ORIGIN

South America

COMMON NAME

Alligator weed

BOTANICAL NAME

Alternanthera philoxeroides

IDENTIFICATION

Alligator weed is a hairless perennial aquatic plant that can also grow in certain terrestrial situations.

The plant has leaves in opposite pairs along a creeping hollow stem, with nodes 50 - 100mm apart. Fibrous roots may be reproduced from the stem, which reach down into the water or soil. Soil roots are often longer and thicker than those in water, often extending to below 500mm in depth. Nodes also produce shoots that can produce new plants. The stem of the plant is green-brown but often red-tinged. Alligator weed leaves are hairless, waxy and green and have a conspicuous midrib. They are 20 - 70mm long and 10mm - 20mm wide. The white flowers resemble those of a clover but are smaller and each cluster is produced on a long stalk.

HABITATS

Alligator weed will not root in water deeper than 2-3 metres, but a marginal weed bed can support a floating mat of vegetation over deeper water. The plant grows best in fertile, nutrient-rich conditions and requires a warm growing season although it does tolerate cold winters. It can tolerate up to 10% sea strength salinity in still water and up to 30% salinity in flowing water, although it mainly occurs in fresh water.

IMPACT TO BIOTA AND ECOSYSTEMS

Alligator weed forms extensive floating mats, which extend out from the banks of a waterbody and effectively cover the water surface. The prolific growth restricts water flow, increases sedimentation, aggravates flooding and acts as a barrier, accumulating debris that would otherwise be flushed downstream.

DISPERSAL ROUTES, VECTORS, INFESTATION SOURCES

Parts of floating mats or stem fragments with nodes, broken from the main body of the weed mass by wave motion, water level change or mechanical interference, move freely in water currents and once lodged on muddy streambanks away from the source, establish new colonies. Cultivation and drain clearing machinery also spread plant fragments to new sites. Alligator weed is not known to set seed in New Zealand.

MANAGEMENT

Physical Control
Control of alligator weed is extremely difficult. Mechanical harvesting with scoops or draglines gives only temporary relief and is not recommended; stem fragments floating in the stream or moved by harvesting equipment also spread the weed.

Disposal
Physical control is not recommended, however plant material should be dried out before burial.

Chemical Control
Alligator weed can be controlled using herbicides, however there are some associated problems with this option. Applications of herbicides to aquatic infestations kill emergent stems but have little effect on submerged stems, hence regrowth from treated areas is rapid. (Requirements of the Resource Management Act must be adhered to when applying herbicides - Contact Environment Bay of Plenty or your local Pest Plant Officer for details about spraying Alligator weed). The herbicides Tordon Gold (Trilopy + Picloram)and Metsulfuron (Escort) are effective on Alligator weed.

Biological Control
Insects have been released on Alligator weed giving some seasonal control of the plant in Northland and parts of Auckland. Native of South America, Alligator weed beetle (Agasicles hygrophila) was first imported to New Zealand in 1981. Alligator weed moth (Arcola malloi) also native of South America was first imported in 1982.

Recommended Approach

FURTHER COMMENT

This native of South America first appeared in New Zealand in the late 1800s and has spread throughout much of Northland and Auckland. Several sites have been discovered in the Bay of Plenty.

For further information on Alligator weed please refer to Environment Bay of Plenty's Fact Sheet pp1300-Aquatic Pest Plants

This plant is prohibited from propagation, sale and distribution within New Zealand!