Eurasia and Africa
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.
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.
IN THE UNITED STATES
Watermilfoil occurs in thirty-three
states east of the Mississippi River and has recently been found in
Colorado. It is abundant in the Chesapeake Bay, the tidal Potomac
River, and several Tennessee Valley reservoirs.
HABITAT IN THE UNITED STATES
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.
Eurasian watermilfoil was
accidentally introduced from Eurasia in the 1940s. Two theories exist as
to how it entered North America: (1) it escaped from an aquarium, or (2)
it was brought in attached to commercial or private boats. A resort
owner is thought to have introduced watermilfoil into the Tennessee
Valley Authority reservoir system in 1953.
BIOLOGY & SPREAD
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. Although seeds are
usually viable, they are not an important means of dispersal.
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 peak biomass is obtained (early summer). Substantial
regrowth may occur if this is done too early. Better results appear
with multiple harvests in the same growing season. If multiple harvests
are not possible, then sustaining annual harvests is an option. All
fragments of milfoil plants must be removed to achieve adequate control.
Where water levels are under manual control,
raising or lowering of the water can be an effective way to 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
Bankside plantings, floating native plant species,
light limiting dyes, or shade barriers are effective ways of reducing the
amount of light reaching the plants and may reduce overall growth rates. Barriers
can be used 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.
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.
USE PESTICIDES WISELY: ALWAYS READ THE ENTIRE PESTICIDE LABEL CAREFULLY, FOLLOW ALL MIXING AND APPLICATION INSTRUCTIONS AND WEAR ALL RECOMMENDED PERSONAL PROTECTIVE GEAR AND CLOTHING. CONTACT YOUR STATE DEPARTMENT OF AGRICULTURE FOR ANY ADDITIONAL PESTICIDE USE REQUIREMENTS, RESTRICTIONS OR RECOMMENDATIONS.
NOTICE: MENTION OF PESTICIDE PRODUCTS ON THIS WEB SITE DOES NOT CONSTITUTE ENDORSEMENT OF ANY MATERIAL.
For more information on
the management of Eurasian watermilfoil, please contact:
SUGGESTED ALTERNATIVE PLANTS
(Nelumbo lutea), pond weed (Potamogeton nodosus), butterweed
(Senecio glabellus) are some alternative plants to consider for the
Tom Remaley, Great Smoky Mountains National
Park, Gatlinburg, TN
Jil M. Swearingen, National Park Service,
Alison A. Dalsimer, Consultant, Legacy Resource
Management Program, Washington, DC
Barry A. Rice, The Nature Conservancy, Davis,
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.