Winnipesaukee Environmental & Community Action Network

Milfoil Management

Geographic Area
Lakes Region
Variable milfoil (Myriophyllum heterophyllum) 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 spike­like flower above the waterline in late June or in July. A cross­section of the stem will reveal “pie­shaped” air chambers. 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. Where this species grows in its native environment, insects and fish may 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 growth in check. Under optimum temperature, light and nutrient conditions, milfoil may grow up to an inch per day. This exotic species of milfoil became established in Lake Winnipesaukee in 1965 in Moultonborough Bay, and the milfoil in this area is the longest standing infestation in New Hampshire. The plants throughout this area of the lake are mature and well-established, are known to form monocultures in many areas, and are generally widespread in others. In addition to well-established stands and mature root crowns, variable milfoil has flowered for a number of years in some areas of the lake, yielding a substantial seed stock in the lake substrate that could germinate and perpetuate growth for many years to come.
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