With the summer vacation season right around the corner, folks are getting ready to launch their motor boats, kayaks, and canoes, excited to enjoy a day on the lake.  They may be unaware that they could be transporting hitchhikers.  Not human hitchhikers, but the green, leafy kind.

Meet Eurasian watermilfoil (Myriophyllum spicatum L.) and variable-leaf milfoil (Myriophyllum heterophyllum), two perennial, submerged aquatic invasive plants that pose a threat to Adirondack lakes.  They can hitchhike from lake to lake on boat trailers, motors, or on canoe and kayak deck rigging.  These plants can degrade aquatic ecosystems, impair recreation, and are expensive to manage.

Eurasian watermilfoil is native to Asia and Europe, and was introduced to New York State in the 1940s.  Variable-leaf milfoil is native to some parts of the United States, but due to its aggressive nature in Adirondack water bodies, has recently been upgraded from a watch species to an invasive species by the Adirondack Park Invasive Plant Program (APIPP).  Without the ecological checks and balances found on their home turf, these invaders can quickly spread to form monotypic populations.

Both species prefer to colonize slow moving rivers as well as the shallow areas of lakes and ponds.  Spread occurs mainly by fragmentation.  Bits and pieces of plants that collect on boats, trailers, and gear may be transferred to new locations where they take root.  Plants may auto-fragment where abscising vegetation develops roots, falls off the main plant, and takes root to produce a new plant.  Water movement also spreads these invaders.

Variable-leaf milfoil and Eurasian watermilfoil are undesirable for a number of reasons.  Because they tolerate a wide range of environmental conditions, they quickly become established.  Their growing season is jump-started early, allowing them to gain headway on native plants.  As invasive milfoil populations increase, native vegetation is outcompeted for growing space, light, and nutrients.  These milfoils congest waterways, making fishing, swimming, paddling and boating unenjoyable.  Valuable habitat for fish, waterfowl, and invertebrates is lost.  Finally, these plants devalue shoreline property and provide a suburb breeding ground for mosquitoes.

It can be difficult to tell invasive and native milfoils apart, but some key diagnostic characteristics are useful for identification.  Eurasian watermilfoil displays branched underwater stems and forms dense surface mats.   Stems may be pink, red, or brown in color.  Feather like leaves are usually composed of greater than 11 leaflets.  The leaf arrangement is whorled, with 3-5 leaves per whorl.  Leaves display a blunt tip, as if someone snipped off the end with scissors.  Tiny reddish flowers bloom in July or August on a flower stalk that emerges above the water’s surface.  Internodal spacing is large.  When held upside down out of the water, leaves droop and collapse around the stem.  In the Adirondack region, populations have been confirmed in Clinton, Franklin, Hamilton, Essex, Warren, Saratoga, and Fulton counties.

Variable-leaf milfoil, also known as bottle brush, displays tubular growth.  Stems are brown, green, or red in color.  Feather-like leaves may be whorled or alternate.  Usually greater than 10 leaflets compose each leaf.  Surface stems form a thick, submerged canopy.  Internodal spacing is small.  In the Adirondacks, populations have been noted in Hamilton and St. Lawrence counties.

The costs associated with managing aquatic invasive plants are expensive.  According to the United States Geological Survey, controlling Eurasian watermilfoil costs the United States millions, while New York State spends approximately $500,000 annually.  Aquatic invasive plants are managed by physical, mechanical, or chemical techniques, according to the characteristics of the infested site.  Herbicides, insects, carp, pathogens, hand harvesting, benthic barriers, and underwater rototilling are examples of management methods for invasive milfoils.

You can help stop the spread of aquatic invaders.  Check your boat, trailer, and gear for plants, animals, and mud and remove at the boat launch.  Drain all water from water craft and bait containers.  Monitor your lake for aquatic invasive plants.  For more information, visit the Hamilton County Soil and Water Conservation District’s website at www.hamiltoncountyswcd.com/ or APIPP’s website at www.adkinvasives.com/.

Photos: Above, Eurasian watermilfoil with feathery leaves and large internodal spacing; Middle, the blunt-tipped leaf of Eurasian watermilfoil; Below, Variable-leaf milfoil displays a brushy, tubular appearance.