2008 Statewide Strategic Plan for Eurasian Watermilfoil in Idaho

B. Description
A number of milfoil species occur in the western United States, and many of these
species are similar in appearance. Eurasian watermilfoil closely resembles its
native relative northern milfoil (M. sibiricum), and was once thought to be a variety
of that species.
Eurasian watermilfoil is a rooted, perennial dicot that is submersed except for the
upper flower-bearing portions. The stem branches underwater and produces
whorls of 4 (rarely 5) leaves around the stem at each node. The leaves have a
grayish cast and feathery appearance. They are finely divided into leaflets, each
of which generally has 14 or more paired divisions. This feature can be used to
distinguish Eurasian watermilfoil from other milfoil species about 70 percent of the
time. However, the number of pairs of leaflet divisions is variable and can range
from 5 to 24,
Eurasian watermilfoil is one of several aquatic invasive weeds that reproduce
primarily by fragmentation. Viable propagules can be as small as a stem portion
carrying a single leaf node. Young plants and free-floating plant fragments often
develop leaflets with fewer than 14 divisions.
In Figure 5, the leaflet on the left is Eurasian
watermilfoil; on the right is northern milfoil, M.
sibiricum. Note the difference in leaflet shape,
number of divisions, and spacing of the divisions
between the two species. Although the two
species appear easy to distinguish in this
photograph, frequently these characteristics are
not as distinctive.
The growing stem tips of Eurasian watermilfoil
(and other milfoil species) are tassel-like and often
Figure 6. Eurasian watermilfoil in
flower
Figure 5. Eurasian watermilfoil
(left) and northern milfoil (right)
red; especially early in the growing season. Tiny
pinkish flowers occur on reddish spikes that stand
several inches above the water and submerge
when pollination is complete (Figure 6). The stem
width of Eurasian watermilfoil almost doubles
below the inflorescence. Lower flowers are
pistillate, upper flowers staminate. Seeds are
produced, but seedlings are rare in the field. In
situations where water evaporates slowly and the
plants become stranded gradually, Eurasian
watermilfoil can develop into a terrestrial form. The
leaves of the terrestrial form are smaller, stiffer,
and have fewer divisions. If these plants are
submerged, new aquatic leaves develop in 7-10
days, but the first leaves formed have relatively
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