Alternanthera is from the Latin words meaning 'alternating anthers' referring to the arrangement of fertile stamens next to infertile ones in the flower.
A perennial aquatic herb with hollow stems to several metres long and opposite, hairless leaves with obvious veins and clusters of 1 cm round, 5-petalled, white, papery flowers in summer. It forms dense floating or rooted mats.
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.
Spherical or cylindrical groups of silvery-white flowers, are produced on stalks, up to 30 mm long, from the upper leaf axils.
Sepals - 5.
Petals - 5. White.
Stamens - Fertile ones alternate with infertile ones.
Not produced in the field in Australia.
Produced at nodes and grow into the soil and mud or hang in the water.
Summer in WA.
Seed Biology and Germination:
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.
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.
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.
Declared plant in WA.
Secondary and prohibited weed.
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.
Some success has been claimed with biocontrol agents.
Common Joyweed (Alternanthera nodiflora) is similar but more delicate and a native 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.