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Alligator Weed

Alternanthera philoxeroides

(Alternanthera philoxeroides) is a stoloniferous and rhizomatous perennial herb that can grow in or on water and also on dry land. In water it either roots on the banks or bottom of shallow water bodies or floats freely on the water surface with roots trailing from the stem internodes. It produces masses of creeping and layering stems up to 10 m long which can root at the nodes. Over water, stems grow up to 60 cm high and have large, hollow internodes that aid in floatation. On land, stems are shorter and internodes are smaller and much less hollow. Tap roots on land can reach depths exceeding 500 mm.

On land, underground stems (rhizomes) are produced which may extend to a depth of one metre in soil (Land Protection 2006). Plants form dense mats of interwoven stems over water or land. Mats may extend 15 metres over the water surface and become so robust they can support the weight of a man. The leaves are shiny, dark green, spear-shaped (lanceolate) to elliptic, opposite in pairs along the stem, sessile (without a leaf stalk), margin entire, 20-90 mm long and 10-20 mm wide, with a prominent midrib. Leaves on aquatic plants tend to be longer and wider than those on plants growing on land. The heads of flowers are small, 8-14 mm wide, white, papery and borne on a stalk up to 50 mm long arising from the leaf axil (Ensbey 2004; Palmer 2007, pers. comm.).
For further information and assistance with identification of Alligator Weed contact the herbarium in your state or territory.

Distribution: Map

Since its initial introduction Alligator Weed has spread throughout many waterways in eastern New South Wales. It has infested seasonally flooded agricultural grazing lands in the lower Hunter River region, and has spread south via creeks and drainage channels in the area to the Central Coast. It was first recorded in the Sydney basin area in 1969 and has spread to many locations infesting several major river areas such as the Parramatta River, Georges River and Hawkesbury/Nepean catchments, and the Botany wetlands. It also occurs in some mangrove areas and just above the high tide mark around Sydney Harbour. In 1994 Alligator Weed was found in Barren Box Swamp, near Griffith in western New South Wales, and threatened to spread to adjacent rice fields and irrigation channels. The infestation has been reduced through an integrated control program but still persists.

Alligator Weed has also been found in Byron Creek, a tributary of the Richmond River on the far north coast with control being complicated by flooding in the area. The Murray River catchment has been threatened by an infestation near Albury that is currently contained but not eradicated. A small floating mat was recently eradicated from Lake Ginninderra, Canberra. In 1995 Alligator Weed was observed in backyard vegetable gardens in Brisbane, being mistakenly grown by the Sri Lankan community as the herb and vegetable Mukunu-wenna (Alternanthera sessilis). Follow up investigations revealed that it was growing in suburban backyard gardens in Queensland from as far north as Port Douglas down to Brisbane and also in New South Wales, Australian Capital Territory, Victoria, Tasmania, South Australia, Northern Territory and Western Australia (Ensbey 2004; Sainty et al. 1998; Gunasekera 1999).
The potential range of Alligator Weed based on climate, includes waterways throughout most of southern Australia, extending south from Bundaberg in Queensland, through New South Wales, Victoria, Tasmania and South Australia, and north to Kalbarri in Western Australia. However, a different model predicts that Alligator Weed could also survive in the tropics, which may explain an infestation surviving in Cairns (CRC 2003a).
Habit: Herb, Aquatic

Key Points:

Alligator Weed (Alternanthera philoxeroides) poses a significant economic and environmental threat.
It can grow on water and on land, and has been mistakenly grown in the past as a food as it has been mistaken for the edible plant Mukunu-weena (Alternanthera sessilis).

Prevention is the most cost-effective form of weed control.

Quarantine, early detection and good hygiene within infestations will prevent its spread.

Mechanical and chemical control, integrated with biological control, is effective on established aquatic growth forms. However, care must be taken because it spreads easily from fragments and ongoing follow-up control will be required.

How it spreads:

Alligator Weed does not produce viable seed in Australia, although it can in its native range. It spreads in Australia through vegetative reproduction, when fragments containing at least one node are moved from one place to another and take root in suitable habitat. It is commonly spread downstream when the plant is broken up into smaller fragments (e.g. by floods, or following mechanical or chemical control). Movement between river catchments is most commonly due to human activities.

It has been spread in garden mulch and landfill, and attached to machinery and vehicles (e.g. bulldozers, trailers, boats and other watercraft) and also by hand and the post in the case of its mistaken identity for the vegetable and herb Mukunu-weena (Alternanthera sessilis) favoured by the Sri Lankan community. Animals may also spread the fragments (e.g. by transport of nesting material by ducks or in cow's hooves).
Where it grows:

One of the reasons that Alligator Weed poses such a dramatic threat is its ability to live in both aquatic and terrestrial habitats. It grows in creeks, rivers, ponds and drainage channels and thrives in nutrient-rich fresh water. It can also tolerate 10% sea-strength salinity or up to 30% salinity in flowing brackish water, e.g. adjacent to or in mangrove and beach areas in coastal regions. Ideal terrestrial habitats include places that are regularly inundated or that have high rainfall or irrigation. Alligator Weed can survive in tropical and sub-tropical regions such as Darwin and Brisbane, and cooler climates such as Victoria and Tasmania where the survival of some stems and rhizomes over winter allows it to regenerate during the warmer months (CRC 2003a; Ensbey 2004).

Flower colour: White

Impacts:

Alligator Weed is a Weed of National Significance. It is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts. It is an especially troublesome weed because it invades both land and water, and is very hard to control.
When growing on land it competes with and displaces native flora species along river and creek banks and in wetlands, and can be harmful to animals. When growing in fresh water, Alligator Weed can cover the entire water surface, preventing flow, blocking up drainage channels and potentially increasing flood damage. Weed mats can impede the penetration of light and also reduce oxygen exchange, affecting aquatic flora and fauna and reducing water quality.

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Alligator Weed has greatly affected primary production having caused the failure of small crop and turf farms in parts of the lower Hunter region in New South Wales and is seriously threatening other turf, vegetable, tea tree and sugarcane industries in the rest of the state.

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Another infestation, in Barren Box Swamp, would have cost irrigation farmers in the Murrumbidgee Irrigation Area up to $250 million annually if left unchecked. So far, more than $3 million has been spent controlling this infestation alone.

Alligator Weed contaminates grazing pastures and dense infestations restrict stock access to drinking water. In New Zealand and Australia, Alligator Weed causes photosensitisation of skin in light pigmented cattle, resulting in cancerous lesions. If present, land and associated production can be quarantined and sales restricted due to W1 weed status.

The extraction industry in the Hawkesbury Nepean is also under threat. This industry supplies most of Sydney's sand, gravel and soil resources. If contaminated the movement of these resources would be severely restricted. The impacts on water resources and infrastructure are great. Weed mats impede stream flow and lodge against structures thereby promoting sedimentation which contributes to flooding and structural damage. It restricts access to and use of water, blocking and damaging pumps.

Alligator Weed promotes health problems by providing habitats for mosquitoes and degrades natural aesthetics. It threatens water storage areas and also waterways associated with tourism and recreation where it creates a dangerous hazard for swimming and other water sports.

Origin:

Alligator Weed is native to temperate regions of South America, especially the Parana River floodplains of northern Argentina.

History:

Alligator Weed was probably introduced into Australia at Carrington, the Newcastle docks area in NSW when ship's ballast was dumped. It was first recorded there in 1946.

Names

Family: Amaranthaceae
Genus: Alternanthera
Species: philoxeroides
Common name: Alligator Weed
Other common name(s) and synonyms: Alligator Weed
Other botanic names: Bucholzia philoxeroides Mart.

Similar species:

Alligator Weed is similar to the 12 other species of Alternanthera in Australia but all these species have sessile flower heads (that is, flowers are not stalked) in the leaf axils. In Alligator Weed the flower heads are borne on a stalk from the leaf axil. In particular Alligator Weed has often been confused with and grown instead of Mukunu-wenna (Alternanthera sessilis) which is favoured as a leafy green vegetable by the Sri Lankan community.

Without flowering material Alligator Weed can be hard to distinguish from other species of Alternanthera. Check with your state or territory herbarium for advice on identification.
Other non-related aquatic plants likely to be confused with Alligator Weed (leaves opposite, margins entire; flowers white, clustered together in a head on an axillary stalk) are: Water Primrose, Ludwigia peploides (leaves alternate, margins entire; flowers solitary, stalked, yellow); Blue Water Speedwell, Veronica anagallis-aquatica (leaves opposite, stem-clasping , margins toothed; flowers on a raceme, blue or lilac); Enydra fluctuans (leaves opposite, glossy, margins serrate or not; flower head sessile in leaf axil, yellowish); Persicaria species (leaves alternate, sometimes with a dark blotch in the centre; flowers on an elongated spike).