Roberta Hill and Scott Williams
Maine Center for Invasive Aquatic Plants and the Maine Volunteer Lake Monitoring Program
Eurasian water-milfoil (EWM), scientifically known as Myriophyllum spicatum, is one of the world’s most notorious and troublesome invasive aquatic plants. Native to Europe and Asia, M. spicatum was first introduced into the waters of North America in the 1940s. It has been spreading here with a vengeance ever since. Eurasian water-milfoil has now been confirmed in nearly every state in the US and in most Canadian provinces.
An extremely aggressive and well-adapted plant, M. spicatum is able to thrive in a confounding array of environmental conditions. It grows well in both still and flowing waters, tolerates mild salinities and can survive under ice cover. The submersed plants grow rooted in water depths from 1 to 10 meters (over 30 feet), generally reaching for the surface in depths of 3 to 5 meters. It is adaptable to a wide variety of substrate types (though fine fine-textured, inorganic sediments seem to be favored).
Eurasian water-milfoil is a powerful competitor that prevails primarily by “outgrowing” its neighbors. The slender stems - many having wintered over, intact -have a significant head start early in the growing season, and may be well on their way to the surface by the time that native plants are just breaking dormancy in the early spring. Where M. spicatum plants do reach the surface they branch profusely, forming a dense floating mat (or canopy) that, according to some accounts, is “thick enough for a mouse to dance on.” The dense canopy typically forms early in the season before native plants are able to reach their optimum growth potential. In this way Eurasian water-milfoil effectively chokes and/or shades out all other plants in the area.
The resulting loss of biodiversity in lake and pond ecosystems cascades through the food web, causing losses in richness from the microscopic zooplankton to the top predator species. Well-established infestations often render waters unsuitable for boating, swimming, fishing, and other recreational uses, and can even interfere with power generation and irrigation by clogging water intakes. The excessive volume of plant material in a waterbody can cause flooding as well as a decline in water quality, as thick mats of vegetation restrict circulation, and decaying organic matter causes oxygen in the water to decline. The stagnant mats of vegetation can also provide a haven for mosquitoes.
It has been known for some time that this plant posed an imminent threat to Maine waters. It has been raising havoc just beyond our borders for decades. It is precisely for this reason that M. spicatum is one of the eleven plants on Maine’s official invasive aquatic plant “watch-list.” Finding this plant actually thriving here in our midst (see story page XXX) has provided new incentive to all of us to take a moment to learn how to spot this plant “in a crowd.” This way, when we are out doing what we love to do on the water, we will be better prepared to recognize a suspicious intruder. Early detection of an aquatic invader provides the best, and often the only, hope of eradication. At the very least, early detection provides an opportunity to manage an infestation in a way that minimizes “collateral damage” to lake ecosystems.
Like most other milfoils, Eurasian water-milfoil is a submersed aquatic plant with feather-like, finely divided leaves. The leaves of EWM are arranged around the stem in whorls (radiating like the spokes on a wheel). Each whorl typically containing 4 leaves. It should be noted that while the leaves of some milfoil species radiate around the stem, they do not appear in clearly defined whorls, but are instead scattered along the stem. This can be an aid in distinguishing EWM from some other milfoils.
EWM has a delicate habit, with the whorls of leaves spaced quite openly along supple stems, (generally with 1 to 3 centimeters between whorls). By comparison, the whorls of variable-leaf water-milfoil, the other invasive milfoil that is known to be present in Maine, are typically tightly packed (1-3 millimeters apart) along robust stems giving this plant its characteristic “bottle brush” appearance.
The leaves of EWM are generally elongate, sometimes (but not always) with a blunt, “snipped off” appearance at the tip. Each leaf is comprised of numerous (12 to 24) pairs of thread-like leaflets, arranged along the mid-vein like the barbs of a feather along the shaft. Since the leaves of most other milfoil species generally have less than 14 leaflet pairs, counting leaflets can be helpful in distinguishing EWM from other milfoil species. However, all leafy milfoils display a wide range of vegetative variability. In some cases Eurasian water-milfoil leaves may have as few as 5 leaflet pairs. Checking numerous leaves from different stems and different locations on the stems will increase the accuracy of the “leaflet count.” In many cases, other structures, such as the emergent flowering stalk, may be needed to confirm identification.
Eurasian water-milfoil flowers form on a spike that emerges from the water surface. The flowers are also arranged in whorls. The leaves that appear with the flowers, called bracts, are shaped like tiny blades and have smooth outer margins. The flowers are typically larger than the bracts, the male flowers occurring closer to the tip of the spike, the female flowers toward the base. Each female flower produces four tiny (2 – 3 mm) nutlike fruits. Flowering spikes may emerge from the water by early summer and may continue to occur for several months, but not all colonies produce flowers.
Eurasian water-milfoil is extremely hardy. It survives winter primarily as rootstalks, but whole plants and stem fragments also commonly survive through the winter. Unlike some other milfoils, EWM does not form winter buds—small compacted leafy structures that over-winter in the sediments. This plant reproduces extremely rapidly and can infest an entire lake within two years of introduction to the system. This suggests that some seeds are surely viable and that sexual reproduction in milfoils is not rare.
Like most of the leafy milfoils, EWM reproduces very successfully and rapidly through the formation of plant fragments. In the late summer and fall the plants become brittle and easily break apart. The fragments will float to other areas, sink, sprout roots and start new plants. Power boats and other activities that disturb the plants can cause fragmentation at any time during the open water period. This is why milfoils are so easily transported from one body of water to another on boat trailers or fishing gear. Once it has been introduced in a new lake or pond, water currents can move new fragments around, resulting in new colonies throughout the waterbody.
It is important to note that Maine is home to five different species of leafy native water-milfoils (as well as one “leafless” species). With all the vegetative variability within each species, and the potential for hybridization between species, distinguishing between a friend and a foe may not be easy. In addition to the other water-milfoils, Eurasian water-milfoil may be confused with other aquatic plants that are native to Maine, including water marigold, coontail, bladderwort, water crowfoot, and mermaid weed. To be on the safe side, all leafy milfoils found in Maine waters should be considered “suspect”, until a positive identification has been confirmed.
The discovery of Eurasian water-milfoil in Maine underscores the importance of vigilance and persistence in efforts to prevent our lakes and ponds from becoming choked with invaders. This discovery also makes it clear that invaders may not always first appear at “high risk” areas, such as public boat launch sites. State and other professional human and financial resources to address this threat are stretched over nearly 6,000 lakes and ponds throughout Maine. Volunteers who have been trained to conduct courtesy boat inspections and invasive plant screening surveys (Plant Patrollers) will play an increasingly important role in the process of protecting our waters from this environmental catastrophe.
To date over 1,250 volunteers have been trained by the Maine Center for Invasive Aquatic Plants (a branch of the Maine Volunteer Lake Monitoring Program) to survey Maine waters for the presence of invasive aquatic plants. Nearly all of the known infestations in Maine waters were first detected by alert and informed citizens. The Invasive Plant Patrol program is made possible with support from the Maine Department of Environmental Protection, and boater participation in the Maine Lake and River Protection Sticker program. For more information on how you can get involved in this important effort please contact the Maine Center for Invasive Aquatic Plants at 207-783-7733.
Be on the lookout for a new resource to help you distinguish “friend from foe.” Maine’s Virtual Herbarium is presently under construction and will be on-line soon with crisp photographs, drawings and descriptions of both invasive and native aquatic plants. Check www.MaineVolunteerLakeMonitors.org.