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Invasive Freshwater Plants and Algae

Eurasian Watermilfoil

  • Scientific Name

    Myriophyllum spicatum - Visit ITIS for full scientific classification. 

  • Description


    • Submerged waterweeds form dense mats.
    • Stems commonly reach 13 (and can be 23) feet long.
    • Feathery leaves are 1" - 3" long and wide, usually in groups of 4 spaced along the stem.
    • Part of plant emerges from water to produce 1" - 3" long flowering spikes with small, pinkish, male and female flowers. 
  • Habitat
    • Usually found in still or slow-moving and sometimes in fast-moving, freshwater habitats. 
  • Invasion Pathways and Distribution
    • Possibly introduced as an ornamental aquarium plant.
    • Found mostly in temperate zone of Northern Hemisphere, but also in subtropical and tropical areas.
    • Occurs in central-western California, San Joaquin Valley, southern Sacramento Valley and Lake Tahoe.
    • Spread by stem pieces carried by boating and fishing activities, on feet of water fowl and mechanical harvesting intended to control it, or discarded from aquariums and ponds. 
  • Life History
    • Produces seeds but seedlngs are rarely seen.
    • Buds break off easily and sprout new plants. 
  • Impacts
    • Dense mats interfere with water flow, boating and other recreational activities.
    • They create mosquito habitat and crowd out native plants. 
  • References and Useful Links

    For references by category and links to other useful AIS sites see our LEARN MORE page.