Eurasian Watermilfoil (Myriophyllum spicatum)

Water-milfoil family (Haloragaceae)

Native Range  

Eurasia and Africa

Description

Eurasian watermilfoil, also called spike watermilfoil, is an emergent, herbaceous aquatic plant. Stems grow to the water surface, usually extending 3 to 10, but as much as 33, feet in length and frequently forming dense mats. Stems of Eurasian milfoil are long, slender, branching, hairless, and become leafless toward the base. New plants may emerge from each node (joint) on a stem, and root upon contact with mud. The grayish-green leaves of Eurasian watermilfoil are finely divided and occur in whorls of three or four along the stem, with 12-16 pairs of fine, thin leaflets about 1/2 inch long. These leaflets give milfoil a feathery appearance that is a distinguishing feature of the plant. Eurasian watermilfoil produces small yellow, 4-parted flowers on a spike that projects 2-4 inches above the water surface. The fruit is a hard, segmented capsule containing four seeds.

Look-alikes

Milfoil may be confused with bladderworts, hornworts, mermaid weeds, water crowfoots, and other leafy watermilfoils. There are a number of native milfoils that look very much like Eurasian milfoil. Make sure your identification is correct before starting control.

Ecological Threat

Eurasian milfoil can form large, floating mats of vegetation on the surface of lakes, rivers, and other water bodies, preventing light penetration for native aquatic plants and impeding water traffic. The plant thrives in areas that have been subjected to various kinds of natural and manmade disturbance.

Distribution and Background

Eurasian watermilfoil was accidentally introduced from Eurasia in the 1940s. Two theories exist as to how it entered North America – it escaped from an aquarium, or was brought in attached to commercial or private boats.

Typical habitat for Eurasian watermilfoil includes fresh to brackish water of fish ponds, lakes, slow-moving streams, reservoirs, estuaries, and canals. It is tolerant of many water pollutants. Eurasian watermilfoil tends to invade disturbed areas where native plants cannot adapt to the alteration. It does not spread rapidly into undisturbed areas where native plants are well established. By altering waterways, humans have created a new and unnatural niche where milfoil thrives.

Alternative Plants (Alternative Native Species)

There are a considerable number of native pondweeds such Elodea canadensis and E. nuttallii.

Whenever possible, use as alternatives plant species that are native and adapted to the ecological region where you live. They will be more valuable to the wildlife species that have evolved with them and depend upon them for food and shelter. Check with your local native plant society for recommendations and sources of native plants.

Biology and Spread

Most regeneration of Eurasian watermilfoil is from rhizomes, fragmented stems, and axillary buds that develop throughout the year. Flower spikes often remain above water until pollination is complete, then resubmerge. Seeds are not an important means of dispersal.

Management Options

Large harvesting equipment can be used to mechanically remove milfoil in larger areas; a sturdy hand-rake can be used for smaller areas. Other available options include manipulation of water level, light penetration and chemical control. Potential impacts to existing native aquatic plant species should be evaluated carefully before implementing any of these techniques. For the single harvest, removal should take place just before the population peaks in early summer. Substantial re-growth may occur if control is implemented too early. Better results appear with multiple harvests in the same growing season. If multiple harvests are not possible, then regular annual harvests over a period of years is an option. All fragments of milfoil plants must be removed to achieve adequate control.

For disposal options, refer to disposal options page.

Plant Control

Manual and chemical control methods are effective in removing and killing milfoil.

Biological

There are no biological controls available in the United States for this plant.

Manual and Mechanical

Raising or lowering the water level can reduce the growth of milfoil. By raising the water level, plants can be "drowned" by not having access to enough light. By lowering the water level, plants can be dehydrated and, at the right time of the year, frozen to death. This type of control is best used in conjunction with herbicides and shade barriers.

Bankside plantings, floating native plant species, light limiting dyes, or shade barriers are effective in reducing the amount of light reaching the plants and reducing the overall growth rates. Use barriers to prevent the movement and spread of aquatic weeds in ponds and lakes. A barrier is usually a suspended blocking screen that hangs vertically from a cable to a depth of about 4 meters; the cable is suspended by drum floats.

Chemical

Fluridone (the active ingredient in Sonar AS) is a selective herbicide for milfoil and several other exotic aquatic weeds. There are no restrictions on swimming, fishing, or drinking after application, and season-long control can be achieved with one application. Fluridone is available in liquid or granular form, and can be used as a spot treatment or on an entire waterway. For best results, applications should be made before or during the early stages of active growth.

Caution:  Any activities in aquatic systems (from removing invasives by hand or by applying herbicides) may require a special permit under your local bylaws. Be sure to contact your local Conservation Commission before you act.

Notice: Mention Of Pesticide Products On This Web Site Does Not Constitute Endorsement Of Any Material.

 

 

More Information

For more information on invasive species in Massachusetts, refer to the Massachusetts Prohibited Plant List at http://www.mass.gov/agr/farmproducts/prohibitedplantlist.htm.

Author

Tom Remaley, Great Smoky Mountains National Park, Gatlinburg, TN.

References

B.C. Ministry of Environment 1989. Eurasian Water Milfoil in British Columbia (Pamphlet).

Gleason, H.A., A. Cronquist. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. 2nd ed. The New York Botanical Garden, 910.

Swearingen, J. 2009. WeedUS Database of Plants Invading Natural Areas in the United States: Eurasian Watermilfoil (Myriophyllum spicatum).  http://www.invasive.org/weedus/subject.html?sub=3055

USDA, NRCS. 2009. The PLANTS Database (http://plants.usda.gov). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.

Plant Conservation Alliance, Alien Plant Working Group http://www.nps.gov/plants/alien/fact/mysp1.htm