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Submodule 3: Invasive Plants
Aquatic Plants
Aquatic plants destroy habitats in coastal waterways and interrupt the flow of water in inland desert irrigation canals. They clog storm canals leading to the flooding of homes and displace native species in our nation’s wetlands. Here is a partial list of some of the worst offenders:
- Hydrilla (Hydrilla verticillata): A freshwater, submersed invasive plant. It has infested almost all parts of the world except Antarctica and the Artic regions. Hydrilla can form large monocultures while pushing out native plants. Its mat-like growth habit clogs waterways and pipes causing millions of dollars for repairs and lost production, and making it difficult for watercraft to navigate infested waters. There were probably multiple introductions in the US.
- Eurasian water milfoil (Myriophyllum spicatum): Occurs primarily in the Great Lakes waterway, along the east coast, and from California to Washington along the Pacific coast; it has been identified in some inland locations such as Colorado and Idaho. It alters habitats by forming large mats on the water surface and blocking light to native plants. It can tolerate many water pollutants, which increases its invasiveness. It is thought that one major pathway for its introduction was from an aquarium. Milfoil is also a major cause of clogged waterways.
- Caulerpa (Caulerpa taxifolia): A green, marine invasive alga. Three fast-growing invasive species of Caulerpa have been identified in the US. It has a very high growth rate, can grow at relatively cool temperatures, and is more competitive for nutrients than native flora. Invasive Caulerpa can reproduce both sexual and asexually, increasing its rate of spread. The plant produces chemicals that may decrease grazing by marine animals.
- Giant salvinia (Salvinia molesta): A water fern that reproduces very rapidly – sometimes doubling its numbers in less than three days. Salvinia can easily reproduce by the detachment and regrowth of small vegetative fragments, which can easily happen when, for example, a boat travels through infested waters. It has been found in Hawaii and throughout the southeast states. Salvinia grows in large mats that can be several feet deep, blocking sunlight to native species. It was probably introduced into this country by the aquarium and landscape trades.
- Water hyacinth (Eichhornia crassipes): A floating, freshwater plant found throughout the southeast, and in California, Arizona, and Hawaii. Water hyacinth is a native of South America and was originally introduced to the US as an ornamental. The dense surface mats formed by the plant decrease sunlight to native plants and animals, and reduce available oxygen. It thrives in rich, warm waters and reproduces vegetatively; it can reproduce form seeds that can survive for over 20 years. Water hyacinth growth can clog waterways and restrict water navigation.
- Water chestnut (Trapa natans): A native of Eurasia, this plant is now found in mid- and northern east coast states. The plant, which produces edible fruit, makes navigation difficult by blocking waterways. Water chestnut growth restricts light penetration for native plants and causes oxygen depletion. Sharp spines on the fruit can cause pain and injury if stepped on.
- Purple loosestrife (Lythrum salicaria): Commonly infests freshwater and brackish wetlands. This plant is particularly destructive in wetland pastures where it replaces plants that are grazed by livestock and native animals. Purple loosestrife destroys hunting and recreational wetlands, again by replacing native vegetation. Reproduction occurs vegetatively as well as by the production of over 2 million seeds per plant. It has some economic benefits in that it is used for honey production and as an ornamental. The plant’s detriments are considered to far outweigh its benefits.
Hydrilla (photo courtesy NPS)
Caulerpa (photo courtesy NOAA)
Water chestnut (photo courtesy NAL)
Purple loosestrife (photo courtesy NPS)
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