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(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,
For further information and assistance with identification
of Alligator Weed contact the herbarium in your state
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;
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
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
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.
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
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.
Alligator Weed is native to temperate regions of South
America, especially the Parana River floodplains of
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
Common name: Alligator Weed
Other common name(s) and synonyms: Alligator Weed
Other botanic names: Bucholzia philoxeroides Mart.
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
Without flowering material Alligator Weed can be hard
to distinguish from other species of Alternanthera.
Check with your state or territory herbarium for advice
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).