Alligator Weed (Alternanthera philoxeroides) is a Weed of National Significance from South America that grows in water and on land in damp soils. It forms dense mats that clog waterways, restrict flow and access, and reduce light and oxygen. It can also invade wetter pastures and irrigation channels, affecting agriculture and infrastructure. In Australia it does not produce viable seed; spread is mainly by stem and rhizome fragments moved by floods, machinery, boats, animals, and soil/turf movement.
Identify it
Field cues
01Grows as dense mats on water or as a creeping mat on damp land
02Hollow stems (especially in water) that root at the nodes
03Opposite, dark green, smooth/shiny leaves with a distinct midrib and little to no leaf stalk
04White, papery, ball-shaped flower heads on short stalks from leaf junctions
Distribution records are indicative. Always confirm current status with your state biosecurity authority.
Found in: freshwater, brackish water, wetlands, creek banks, stream banks, irrigation channels, damp soil, riparian zones.
Control is difficult because plants tolerate many control measures and readily re-establish from very small stem or rhizome fragments. Disturbance, mechanical removal, and some treatments can increase fragmentation and spread, including downstream movement in waterways. Effective management commonly relies on integrated approaches with ongoing follow-up, strong hygiene, and secure handling and disposal of plant material to prevent re-infestation.
1
Prevent fragment spread
Avoid moving soil, turf, or plant material from infested areas, and keep removed material contained so fragments cannot wash or fall into clean areas.
2
Clean machinery and watercraft
Remove all mud and vegetation from equipment used in infested areas and inspect it thoroughly before moving to another site.
3
Check for regrowth
Revisit treated or disturbed sites regularly and act quickly on any regrowth to prevent re-establishment.
Registered herbicide options
ImazapyrGroup 2 · 250 g/L
Foliar application for terrestrial situations only including riparian zones, limited to once per annum under permit conditions. Source ↗
DichlobenilGroup 29 · 40 g/kg
Granular application for home gardens in residential areas, treat dormant plants. Source ↗
GlyphosateGroup 9 · 360 g/L
Spot spray actively growing plants for home gardens in residential areas, use coarse droplets and low pressure and avoid run-off. Source ↗
Metsulfuron-MethylGroup 2 · 600 g/kg
Hand-directed spot spray in aquatic and terrestrial areas across NSW under permit conditions, do not apply more than 3 applications per growing season. Source ↗
FlumioxazinGroup 14 · 15 g
Aquatic use in enclosed water bodies or margins of still water, applied by throwing tablets into water, injecting dissolved solution, or spraying solution depending on water body characteristics. Source ↗
AminopyralidGroup 4 ·
Used in a mixture product for spot spray application in terrestrial situations only, follow-up applications over at least two seasons are essential for complete control. Source ↗
Use chemicals to the label. Always read the current APVMA-approved label before application.
Check permit conditions for your state, as some uses are limited to council or government staff.
Aquatic situations require products registered for use in or near water.
Why it matters
Impacts
Downstream spread from fragments
Small stem or rhizome fragments can regenerate, and fragmentation during floods, mechanical removal, or after treatment can create new infestations downstream.
Waterway and infrastructure impacts
Dense mats can restrict water flow, impede drainage, increase sedimentation and flooding risk, reduce water quality, and damage pumps and irrigation equipment.
Access and safety impacts
Infestations can restrict access for people and animals, impede boating and water sports, and create favourable habitat for mosquitoes.
Toxicity note (cattle): May cause photosensitisation in light-pigmented cattle.
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