Non-native to Chesapeake Bay; invasive
- Family - Haloragaceae
- Distribution - Introduced from Europe and Asia, Eurasian
watermilfoil is now found throughout the United States. Explosive growth of
Eurasian watermilfoil during the late 1950's covered large areas of the
Chesapeake Bay and its tidal tributaries. Eurasian watermilfoil choked waterways
until the epidemic came to an end in the early 1960's, possibly due to spread of
a virus-like organism in combination with pollution, grazing, and herbicide and
harvesting programs. Eurasian watermilfoil is still present in the Chesapeake
Bay and its tributaries today, inhabiting non-tidal fresh to moderately brackish
tidal water and preferring soft mud to sandy mud substrates in slow moving
streams or protected waters. Eurasian watermilfoil does not tolerate strong
current or wave action. It is often the first species to appear in the spring in
tidal tributaries with fairly degraded water quality and may be followed by
other native species.
- Recognition - Up to 2.5 m (9 ft) tall, leaves in whorls
of 4 or 5, finely divided (pinnate), 0.8 cm to 4.5 cm (1/3 in to 2 in) long with
9 to 13 hair-like segments per side. When removed from water these delicate
leaves compress and lose their shape. Lower portions of the stems may be devoid
- Ecological Significance - Eurasian watermilfoil was introduced
to the Chesapeake Bay in the early 1900's. In the early 1960's, the population
exploded in the Bay and could be found in almost all the tributaries. By 1970
the populations had died back and stabilized. This plant, while not considered a
great food source for waterfowl, provides excellent cover for young fish, crabs
and invertebrates. Fishermen recognize watermilfoil beds as excellent places to
catch large mouth bass, which are often found lying in ambush near and amongst
the stems of watermilfoil plants.
- Similar Species - Eurasian watermilfoil usually has whorls of 4
pinnate leaves whereas parrot feather (Myriophyllum
brasiliense) usually has whorls of 5 pinnate leaves. Appearance is similar
to coontail (Ceratophyllum demersum),
however, coontail has whorls of 9 to 10 leaves at stem nodes, has stiffer leaves
(especially when taken out of the water), and lacks a root system.
- Reproduction - During late summer watermilfoil grows flower
spikes on stem tips that protrude above the water surface. Self-pollination does
not occur because the pistillate flowers on each individual reach maturity
before its staminate flowers. Aerial pollination produces nut-like fruits that
sink to the bottom where they can remain viable for years. Asexual reproduction
occurs by fragmentation.
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