Alligator Weed

Alternanthera philoxeroides

Family: Amaranthaceae

Type of weed: ,

Noxious Weed Class 2 and WoNS (Weed of National Significance). (See more , noxious weeds).

Flowering Months: , , , ,


A perennial herb growing either as a terrestrial or free floating aquatic plant.

Stems are hollow, leaves are dark green, fleshy, waxy and linear to 70 mm long and 40 mm wide narrowing at the tip.

Silvery white flower heads appear on stalks approximately 7 cm long, rising from leaf axils, papery when mature, mid summer to autumn.

Seeds from the fruit are not viable in Australia.

Don’t confuse with…

Alligator weed can be confused with the native Alternanthera denticulata.


Dense mats produce roots at nodes, which are easily broken off from the main plan and take root downstream, establishing new colonies.

On land, dispersal is usually by stem fragments, or in contaminated turf or in dumped garden waste.

Impact on bushland

On creek lines and in water bodies the plant forms a dense, interwoven mat that covers the surface, blocks water flow, changes water chemistry and adversely affects aquatic plants and animals.

On land the plant becomes dominant in moist areas, replacing native species.


. Lower Mountains including creek lines on the escarpment. Also in South Woodford and South Katoomba.

Alternative planting

Native plants

Alternative plants for moist areas on land include:

  • Bidgy Widgy (Acaena novae-zelandiae)
  • Native Violet (Violaceae hederacea)
  • Water Fern (Blechnum nudum)

Council provides a tool, on its Mountain Landscapes website, to help you choose native alternative plantings. Choose your village, soil, vegetation community and the purpose of your planting, and the tool will give you suggestions.

There are native nurseries in several Blue Mountains villages, including Glenbrook, Lawson and Katoomba. Please also ask at your favourite local nursery.


Do not move the plant; it will grow from the tiniest piece.

For more info

For key points on these techniques:

Noxious Weed Class 2

Regionally Prohibited Weeds

Characteristics: Class 2 noxious weeds are plants that pose a potentially serious threat to primary production or the environment of a region to which the order applies and are not present in the region or are present only to a limited extent.

Control objective: Prevent the introduction and establishment of those plants in parts of NSW.

Example control requirements: These plant must be eradicated from land and not allowed to re-establish. They are also ‘notifiable’, with a range of restrictions on their sale and movement.

NSW Noxious Weeds Act 1993

Refer to the NSW Department of Primary Industry’s Noxious and Environmental Weed Control Handbook.

WoNS (Weeds of National Significance)

All of the Australian governments have agreed on a list of thirty-two Weeds of National significance.

These weeds were chosen because of their:

  • invasiveness,
  • potential for spread,
  • environmental, social and economic impacts, and
  • the potential to manage them successfully.

Managing Weeds of National Significance

If you have any of these weeds on land you own or manage, you have a responsibity to manage them.

The reason these weeds have special status is that managing them requires coordination by every level of government, organisations and responsible individuals.

There is a strategic plan for each of the Weeds of National Significance, making clear how everyone is to work together, from research through to on the ground action.

For more information, visit the Federal Environment website.