sustained effort is often required to ensure control.


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