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   Hydrilla verticillata (aquatic plant)  français     
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      Hydrilla verticillata in Lake Tutira, NZ (Photo: R. Wells, NIWA) - Click for full size   Hydrilla community (Photo: John Clayton, NIWA) - Click for full size   Hydrilla signage in Hawkes Bay, NZ (Photo: John Clayton, NIWA) - Click for full size   Hydrilla verticillata turions (green) and tubers (Photo: Visual Arts, MAFTech) - Click for full size   Hydrilla tubers and turions with scale (Photo: Visual Arts, MAFTech) - Click for full size
    Taxonomic name: Hydrilla verticillata (L. f.) Royle
    Common names: Florida elodea (English), hydrilla (English), oxygen weed (English), water thyme (English), water weed (English)
    Organism type: aquatic plant
    Hydrilla verticillata is a submerged freshwater aquatic weed that can tolerate salinity up to 7%. It crowds out native plants by shading them and out-competing them for nutrients. The dense masses it forms interfere with recreational activities such as boating, fishing and swimming. Hydrilla verticillata can be dispersed by river flow, waterfowl and recreational activities and is sold as an aquarium plant.
    H. verticillata is a submerged aquatic perennial with heavily branched stems towards the water surface. Stems are slender and can grow up to 9m long. Leaves are 6 to 20mm long, 2 to 4mm wide. The leaves are strap-shaped with pointed tips and saw-tooth edges, and they grow in whorls of 4 to 8 around the stem. Leaf colour can vary from green, translucent, yellowish, to brown. Hydrilla produces turions (over-wintering dense vegetative buds) in the axils of leaves and tubers within the sediment. The plant sometimes produces flowers. Small white flowers on long slender stems are female, and small, green, free-floating, inverted bell-shaped flowers are male. The plant is usually rooted to the substrate but sometimes grows as floating mats at the surface.
    Similar Species
    Egeria densa, Elodea canadensis

    Occurs in:
    estuarine habitats, lakes, water courses, wetlands
    Habitat description
    H. verticillata is found in freshwater but can tolerate salinities of up to 7% salinity of seawater. It has been found in springs, lakes, marshes, ditches, rivers, and tidal zones. It can grow in relatively low light and CO2 conditions. H. verticillata prefers temperatures between 20 and 27 degrees Celsius.
    General impacts
    H. verticillata competes with native plants by growing to the water surface and forming dense mats that totally exclude sunlight from other plants, which in turn can significantly reduce aquatic plant and animal biodiversity. Large populations of H. verticillata may affect fish size and population levels where predatory fish cannot hunt effectively within the thick mats. The dense mats also affect recreational activities. Apart from interfering with fishing, boat motors can become tangled with them and swimming areas choked. H. verticillata often slows or clogs rivers, irrigation ditches, and flood control canals, creating stagnant water that is prime mosquito breeding habitat. Dense stands can even cause flooding, alter water quality by decreasing oxygen levels and increasing pH and water temperature.
    H. verticillata provides a food supply for waterfowl in areas where wetland degradation has lowered their food supply, such as in Florida. Up to 30% H. verticillata cover is beneficial to most fisheries because it allows for an increase in the population of prey fish that game fish feed on.
    In North America, all dioecious plants are female. In New Zealand, all H. verticillata plants are male.
    Geographical range
    Native range: Hydrilla is native to Asia and northern Australia (Rod Randall, pers. comm., 2003). The dioecious plant (having female and male flowers on different plants) is native to southern India and the monoecious plant (having both female and male flowers on the same plant) is probably native to Korea.
    Known introduced range: The dioecious and the monoecious plant are now found on every continent except Antarctica.
    Introduction pathways to new locations
    Floating vegetation/debris: Plant fragments dispersed by river flow.
    Ignorant possession: Shipments of water lilies have been found contaminated with Hydrilla.
    Pet/aquarium trade: Sold as an aquarium plant.

    Local dispersal methods
    Boat: Plant fragments carried by boats (e.g., on motors and in live wells), trailers and fishing equipment (e.g. nets), kayaks .
    Consumption/excretion: Tubers survive digestion and regurgitation by waterfowl.
    Water currents: Dispersed via plant fragments.
    Management information
    Preventative measures: H. verticillata is on the United States Federal Noxious Weed List, but aquarium supply sales continue through the Internet. It has been classified as a Nationally Banned Plant List species in New Zealand. A Risk assessment of Hydrilla verticillata for Australia was prepared by Pacific Island Ecosystems at Risk (PIER) using the Australian risk assessment system (Pheloung, 1995). The result is a score of 20 and a recommendation of: reject the plant for import (Australia) or species likely to be a pest (Pacific).

    Physical: Harvesting and use of motorised boats is not recommended in partially infested lakes or where uncontamainated waterbodies occur nearby, because this can chop the plants and facilitate spread of shoot fragments (NIWA, 2003). In ponds and small lakes, water draw-downs, which expose and kill the plants, have been found effective. Weed mats in public access sites have been used to contain spread,and signage to increase public awareness are some of the containment methods adopted (NIWA, 2003).

    Chemical: Aquatic herbicides are effective at temporarily controlling the weed but do not kill the tubers, turions (overwintering structures that detach and geminate in the spring), and seeds. Some of the herbicides which have been used are Fluridone and endothall (dipotassium).

    Biological: Biological controls include Chinese grass carp (Ctenopharyngodon idella), tuber-feeding weevils, and leaf-eating flies. Chinese grass carp have been found effective, but these fish are vegetative generalists, so they should be used with care so as not to destroy native aquatic vegetation. Tuber-feeding weevils and leaf-eating flies are still under evaluation for their effectiveness. The tuber-feeding weevil (Bagous affinis) only attacks the tuber when the plant is not submerged beneath the water. Leaf-eating flies, such as Hydrellia pakistanae , attack the weed by feeding on it as larva.(NIWA, 2003).

    Integrated management: An integrated approach of fish, mechanical, and manual methods to eradication has been found to achieve maximum success.

    H. verticillata reproduces mostly by asexual vegetative fragmentation (from stem fragments), but it also grows new plants from tubers and underground tubers and reproduces sexually with flowers. One H. verticillata tuber can lead to the production of 5,000 new tubers per square m. It spreads faster in flowing water habitats because the fragments are more efficiently dispersed.
    Lifecycle stages
    Tubers and turions can survive ice cover, drying, ingestion, and regurgitation by waterfowl. Tubers may remain viable in the sediment for several years.
    Reviewed by: Dr John Clayton. NIWA, National Institute of Water and Atmospheric Research. Hamilton, New Zealand.
    Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
    Last Modified: Friday, 31 March 2006

ISSG Landcare Research NBII IUCN University of Auckland