Eurasian water-milfoil (EWM) is an introduced invasive aquatic plant that has been identified in Shawnigan Lake and has been here since the late 70s. EWM grows rapidly and can form densely tangled mats of vegetation near the water surface that shade out native aquatic plants. In addition to ecological and water quality impacts, dense growth of EWM can negatively impact recreational and aesthetic values.
In Shawnigan Lake, EWM is typically found along most of the shoreline in shallow water that is between 1m and 4m deep (3 to 13ft). This area represents approximately 13% of the Lake.
The recent increase in EWM may be the result of human introduced nutrients from erosion (forestry, soil dumping, etc.), leaking septic infrastructure, livestock and the application of fertilizer to ornamental planting (e.g. grass, flowers, shrubbery etc.), agricultural fields or pastures.
EWM is spread by fragmentation. The dispersal of seeds is considered to be of minor importance compared to fragmentation. Stem fragments are readily dispersed by water currents within lakes, and by recreational boat trailering between lakes. Stem fragmentation occurs naturally after flowering, and also occurs as a result of mechanical damage from recreational activities as well as from milfoil harvesting. Fragmented EWM in water temperatures consistently less than 10°C will decompose within a relatively short timeframe.
In order to reduce fragmentation of the plant, it is vitality important that users of the lake (a) do not operate boats in shallow water and (b) collect ALL of the plant fragments when trying to remove the plant.
Widespread management of EWM in Shawnigan Lake will not be implemented in the short term but concerned residents may wish to begin controlling the plant in priority areas around docks, boat ramps and designated swimming areas.
In addition to the traditional methods of controlling the plant, the Shawnigan Research Group is currently exploring the viability of a winter removal program as well. Hopefully this will increase the options available to those wishing to control the plant along their waterfront. However, removal activity is currently only permitted during the timing window (see instructions below.)
There are up to 10 native species of plants that appear similar to EWM and may be found in Shawnigan Lake. Native milfoil species can be distinguished from EWM by the arrangement of its leaves in whorls of four around reddish stems (or green in summer). Usually EWM has 14 to 21 leaflet pairs per leaf, which is more than native milfoils.
The images below were sourced from http://bomi.ou.edu/ben/428/myriophyllum_ben428.pdf The text is sourced from the Madrone Report, Appendix II.
INVASIVE Myriophyllum aquaticum (Vell.) Verdc. - Parrot's-Feather Syn.: M. brasiliense Camb., M. proserpinacoides Gillies ex Hook. & Arn. Introduced in North America from South America. Dioecious, only female plants known in North America. Easily recognized by thick stems, firm leaves and overall bright green colour. Winter buds absent.
NATIVE Myriophyllum hippuroides Nutt. ex Torr. & Gray - Western Water-Milfoil Occurs from California to the Lower Fraser Valley in British Columbia; there it forms dense stands in sloughs of the Fraser River and its tributaries. Easily recognized by deep green colour, dense foliage with additional numerous leaves outside the whorls. Winter buds absent.
NATIVE Myriophyllum sibiricum Komarov - Siberian Water-Milfoil Syn.: M. exalbescens Fern.; M. spicatum subsp.exalbescens (Fern.) Hultén Native to North America, northern Europe and eastern Asia, widespread in North America (except Texas and SE states). Stem whitish, leaves with smaller number of "untidy" segments. Winter buds frequent, cylindrical.
INVASIVE EWM Myriophyllum spicatum L. - Eurasian Water-Milfoil introduced and invasive. North American invasive populations are considered to be of a hybrid origin [M. spicatum x M. sibiricum] by Moody & Les (2002, 2007). Stem drying reddish. No winter buds.
The Shawnigan Residents Association has received permission from the Ministry of Forests, Lands, Natural Resource Operations and Rural Development (FLNRO) for our members to help control EWM in Shawnigan Lake this summer. Please follow these guidelines if you intend to remove the plant from your waterfront;