Invasives Watch

The Four Worst Invasive Aquatic Species currently on Lakes of the Great Raft

  • Giant Salvinia (Salvinia molesta)
    and the related Common Salvinia (S. minima)

  • Giant Salvinia
    Photo: Caddo Lake Institute

    Giant salvinia is an invasive, aquatic, floating fern native to Brazil that is considered one of the world’s worst weeds. It was first found in the US in South Carolina in 1995, and is now found throughout the southeast and portions of the southwestern United States. Giant salvinia shades out native aquatic vegetation, reduces dissolved oxygen levels leading to fish kills, hinders recreational boating and fishing, and obstructs water and power intake structures.

    Giant salvinia varies in color from green to gold to brown with floating leaves in pairs about ½ inch (in) wide and 1 in long. The upper portion of the leaf is covered in dense, white hairs joined at the tip in an eggbeater shape that repels water.

    This species is very prolific with individual plants growing together to form dense mats that can form layers of vegetation up to 2 feet thick. Under favorable conditions, salvinia mats can double in size in 1-2 weeks primarily through vegetative budding from attached nodes or stem fragments.

    Giant salvinia was first reported in Texas in 1997. It reached Lake Bistineau and Caddo Lake in 2006 and is now infesting nearly all of the Lakes of the Great Raft. Its spread is being controlled by the use of herbicides and through bio-control by the weevil Cyrtobagous salviniae.

  • Water Hyacynth (Eichhornia crassipes)

  • Water Hyacynth
    Photo: University of Florida

    Water hyacinth, a free-floating aquatic plant from South America that was introduced in the US in 1884 at New Orleans and is now naturalized over the southern US. Water hyacinth can form dense mats of vegetation that severely impact aquatic ecosystems by affecting boating access, irrigation canals, preventing swimming and fishing, lowering oxygen levels, and lowering sunlight to submerged vegetation.

    Hyacinth can grow up to 3 feet ft. high with thick, shiny, bright green, oval to elliptical leaves from 1-5 in wide and attached to a spongy, inflated petiole. It has showy lavender flowers with 6 petals, and a central lobe with a yellow spot.

    Water hyacinth reproduces mostly by vegetative propagation, but also produces viable seeds that help in colonization. This species now infests most, if not all, of the Lakes of the Great Raft. Control is by herbicides, harvesting, and biological means.

  • Alligatorweed (Alternanthera philoxeroides)

  • Alligator Weed
    Photo: University of Florida

    Alligatorweed is a perennial, emergent or rooted floating invasive that occurs throughout the southern US. It is a native in South America and was introduced in the US around 1900. This species forms thick mats that compete with native vegetation, retards water flow and increases sedimentation, and lowers oxygen levels.

    Alligatorweed has hollow stems with opposite, lance-shaped leaves up to ¾ in wide and 5 in long with roots sometimes developing at the nodes. Flowers are white, fragrant spikes resembling clover. This species grows rapidly with each node or fragment capable of producing another plant.

    Alligatorweed is present in all the Lakes of the Great Raft. It can be successfully controlled with herbicides, and several means of bio-control have met with some success including the flea beetle Agasicles hygrophila.

  • Hydrilla (Hydrilla verticillata)

  • Hydrilla
    Photo: University of Florida

    Hydrilla is a hardy, tolerant, perennial (in the south), submerged aquatic plant that can grow up to 20-25 ft thick to the water surface. It is a native of Europe and Asia that was introduced in the US for use in the aquarium trade in the late 1950s. Hydrilla is present throughout the south, most of the eastern US, and along the west coast. This species can raise the temperature and pH of water, lower dissolved oxygen potentially causing fish kills, impact boating and fishing recreation, and clog water and power plant intakes.

    Hydrilla has leaf blades that are about 1/8 to 3/8 in long with toothed margins and spiny midribs making the plant rough to the touch; leaves are in whorls of 4-8. Flowers are solitary, tiny, and white.

    Hydrilla can grow up to 1 in per day under favorable conditions and can produce dense mats. Hydrilla is a highly problematic invasive because it can reproduce in many ways: from fragmented stems, budding of new plants from leaf axils, and from tubers that can remain viable for up to 4 years.

    Hydrilla is present in all the Lakes of the Great Raft. Herbicides, biological control agents, and drawdowns can achieve some control of HY.

Other Aquatic Invasive Species in Lakes of the Great Raft

Other Terrestrial Plant Species along Lakes of the Great Raft

Watch List of Aquatic and Adjacent Wetland Plant and Animal Species

Below are plant and animal species that are not yet well established in the Lakes of the Great Raft, but are aggressive invaders that are rapidly extending their range and should be closely monitored.

Invasive Plants

Invasive Mollusks

Invasive Fishes

Additional Information

Special attention should be paid to the Zebra Mussel and the Bighead, Silver, Black, and Grass Carps, collectively known as Asian Carps. These species are especially aggressive invaders and have been reported in areas near the Lakes of the Great Raft.

Further information on these and other species can be obtained from:

Report Invasive Species

To report invasive species in Texas, go to:

To report invasive species in Louisiana, go to: