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Milfoil is a submerged aquatic plant with fine, densely packed, feather-like leaves whorled around a main stem.  It can grow up to 15 feet and may exhibit a three- to six-inch green spike-like flower above the waterline in late June or in July.

A cross-section of the stem will reveal “pie-shaped” air chambers. This exotic species of milfoil has been in the state since the late 1960s, and can currently be found in over 75 waterbodies in New Hampshire.  There are six native milfoil species present in the state that do not cause problems, as they are low growing and do not form monocultures, unlike variable milfoil.  Eurasian milfoil is another non -native milfoil found in New Hampshire, but it is less of a threat than variable milfoil due to our water chemistry (Eurasian milfoil tends to be found in waters with higher pH ranges than those found in

New Hampshire)

Why is Milfoil Considered a Nuisance Species? 


This species is not native to our state and is very difficult to control once it becomes fully established. Milfoil reproduces through fragmentation, whereby plant fragments break off from the parent plant through wind or boat action, grow roots, and settle in a new location.  Seeds are also a means of spread within an infested waterbody.  Milfoil spreads rapidly and displaces beneficial, native plant life, often forming monoculture of growth around the shallows of a waterbody. It makes swimming difficult and can devalue waterfront property. In its native environment, insects and fish feed on this plant at such a rate as to control its growth. In New Hampshire, variable milfoil has no abundance of natural predators to keep its population in check. Under optimum temperature, light and nutrient conditions, milfoil may grow up to an inch per day.

How Did Exotic Milfoil Become Established in This State?

Most likely, a “stowaway” fragment of variable milfoil made its way to New Hampshire attached to a boat or trailer

from another waterbody out of the state.  Milfoil can live out of water for many hours if it remains moist, like when it’s wound around a wet carpeted bunk on a boat trailer or in a live well. Milfoil is usually first found near boat launch sites when it infests a new waterbody, a sure sign that transient boaters are the leading means of spread.  Another theory is that milfoil was introduced to a New Hampshire waterbody through the dumping of a home aquarium.  This plant has sometimes been used as an ornamental plant in aquariums.

Once Established, How Does Milfoil Infest Other Areas of a Waterbody?

Boat propellers chop milfoil plants into small fragments. These fragments float on the surface and are at the mercy of the wind and lake currents. In a short time, roots form on these fragments. If washed ashore, these plants eventually take hold, creating a new colony of milfoil. The cycle goes on until every suitable area is filled in with these weeds. An alternative form of the plants develops during low water. This vegetation type is more succulent than the submersed form and can persist for moderate periods of low water.

NH DES has recently collaborated in a study to evaluate the viability of milfoil seeds and the research showed that milfoil seeds are very viable and have a high regeneration rate, though survival of the seedlings is thought to be relatively low.

Regardless, seed production in a dense milfoil bed is high, and seeds are thought to be a probable source of new plants, even following extensive control measures (seeds are resistant to herbicides).  Data suggest that long-term monitoring and appropriate follow -up activities are needed to truly reduce or potentially eradicate infestations.  Regular surveys by

NH DES biologists or volunteer Weed Watchers are needed to find new growth early, so that small-scale control measures can address the problem before it spreads. It is unlikely that seeds are responsible for lake -to-lake spread; fragments are still the primary cause of that problem.


What Methods Are Currently Being Used to Control Milfoil?

NH DES implements an integrated plant management (IPM) approach for control. Each infestation is assessed and a long-

term management plan is prepared to guide control activities for a number of years.  Waterbody-specific goals range from reduction of the infestation to control to possible eradication, depending on the status of the infestation and characteristics of the waterbody.  All available control options are considered and actions are chosen that best suit the size, density and character of the infestation. Hand -pulling, diver -assisted suction harvesting, benthic barrier placement, herbicide treatment and other strategies are evaluated for each infestation, including a review of a ‘no control’ option , and often a combination of approaches is recommended.

Have Chemicals Been Used to Effectively Control Exotic Milfoil?

Yes.  NH DES has collaborated on a number of research projects focusing on chemical control of milfoil.  Through that research , 12 aquatic herbicides were evaluated and results indicate which products work best on milfoil, with limited

impacts on non -target species. Used at lower concentrations to target milfoil, native aquatic plants and other aquatic life are not significantly impacted by an herbicide. 


Herbicide treatment is a science, and therefore should be conducted by trained professionals . It is illegal to apply chemical herbicides to any New Hampshire waters unless you contract with a licensed applicator.  The use of chemicals by an untrained person could jeopardize the health and welfare of the lake and its ecology.  Inappropriate or inaccurate use of chemicals is life- threatening to people, mainly due to over -dosing as a result of the unwise “more is better” approach.  It should be noted that the state has been conducting herbicide applications under permit and through licensed applicators

for several years, and no negative impacts to non- target plants, animals or humans have been observed.

For more information on milfoil or other Exotic Species, please contact the Exotic Species Coordinator at (603)271-2248 or Also, visit the NH DES website for more information about exotic species at


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