Family: Ceratophyllaceae

Ceratophyllum demersum, commonly known as common hornwort or coontail, is a free-floating aquatic plant with a nearly worldwide distribution, native to many regions of North America, Europe, Asia, and Africa. It has no true roots and drifts just below the water’s surface, anchored loosely by stems that tangle in soft sediments. Its dark green, finely divided leaves form dense, bushy whorls that resemble a raccoon’s tail. Common hornwort is often used in aquariums, ponds, and water gardens because it provides habitat for fish and invertebrates, offers shade that can reduce algae growth, and helps oxygenate the water.

Despite its popularity, common hornwort can pose ecological risks where it spreads beyond managed settings. It can grow rapidly and form thick underwater mats that crowd out native aquatic plants, alter water flow, and interfere with recreation such as swimming, fishing, and boating. These dense growths can also reduce oxygen levels when large amounts of plant material die back, negatively affecting fish and other aquatic life. Because it spreads easily through stem fragments and tolerates a wide range of conditions, careful use and proper disposal are important to prevent unintended spread into natural waterways.

High Risk Traits:

• Broad native distribution and climatic range
  
• Grows in temperate and tropical climates
  
• Naturalized on Maui, Hawaii, and Oahu
  
• An aquatic weed that can reduce stream flow, interfere with navigation, fishing and hydro-electric output
  
• Spiny fruit
  
• Can accumulate water toxins
  
• Can act as a host of disease-causing organisms
  
• Able to smother water surfaces
  
• Reproduces by seeds and vegetative fragments
  
• Self-compatible
  
• Able to reproduce in first year of growth
  
• Propagules dispersed by water and externally on fur, feathers, or feet of animals as well as by on clothing or water craft
  
• Sold as an aquarium or pond ornamental
  
• Mechanical control can result in inadvertent dispersal

Low Risk Traits:

• Palatable to fish and other aquatic animals
  
• Ornamental
  
• Does not produce prolific numbers of seeds (although able to spread vegetatively)
  
• Seeds presumably do not form a persistent seed bank
  
• Herbicides may provide effective control