Alligator weed - An early detection guide for farmers

Free printed version available

Cover of Alligator weed farmers guide

How to get your free copy:

What is alligator weed?

Alligator weed is a highly invasive weed that grows both on land and in water.

It can tolerate herbicides and spreads easily by fragments, making it a serious weed of waterways, wetlands and floodplains in Australia.

Irrigated and floodplain-based farming is threatened by alligator weed. The worst infestations currently affect turf farming, cropping and grazing in the Hawkesbury and Hunter regions of NSW.

Alligator weed has the potential to spread to most non-arid parts of Australia including inland river systems. In catchments with high rainfall alligator weed can persist and grow above the floodplain.

Impacts of alligator weed

Impacts of having alligator weed on your farm include:

  • Reduced yields and production losses
  • Contamination of crops and pastures
  • High costs of ongoing control
  • Loss of use of irrigation bays, paddocks and pastures for periods up to 5 years (during suppression and eradication programs)

Impacts for graziers include:

Small papery white ball shaped flower on short stalks with dark green glossy spear shaped leaves.
Small papery white ball shaped flower on short stalks with dark green glossy spear shaped leaves.
  • Photosensitivity in livestock (leading to skin lesions, liver damage, and death)
  • Malnutrition due to lack of essential amino acids (alligator weed should not make up more than 15% of a ruminant diet)
  • Loss of pasture productivity (alligator weed will slowly dominate a pasture over a number of years)

Impacts for irrigators include:

  • Floating plant masses reducing downstream supply flows and efficiencies. (Infestations of between 2m2 and 10m2 can reduce flows by at least 50%)
  • Obstruction of supply channels causing collapse or channel bank breaching and flooding
  • Reduced flows and pressure at the discharge side of the system
  • Increased maintenance and replacement costs of foot valves, strainers and pumps
  • High risk of spread to other irrigated areas

Alligator weed does not produce viable seed in Australia. Spread is via plant fragments.

A new infestation can start from a single plant fragment.

The following table describes how alligator weed spreads onto and within farms.

Alligator weed can spread ONTO a farm when: Flood irrigation
(rice, wheat, grains)		 Pressurised irrigation
(turf, crops, horticulture)		 Grazing Dryland
cropping
Irrigation water from infested supply channels carries fragments into a crop or irrigation bay Yes      
Irrigation pipelines pumping from an infested water source carry fragments into a crop   Yes Yes  
Fragments are carried in stock hooves     Yes  
Slashing or cultivation occurs close to infestations Yes Yes Yes Yes
Plants grow into paddocks from aquatic bankside infestations Yes Yes Yes Yes
Contaminated feed, soil, gravel, sand or turf is introduced to the farm Yes Yes Yes Yes
Fragments are carried on watercraft, vehicles and machinery Yes Yes Yes Yes
Flood waters deposit fragments Yes Yes Yes Yes
Alligator weed can spread WITHIN a farm when: Flood irrigation
(rice, wheat, grains)		 Pressurised irrigation
(turf, crops, horticulture)		 Grazing Dryland
cropping
Fragments are carried in stock hooves and mouths     Yes  
Fragments are moved in soil (from banks into cropping bays or paddocks) Yes Yes Yes Yes
Plants are slashed or cultivated over Yes Yes Yes Yes
Fragments are carried on watercraft, vehicles and machinery Yes Yes Yes Yes
Fragments are carried in water Yes Yes Yes Yes

Early detection is critical for eradication

Size demonstration of alligator weed
Plants growing on land in cooler months have creeping runner-like growth.

Early detection increases the chances of successful eradication. Years of expensive control will be necessary before an established infestation may be eradicated.

What to look for:

  • Floating mats of stems on water surfaces
  • Creeping, layering and upright stems on land
  • Upright growth in warmer months
  • Runner-like stems in cooler months
  • Small papery white ball-shaped flowers (1.2 –1.4cm) on short stalks
  • Dark green, glossy, spear-shaped leaves (2-12cm) in opposite pairs along stems
  • Hollow stems

When to look:

Alligator weed growing on land
Plants growing in aquatic situations in warmer months have upright growth.
  • Regularly, during the growing season from September to May (flowering occurs mid to late summer)
  • After floods and heavy rains
  • Prior to cultivation or planting
  • After irrigation

Where to look:

  • Drainage lines, waterways, wet depressions, swampy areas and floodplains
  • In bankside vegetation – check from the bank to the paddock
  • Where earthmoving equipment has been operated
  • Where feed, soil, gravel, sand or turf has been introduced

Be vigilant and check regularly and repeatedly

Alligator weed in a creek bed, climibing the bank.
Plants grow into paddocks from aquatic bankside infestations.

Contact your local weed officer immediately if an infestation is suspected.

  • Don’t attempt to control or remove the plants until you have contacted your local weeds officer
  • Mark infestations with high visibility markers
  • Reporting an infestation enables local weed officers to assist with control and disposal

Further Information

For advice on identification, prevention of spread and control of alligator weed contact your local weeds officer.

Detailed information is provided in: “Alligator weed control manual – Eradication and suppression of alligator weed (Alternanthera philoxeroides) in Australia”.

For copies contact NSW DPI Bookshop 1800 028 374 or your local weeds officer.

The Alligator weed: An early detection guide for farmers is available for download below.