If a ‘top ten’ list of aquatic invasive plants existed, Eurasian watermilfoil (Myriophyllum spicatum) would top the list of non-native water plants that pose the greatest threat to Georgian Bay and its watershed.Native to Europe, Asia, and northern Africa, Eurasian watermilfoil (EWM) was first discovered in a Washington, DC pond in 1942. EWM has since spread throughout the continental United States. The invasive plant is present in the Canadian waters of British Columbia, Manitoba, Ontario, and Quebec. The 2018 University of Georgia Early Detection and Distribution Mapping System has identified 72 sites along the Georgian Bay shoreline where EWM has been observed. However, it is believed that many more sites exist between Bayfield Inlet and Collingwood Harbour due to the rapid growth and spread of the plant species and the lack of knowledgeable observers.
What does EWM look like?EWM is an aggressive rooted aquatic perennial plant. It has delicate, feathery leaves. The leaves are mostly the same length; the tips have a snipped off appearance. Leaves are arranged in whorls (circles) of three to five leaves around each stem. The stem is thick or thicker than a pencil, and is long and spaghetti like in appearance. Stems typically grow .9 – 3 metres (3 -10 feet) in length. Some EWM plants can grow to 9 metres (30 feet) in length with the majority of the plant branching at the surface.Due to the similarity between EWM and two other milfoil species…Northern Milfoil (a native aquatic plant) and Hybrid Milfoil (a cross between EWM and Northern Milfoil), verification of EWM should be undertaken by a biologist or an ecologist.
How does it grow and spread?EWM grows quickly and spreads easily. Studies have shown EWM to grow 5 - 7 cm (1 – 2.75 in) per day in ideal growing conditions. It grows in depths up to 8 metres (26 feet) of water…deeper depths if the water is clear. EWM grows in any type of aquatic substrate; however, it prefers sand and silt substrates. EWM has the ability to grow over a broad range of temperatures (15⁰ C - 35⁰ C).Although EWM produces seeds, the plant primarily spreads through its root extension (runners) and through plant fragmentation. Fragmentation can occur by breakage from wind and wave action, contact by waterfowl, animals, and fish as well as human activity such as boating, angling, and swimming. EWM also ‘auto-fragments’ towards the end of its growing cycle; stem and leaf parts are naturally separated and dispersed. Localized spread is primarily through its root extensions, intermediate distance spread is by plant fragments, and long distance spread between water bodies is primarily through the transport vector of PWCs, boat motors, boat trailers, fishing gear and bait buckets. Each plant fragment is capable of growing roots and developing into a new plant.An example of the spread capability of EWM is a 36 island archipelago in Lake Huron known as Les Cheneaux Islands. A small 250 square foot patch of EWM was observed to grow to 400 acres over a six year period.
The ugly facts…why EWM is a concern.Some of the problems associated with EWM include:
- Recreational activities such as boating, fishing, and swimming are impeded by dense growth at or near the surface.
- Water related tourism, hospitality, and retail services such as marinas can be impacted.
- Shoreline property values can be de-valued. EWM choked water bodies can depress real estate values up to 20%.
- The water body’s ecosystem will be altered. EWM will displace important native plants thus harming fish and wildlife habitat. Dense floating vegetation becomes breeding habitat for mosquitoes.
- Stagnant oxygen-depleted conditions are often found in association with dense beds of EWM. Fish mortality may occur and degraded water quality could contribute to algae blooms.
- Costs to manage EWM growth are borne by local citizens and businesses, lake management organizations, and/or local governments.