What is Milfoil?   

Milfoil is an aquatic plant that lives submerged in the shallow waters of freshwater ponds, lakes and streams.  The plant is not uncommon in New Hampshire, there are actually six native species, -but these native species typically don’t grow to dominate the shallow water regions.  There are also two non-native species of milfoil becoming more prevalent in New Hampshire waterways, Variable Water-Milfoil and Eurasian Water-Milfoil.1 These are invasive exotic species that are very difficult to control once established.  These species are typically referred to as “nuisance” weeds due to their ability to displace native plant species, changing the natural habitat of lakes, ponds and streams.2  When a New Hampshire waterway is said to have "milfoil" it is typically a reference to one of these nuisance species.

A Growing Problem:  Invasive milfoil was first identified in New Hampshire’s Lake Winnipesaukee in 1965.  Since then it has spread throughout the state at an ever-increasing rate.  An accounting of invasive aquatic species in 2008 identified 64 major New Hampshire waterways with invasive milfoil.1,3,4

Flints Pond is the unfortunate host to Variable-Water Milfoil, the most pervasive of the exotic species infesting NH waters.  Variable milfoil reproduces rapidly by fragmentation –small pieces that break off a main stem, sprout roots, and establish themselves throughout a body of water.  Boat traffic is cited as the most likely means for the spread of milfoil throughout the state.2,4  Small fragments of weed or seeds are unwittingly transported, attached to boats or trailers.  In areas where milfoil is already established, propellers chop the weeds into fragments, allowing the weeds to spread throughout the water body.    


Why is Milfoil Bad for Flints Pond?

Significant Loss of Natural Habitat:  Exotic milfoil is invasive and dominates in shallow areas, displacing the natural aquatic plant habitat.  It thrives in the “submergent zone”, where sunlight penetrates to the bottom of the pond.1  Studies of depth and water clarity in Flints Pond show that sunlight can reach the bottom of over 80 percent of the pond5-which means nearly the entire pond is susceptible to milfoil infestation and loss of natural habitat.

Increased Stress on Native Aquatic Plants and Animals:  Under ideal conditions, milfoil can grow up to an inch a day and produce large amounts of biomass.1  Each winter the weeds die off and the mulch sinks to the bottom of the pond or it tangles with other pond weeds like water lilies.  Eventually large vegetative masses form.  The decaying vegetation uses oxygen, competing with native plants and animals.

Increased Flood Risk:  Large tangles of biomass and water lilies form “floating islands” that threaten pond outflows.  Blockage of the outflow can cause flooding of the pond and could cut off the flow to Flints Brook, an important tributary to the Nashua River watershed and local aquifers.

Increased “Lake Aging”:  Lake aging is the natural process of a water body filling in over geologic time, changing from a lake, to a pond, to a marsh, to a meadow, and finally to dry land.  The large vegetative masses produced by milfoil fill in shallows, prematurely aging the water body.  The shallow depth of Flints Pond means that significant areas are in danger of prematurely transitioning to marshland.


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Want to Learn More?

The State of New Hampshire Department of Environmental Services put together a comprehensive “Fact Sheet” on milfoil.  Check out the link on the left for answers to some common questions





1Aquatic Plants and Algae of New Hampshire’s Lakes and Ponds”, Amy P. Smagula And Jody Connor, New Hampshire Department of Environmental Services, 2007, www.des.nh.gov.

2 Environmental Fact Sheet WD-BB-23, “Variable milfoil”, New Hampshire Department of Environmental Services, 2010, www.des.nh.gov.

3Exotic Aquatic Plant Species in New Hampshire”, New Hampshire Department of Environmental Services, 2008, www.des.nh.gov.

4New Hampshire Water Resources Primer”, R-WD-08-23, Chapter 3, “Lakes and Ponds”, Thomas S. Burack, Commissioner, Michael J. Walls, Assistant Commissioner, Harry Stewart, P.E., Director, Water Division, New Hampshire Department of Environmental Services, December, 2008. 

5Total Maximum Daily Load for Flints Pond”, Prepared by AECOM, 171 Daniel Webster Hwy, Suite 11, Belmont, NH 03220, July 2009, Document Number: 09090-107-14.

6 Environmental Fact Sheet WD-BB-3”, Lake Eutrophication, New Hampshire Department of Environmental Services, 2010, www.des.nh.gov.

Illustrations courtesy of the New Hampshire Department of Environmental Resources

2011-01-08 DJC