Alligator weed

Alternanthera philoxeroides (Martius) Griseb.

Family: Amaranthaceae.


Alternanthera is from the Latin words meaning 'alternating anthers' referring to the arrangement of fertile stamens next to infertile ones in the flower.
Alligator Weed.


A perennial aquatic herb with hollow, sprawling stems up to 10 metres long. It forms dense floating or rooted mats. The opposite leaves are dark green, hairless, elliptic, up to 10 cm long and conspicuously veined. The flower heads are stalked, spherical or cylindrical with white papery flowers each with 5 petal-like sepals 4-4.5 mm long. The fruits are tiny and fall with the sepals.
Alligator Weed is native to South America and grows in still or slow flowing water or swampy land and is a potentially troublesome water weed. It flowers in summer.





Stipules -
Petiole - Short and clasping.
Blade - Short. Up to about 100 mm long, elliptical in shape, tapering to a short stalk below and to a rounded or pointed apex.


The prostate stems are much branched and may be several metres in length. The tips of the stems and their branches produce vertical shoots of up to 500 mm in height. They are green to brown, hollow and round in cross section.

Flower head:

Spherical or cylindrical groups of silvery-white flowers, are produced on stalks, up to 30 mm long, from the upper leaf axils.


Papery white.
Sepals - 5.
Petals - 5. White.
Stamens - Fertile ones alternate with infertile ones.
Anthers -



Not produced in the field in Australia.


Produced at nodes and grow into the soil and mud or hang in the water.

Key Characters:

Leave opposite
Flowers in axillary clusters
Anthers 1 celled
Stigma capitate


Life cycle:




Flowering times:

Summer in WA.

Seed Biology and Germination:

Vegetative Propagules:

Shoot fragments.



Population Dynamics and Dispersal:

Except in laboratory situations Alternanthera does not generally produce viable seeds. All local spread is by stem elongation and rooting from the nodes. Distant dispersal is the result of fragments breaking off and being carried downstream by the water flow, or being transported in mud after mechanical clearing of infested waters. In tropical and sub-tropical waters growth is fairly constant but is seasonal in colder waters. In California stem elongation of 17 metres has been recorded in one year. A single node is capable of establishing a separate viable plant. It is not limited to purely aquatic habitats and may colonise low lying areas alongside bodies of water from which it has been removed.

Origin and History:

South America. Probably imported into NSW in ship ballast from South America.
610 infestations had been recorded in Victoria in 1997 and its is estimated that there are over 1000 infestations present.



Courtesy Australia's Virtual Herbarium.


Normally grows in still or slow flowing fresh water but will tolerate 10 per cent sea water in still situations or up to 30 per cent sea water in flowing water. It is rooted in the bottom mud and cannot survive in water exceeding 2500 mm in depth.


Tropical, subtropical and temperate.


Flood plains and aquatic areas.

Plant Associations:




The main nuisance results from the restriction of water flow and the acceleration of siltation. Impedes navigation and may favour mosquito breeding.


Not recorded as toxic.


Weed of National Significance.
Declared plant in all states.

Management and Control:

It has proved resistant to many herbicides. The nodes on the stems appear to inhibit the movement of the herbicides and unless full cover of all leaves is achieved regrowth is almost certain.


Eradication strategies:

Herbicide resistance:

Biological Control:

Some success has been claimed with biocontrol agents.

Related plants:

Common Joyweed (Alternanthera nodiflora) is similar but more delicate with stems usually less than 1 m long, leaves only 13-45 mm long and stalkless flower heads. It is a native of WA and a terrestrial or semi-aquatic herb fringing watercourses, lakes or winter-wet depressions in the south west of WA.
Hairy Joyweed (Alternanthera nana)
Khaki weed (Alternanthera pungens) has a large carrot-like roots, spiny fruits and flowers with a pungent odour and the leaves are broadly oval rather than narrow.
Lesser Joyweed (Alternanthera denticulata)
Narrow leaved Joyweed (Alternanthera angustifolia)

Plants of similar appearance:


Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P68 (photo).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #63.5.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P154-157 (photos).


Collated by HerbiGuide. Phone 08 98444064 or for more information.