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Milfoil madness

September 20, 2011
By HILARY SMITH ( , Adirondack Park Invasive Plant Program

As I thought about what to feature this week, I realized that I have not yet written about one of the most high-profile invaders in the region - milfoil. This is likely because milfoil receives so much attention through press releases, news coverage, signage, fact sheets, brochures, etc. But the fact remains that many may still not know about milfoil or, most importantly, how to tell milfoils apart from the many other aquatic plants in our waterways.

First, not everything green in the water is milfoil. Many Adirondack waters have healthy populations of native aquatic vegetation, which oxygenates the water and provides food and habitat for fish and wildlife.

Second, not every Adirondack lake is infested with milfoil. Lake associations and volunteers are tracking where milfoil and other aquatic invasive plants are and where they aren't. Surveys show that, in fact, three out of every four waters checked are free of invasive aquatic plants. That's great news.

Article Photos

Eurasian watermilfoil has finely divided leaves like a feather that whorl around the stem.
(Photo — Adirondack Park Invasive Plant Program)

Now for the tricky part: identification. Distinguishing milfoil would be easier if there was only one species, but there are at least five different native milfoils in the region (northern, Farwell's, verticillatum, humile and little) and three invasive milfoils, two of which are here (Eurasian and variable-leaf) and another that is on the move downstate (parrot feather). This makes proper identification critical since management efforts focus on the invasive milfoils.

Nearly all of the milfoils, native or invasive, share some similar characteristics. They are rooted plants that often reach the water's surface. Their leaves are finely divided like a feather and either whorl, alternate or both whorl and alternate around the stem. Plants grow in a columnar shape in the water, similar to a bottle-brush in appearance. Some also produce flower spikes that resemble a pipe cleaner sticking out of the water.

Milfoils are perennial, so they return each year from roots in the sediment. They also spread through fragmentation, and fragments are often seen floating in the water. Most milfoils tend to grow thickly in clumps; however, the invasive milfoils are not native to the Adirondacks and, with no predators to keep populations in check, grow uncontrollably. Invasive milfoils survive in a range of environmental conditions, spread easily and quickly form expansive monocultures and dense tangled mats that degrade the environment and recreational opportunities.

First introduced in the 1940s through either ballast water or aquarium dumping, Eurasian watermilfoil is one of the most widespread aquatic invaders in the U.S. and is in at least 50 Adirondack waters. Variable-leaf milfoil is a more recent newcomer to the invasive species arena. It is actually native to the Midwest but listed as a non-native invasive plant in New England. Its date of origin in New York is unknown, but it was present in at least 20 waters in the region so it was on a "watched" species list. When a new population was discovered in Lake Placid in 2009, resource managers advised including variable-leaf as an invasive species, too.

Eurasian watermilfoil, I think, is easier to distinguish from native milfoils than is variable-leaf. Eurasian has a whorl of four feathery leaves which are often bright green and whose ends each have a blunt top, as though clipped. It often grows with more space between each whorl and sometimes has a reddish stem or growing tip. It grows horizontally when it reaches the water's surface. Also, if you pull the plant out of the water, the leaves collapse around the stem.

As the name suggests, variable-leaf watermilfoil is variable in appearance, and its identification has stumped many botanists. Sometimes four to six leaves whorl around the stem; sometimes they both whorl and alternate. Its leaves are more densely packed together, so it appears bushier than Eurasian but very similar to native milfoils. To further complicate things, research now shows that variable-leaf hybridizes with several native milfoils. One of the best ways to identify the plant is by its flower - a spike about as thick as a pencil that grows upright out of the water. Without the flower, identification can be so deceiving that scientists may need to send a specimen out for genetic testing.

You are critical to helping stop the spread of invasive species. Take notice of the plants around you. If you see a suspicious plant, take a good photo and email it to Or if you can, collect a sample of the entire plant, float it in a bucket, and call APIPP at 518-576-2082.


"Eye on Invasives" is a biweekly column that spotlights a top invader when it is easiest to identify. The Adirondack Park Invasive Plant Program is a partnership program housed at the Adirondack Chapter of The Nature Conservancy in Keene Valley. Find out more about this award-winning program online at



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