Weed of the Month: Eurasian Watermilfoil
Eurasian Watermilfoil is an aquatic invasive plant that’s considered a priority one species in Teton county. This means that there is no known infestation and it is a considerable concern of ours. It spreads locally by rhizomes, stem fragments, and seeds, and can be spread to other bodies of water as vegetative material attached to watercraft and equipment.
This invasive aquatic plant is native to Europe, Asia, and North Africa and was likely accidentally introduced to North America in the late-1800s or early-1900s. Although its exact origin of spread is unknown, researchers believe that it was unintentionally introduced through the aquarium trade or on watercraft.
Eurasian Watermilfoil Identification
Eurasian watermilfoil is a perennial, primarily submerged mat forming aquatic plant in the Haloragaceae (watermilfoil) family.
Leaves are finely dissected with 12 to 21 leaflet pairs giving the plant a feathery appearance. Leaflets are ½ to 2 inches long. Leaves are arranged in whorls of 3 - 6 leaves on stems. Leaves are grayish-green, and stems may be white, pink, brown, or reddish-brown.
Stems may be 3 to 12 feet long and are branched near the water surface. Stems are easily fragmented.
Eurasian watermilfoil has both male and female flowers. Flowers are very small and may be pink, yellow, or red. Flowers occur in small spikes that emerge 2 to 4 inches above the water surface. Male flowers have four petals; female flowers lack both petals and sepals.
Eurasian watermilfoil produces seeds in segmented capsules each containing four seeds.
Roots are an extensive network of rhizomes.
Eurasian Watermilfoil Control
We’ll say it once, and we’ll say it again—prevention is the best control! Aquatic invasives are difficult to quickly identify and easily control. Once an aquatic plant is established, it can be unknowingly transported by recreators. Plant material can hitch a ride on surf boats, pontoon boats, drift boats, paddleboards, kayaks, paddles, life vests, water shoes, and more. Plus, it could hitch a ride on your fishing gear if it’s not properly checked and drained after each use. Don’t forget to inspect and clean your boat trailers as well!
If eurasian watermilfoil is already established, you can attempt cultural control strategies to minimize its spread. Cultural control strategies include maintaining a healthy nutrient balance by reducing runoff from lawns and pastures, but these strategies are not always viable in natural systems.
You can rake, cut, or pull by hand to reduce small stands of Eurasian watermilfoil. All fragments must be removed to prevent reestablishment. Mechanical control can eradicate eurasian watermilfoil in small, isolated ponds but probably wouldn’t be effective in a large body of water like Jackson Lake.
Dense infestations can be managed with herbicides. Because of the complexity of using herbicides in aquatic systems, TCWP recommends that you contact our office or Wyoming Game and Fish before conducting aquatic treatments.
Why Eurasian Watermilfoil is a concern in Teton County — and what you can do about it
Eurasian Watermilfoil is a deceptively delicate and fragile looking plant. It can be easy to overlook if it’s in a nearby pond or a lake that you typically hike past in the summer. However, this plant threatens biodiversity in Teton county because it forms thick mats in shallow areas, grows quickly, and blocks sunlight —killing off native fish, aquatic plants, and other underwater species.
Another reason why we’re desperately trying to keep it out of our wetlands is that Wyoming is the only state where Eurasian watermilfoil has never been found. We’re surrounded by states with known infestations, yet we’ve been able to prevent it so far. So, help us keep our waterways safe this summer. CleanDrainDry!
If you see an aquatic plant that matches the description of Eurasian watermilfoil, go to our website and fill out our simple plant identification form and submit a photo of it. We can verify if it is in fact Eurasian watermilfoil or a native lookalike.