Quick Facts

Quick facts

• Originally from South America, Alligator Weed (Alternanthera philoxeroides), a Weed of National Significance, is a soft aquatic or riparian terrestrial herb.
• It poses a significant economic and environmental threat in cool to warm temperate, to sub-tropical to tropical areas, forming large dense mats that can be free floating, rooted to bottom of water bodies, or grow in damp soil.
• Plants clog waterways restricting access for people and animals, impede flows, and limit light and oxygen.
• Grows year-round in tropical to sub-tropical areas. In cooler climates growth starts in spring, growing rapidly in summer, becoming dormant in cold winters with frosts killing exposed parts. Plants re-growing from dormant submerged vegetative parts or roots.
• Control is difficult. Mechanical and chemical control, integrated with biological control, can be effective on established aquatic growth forms. Management requires on-going follow-up and care must be taken as Alligator Weed spreads easily from fragments.
• Prevention is the most cost-effective form of weed control. Quarantine, early detection and good hygiene within infestations will prevent its spread.
• Previously Alligator Weed has been grown and spread by people as it was mistakenly thought to be the edible plant Mukunu-weena (Alternanthera sessilis).

What Does It Look Like?

What is it?

Alligator Weed (Alternanthera philoxeroides) is a spreading perennial herb that forms dense mats of interwoven stems up to 15 m long and 0.6 m thick, in or on water, and also on dry land. The creeping and layering stems can root at every joint (node) where the opposite leaves are produced. The spreading stems run along the ground and root (i.e. stoloniferous), or creep below the surface of the ground in soil and root (i.e. rhizomatous). Alligator weed rooted in soil has an extensive underground starchy rhizome-like root system and these roots allow the plant to survive long dry periods, regrowing when water and warmth return. In water the roots become relatively fine and short, with floating plants able to survive without being rooted. Leaves are in opposite pairs, shiny, dark green, spear-shaped (lanceolate) to elliptic (egg-shaped), sessile (without a leaf stalk), margin entire (smooth, not toothed, notched or divided), 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 (where the leaf joins the stem) (Ensbey 2004; Palmer 2007, pers. comm.).  Not known to produce viable seed in Australia.

Recognition: This species can normally be recognised by the combination of the following characters; small white papery heads of flowers 8-14 mm wide, on  stalks to 50 mm, shiny green leaves in opposite pairs; hollow stems; ability to form dense mats either on land or on water. For further information and assistance with identification of Alligator Weed contact the herbarium in your state or territory.

Flower colour

White

Growth form (weed type/habit)

Herb, Aquatic

Where it currently grows? Preferred habitat

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

In water it roots either on the banks, or bottom of shallow water bodies, or floats freely on the water surface, with roots produced and trailing from the nodes on the stem . Over water, stems are up to 60 cm high and the stems between the leaves (internodes) are hollow aiding in flotation. Mats may extend 15 meters over the water surface and become so robust they can support the weight of a man (Ensbey 2004).

On land, stems are shorter and stems between the leaves (internodes) are smaller and much less hollow. Tap roots on land can reach depths exceeding 500 mm (CRC 2003a; Ensbey 2004).

Are there 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. to 50 mm long. 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 (Sainty 1998; Ensbey 2004). Without flowering material Alligator Weed can be hard to distinguish from other species of Alternanthera. 

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 (a stalk growing from the leaf joint (node) 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) (Sainty et al. 1998).

Check with your state or territory herbarium for advice on identification.

Why Is It A Weed?

What are its impacts?

Alligator Weed is a Weed of National Significance (WoNS). 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 (CRC 2003a).

Agriculture: 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 (Ensbey 2004).

Another infestation, in Barren Box Swamp, would have cost irrigation farmers in the Murrumbidgee Irrigation Area up to $250 million annually (if left unchecked (DPI, NSW 2019). So far, more than $3 million has been spent controlling this infestation alone (CRC 2003a).

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 (Ensbey 2004).

In other parts of the world it is a major weed of transplanted rice wherever it is grown in the world. In China crop production is reduced between 20 to 63%. It impacts on hydro electric power production, fishing and has seriously degraded famous scenic spots. 

Native ecosystems, urban areas & infrastructure: 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 (CRC 2003a; Ensbey 2004).

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 (CRC 2003a; Ensbey 2004).

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 (Ensbey 2004).

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 also has social impacts restricting 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 (Ensbey 2004).

DPI NSW (2019) states that Alligator Weed is a problem in 30 countries. It is a serious weed in eight of these and a major weed in the others. In the USA floating alligator weed caused major impediments to navigation on the Mississippi River. In North Carolina aquatic infestations increased from 152 ha in 1963 to 1000 ha in 1999 along with a conservative estimate of 4000 ha of infested cropping land.

How does it spread?

Alligator Weed spreads in Australia through vegetative reproduction, either on land or in water. Alligator Weed does not produce viable seed in Australia, although it can in its native range. When fragmented small parts of the plant is able to grow in to a new plants and populations. Broken stems with a single node (where the leaves are produced on the stem) are able to produce roots capable of growth into new plants. Portions of thicker roots and underground stems are also all capable of growth into new plants (Julien et al. 2002). It spreads when these fragments are moved from one place to another and take root in suitable habitat. In aquatic environments plants are commonly spread downstream or to other areas when the plant is broken up into smaller fragments (e.g. by floods, or following mechanical or chemical control).

Movement between river catchments or riparian zones is most commonly due to human activities, on machinery, equipment, and/or vehicles (e.g. bulldozers, trailers, boats and other watercraft). Alligator Weed has also been spread intentionally by people has it has been mistaken as another edible species, 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) (CRC 2003a; Ensbey 2004; Sainty et al. 1998).

What is its history in Australia?

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 (Ensbey 2004), although herbarium records indicate it was first collected from NSW in Parramatta in 1969. 

How To Manage It?

Best practice management

Seek advice from your local council or state/territory management agency to help you control Alligator Weed. The principle control methods are often a combination of chemical, physical, and biological methods.

Chemical control: DPI NSW (2019) report that Alligator weed is able to tolerate most herbicides, many herbicides have been trialled over the years for Alligator Weed control. It is now clear that there are important roles for specific herbicides in suppressing and depleting alligator weed and in assisting with eradication. Repeated herbicide treatments on land infestations have had the most success but they can be costly and must be ongoing. In water Alligator Weed is rarely completely killed by herbicides. Also, plants often break up after treatment leading to new infestations downstream. Using herbicides near waterways raises concerns about impact on non-target plants and the environmental health of the waterway (CRC 2003a; Ensbey 2004). Please see the DPI NSW WeedWise site for herbicide options for NSW: https://weeds.dpi.nsw.gov.au/Weeds/AlligatorWeed#biosecurity.

Non-chemical control: Physical control: The value of physical controls (hand-pulling  or manual) is limited to small and isolated situations and could be particularly useful in removing initial invaders of a catchment if they can be located early enough. Mechanical control: Excavation of a small infestation will require at least the removal of 1-2 metres of soil from beneath the plant to ensure all root material is removed. However, other advice states that excavations should only be made to a depth of 20 cm due to the sheer volume of contaminated soil to be disposed of. An excavated site is then inspected regularly for signs of regrowth, which are then either treated with herbicide or removed by deep manual digging, depending on the management aim. Mechanical removal, particularly in waterways, is problematic and may contribute to downstream spread. Cultivation or slashing: Once alligator weed is established on land it cannot be controlled by cultivation or slashing. Any cultivation or slashing of infested land will only spread the infestation further as stem fragments root in soil or water and must be removed.  Councils and landowners in all states and territory may require a permit to remove and transport Alligator Weed.

Disposal: The disposal of the collected material poses specific problems and material must be treated and disposed of securely. Plant material can be dried and incinerated, boiled or microwaved. Do not dispose of alligator weed in green waste or composting facilities. Large volumes of excavated contaminated soil are difficult to process, and if possible need to be spread on an impenetrable surface and dried prior to burial (preferably sealed in containers) at a secure disposal site that can be monitored for any signs of regrowth. Machine hygiene is essential when performing physical control to avoid accidental re-introduction or spread to new areas.  

Biological control: Alligator Weed was recognised as a target for biological control through a cross-jurisdictional government process. This allowed activities to be undertaken to develop effective biological controls. As a biological control the Flea Beetle (Agasicles hygrophila) was first introduced to Australia in 1976. It provides good control in aquatic environments in the Sydney region. Over the years floating mats of Alligator Weed has been successfully reduced the area of in the Georges River and in parts of the Hawkesbury Nepean system. This is a truly aquatic insect and is only effective in warm temperate and sub tropical areas that allow it to breed up to high numbers in early summer. One criticism of the flea beetle is the tendency for Alligator Weed to fragment when under attack, causing downstream spread. Plant fragments should be contained if downstream spread is an issue in areas where the flea beetles are active (NSW DPI 2019). A moth (Arcola malloi) contributes to control in aquatic habitats and is established but, like the Flea Beetle, has no impact on terrestrial Alligator Weed (Ensbey 2004). The role of these agents is therefore limited to ongoing suppression of extensive aquatic infestations. Biological control is not appropriate for eradication strategies. Further biological control programs are being explored (DPI NSW 2019).

Alligator Weed has rarely, if ever, been successfully eradicated once it has infested a water body, despite numerous costly attempts. There has been some success with small infestations on land in suburban backyards. The highest priority for the management of Alligator Weed in Australia is an effective system of early detection and eradication before infestations become established. Management plans for large infestations of Alligator Weed should aim to contain and reduce population size, and prevent spread and establishment of new populations. Weed spreads so readily from fragments, and control methods are varied depending on site and size of population, so please contact your local council or state or territory weed management agency for advice (CRC 2003a).

Does it have a biological control agent?

YES. Three agents released. Two have established: the alligator weed flea beetle (Agasicles hygrophila) and the stem-boring moth (Macrorrhinia endonephele syn. Arcola malloi) (Harvey, et al 2023).

When does it grow? (lifecycle/growth calendar)

Alligator Weed can survive in tropical and sub-tropical regions such as Darwin and Brisbane, and cooler climates such as Victoria, South Australia and Tasmania.

In tropical to sub-tropical regions growth can be year round.  

In cooler regions, temperature is an important factor affecting growth with peak growth occurring in summer. Alligator Weed increases or starts growing again during warmer periods, normally in spring. New stems formed in spring grow quickly during summer, producing flowers by midsummer. Flowering continues until March when the weather becomes cooler and the days shorter and growth slows. In winter, growth is often limited by frosts whereas in aquatic habitats growth can continue slowly. In winter, leaves drop off terrestrial plants and those exposed stems of aquatic plants, and frost kills most of the exposed vegetation. However, some roots, rhizomes and protected stems survive over winter and allows it to regenerate during the warmer months with growth starting again during the following spring (CRC 2003a).

Growth in aquatic areas always exceeds terrestrial growth. At terrestrial sites, cold, moisture stress, limited nutrients, and competition with species growing nearby limits growth. No viable seed is produced.

Where Is It Found?

Which states and territories is it found?

ACT, NSW, QLD, VIC. Also known from cultivated sources in TAS and WA.

What areas within states and territories is it found?

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).

Where does it originate?

Alligator Weed is native to temperate regions of South America, especially the Parana River floodplains of northern Argentina (CRC 2003a; Ensbey 2004).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

YES

Where is it a declared weed?

Declared in all States and Territories.

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Alternanthera philoxeroides

Other scientific names (synonyms)?

Bucholzia philoxeroides Mart.