Eurasian watermilfoil

Myriophyllum spicatum L.

Description from Invasive.org

Appearance: Myriophyllum spicatum is a submersed aquatic plant that invades lakes, ponds, and other aquatic environments throughout the United States. The plants are rooted and the stems grow up to the water surface, usually reaching 3-10 ft. (0.9-3 m) in length and can be as much as 30 ft. (9.1 m) long.
Foliage: Leaves are bright green, finely dissected, and whorled. The delicate leaflets give this plant a feathery appearance.
Flowers: Myriophyllum spicatum has both male and female flowers on the same inflorescence. The female flowers are basal while the male flowers are located distally. The female flowers have a 4-lobed pistil and lack sepals and petals. The male flowers have 4 pink petals and 8 stamens.
Fruit: The very small globular fruit of Myriophyllum spicatum are indehiscent, and contain 4 seeds.
Ecological Threat: Myriophyllum spicatum requires stagnant to slowly moving water and can tolerate brackish conditions. It forms dense mats of leaves restricting light availability, leading to a decline in the diversity and abundance of native macrophytes. In addition, Myriophyllum spicatum displaces the native species of watermilfoil and reduces habitats for fish spawning and feeding. It is native to Europe, Asia, and northern Africa.

Resources

Biological Control of Invasive Plants in the Eastern United States - USDA Forest Service
Southeast Exotic Pest Plant Council Invasive Plant Manual - SE-EPPC
Plant Invaders of Mid-Atlantic Natural Areas - National Park Service and U.S. Fish and Wildlife Service
Weeds Gone Wild: Alien Plant Invaders of Natural Areas - Plant Conservation Alliance
Global Invasive Species Database - Invasive Species Specialist Group
Images, Video and Information - University of Florida - Center for Aquatic and Invasive Plants
Fact Sheet - Ontarios Invading Species Awareness Program

Selected Images from Invasive.org

View All Images at Invasive.org

Infestation
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
Additional Resolutions & Image Usage

Plant(s) Native to Africa, and Europe, invades lakes, ponds, and other water bodies throughout the United States.
Alison Fox, University of Florida, Bugwood.org
Additional Resolutions & Image Usage

Flower(s) Inflorescence
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
Additional Resolutions & Image Usage

Foliage
Graves Lovell, Alabama Department of Conservation and Natural Resources, Bugwood.org
Additional Resolutions & Image Usage

Plant(s)
Graves Lovell, Alabama Department of Conservation and Natural Resources, Bugwood.org
Additional Resolutions & Image Usage

Plant(s)
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
Additional Resolutions & Image Usage

Plant(s) a submersed aquatic plant at Cornell University Research Ponds
Robert L. Johnson, Cornell University, Bugwood.org
Additional Resolutions & Image Usage

Seed(s)
Steve Hurst, USDA NRCS PLANTS Database, Bugwood.org
Additional Resolutions & Image Usage