Alligator weeds


Sprawling emergent perennial. Forms floating mats on water surface or grows rooted in soil

at water’s edge or in shallow water, extending

many metres across the water surface. Also

grows on land.


  • Opposite
  • Glossy, spear-shaped, 2–7 cm long
  • Smooth margin


  • Single, white, papery, ball-like, 1.2–1.4 cm diameter
  • On short stalk in leaf axils (stem and leaf junction)


  • Aquatic alligator weed: completely hollow
  • Terrestrial alligator weed: reddish-brown

Similar looking species

Water primrose (Ludwigia peploides ssp. montevidensis): yellow flowers, alternate glossy leaves

Smart weed (Persicaria decipiens): alternate hairy leaves, dark blotch in centre, small pink

or white flowers on spike.

Senegal tea (Gymnocoronis spilanthoides): irregularly toothed leaf margins, ribbed stems

that are hollow between the joints, halfsphere-shaped (pom-pom-like) white or pale

purple flower heads in clusters

Hygrophila (Hygrophilacostata): stems fourangled, whorled flowers around stem and leaf


  • Alternanthera spp: no flower stalks


Alligator weed is native to South America and was first discovered in Australia during the 1940s

in the Hunter River, NSW. It is believed that plant fragments were accidentally introduced via ship’s

ballast water. Alligator weed is regarded as one of Australia’s worst weeds due to its impact invasiveness, capacity to spread and regenerate from fragments, and ability to tolerate a range of

control treatments.


About 5000 hectares in the Greater Sydney and Hunter regions in NSW is infested with alligator

weed. Smaller infestations are found in Vic, Qld, ACT and regional NSW.

Means of spread

Alligator weed does not produce viable seed, and spreads by fragments. Earthmoving equipment, boating equipment and water movement have been responsible for much of the spread, and some infestations may have been deliberately planted.

Declaration status

WoNS. Vic: S; NSW: C2(84)/C3(44); SA: 1@; WA:

P1/2; Tas: D; Qld: C1; NT: A/C; ACT: C1/4

If found, report this weed to your local weed authority.

Source: Recognising Water Weeds

Plant Identification Guide Aquatic Weeds Early Detection Project
Compiled by Jessica Grantley, Fiona McPherson and Andrew Petroeschevsky,

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