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Alligator weed

Alligator weed

Alligator Weed

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Why it is a pest plant

Production threat Environmental threat Public threat

Identifying features

Alligator weed can be difficult to identify. It is a low-growing, herbaceous perennial which is easy to confuse with two other, similar-looking plants: primrose willow (Ludwigia peploides) and willow weed (Polygonum persicaria). Alligator weed has long horizontal stems (stolons), up to 10m long. These stems are hollow and often reddish in colour. Coverage can be very compact when growing in clumps on land and much larger when growing as floating mats in water.


  • Papery flowers, from December to March.


  • No fruit or seeds are produced in New Zealand. Instead, it spreads aggressively from even the smallest stem fragments.


  • Leaves grow in opposite pairs or whorls along a stem.

Alligator weed is one of the world’s worst weeds. This fast-growing weed can grow both on land and in water, where it forms floating mats. It can also tolerate certain amounts of sea water when growing in flowing water.

Alligator weed is a native of South America and was accidently introduced to New Zealand in the 1880s in ballast water discarded from ships. It has since spread through much of Northland and is now found in several parts of the Waikato region. In the Waikato, alligator weed is found in the Waikato River, on several farms, in market gardens and on urban properties. It’s also in the Ruahorehore Stream near Waihi.  Alligator weed has become a significant problem in new subdivisions in Hamilton. Although stock will eat it, alligator weed is actually toxic and can cause blindness and other health problems.

Alligator weed threatens farms, market gardens and urban properties(often dominating lawns). It clogs waterways and drains, increasing sedimentation and flooding risk. Access to waterways for recreational purposes (boating, fishing) can be blocked and plants may affect whitebait spawning areas.Alligator weed does not set seed in New Zealand but spreads aggressively from even the smallest stem fragments. It can double in area in less than two months. It can out-compete pastures and crops, affecting farm production and profit.The agricultural sector is at increasing risk of alligator weed infestation due to movement of crops and agricultural contracting equipment. Farm owners are urged to have a weed hygiene plan in place to minimise the risk of invasion from alligator weed and other serious pest plants.

Terrestrial alligator weed

Responsibility for control

Waikato Regional Council is responsible for the control of alligator weed. If you think you’ve seen it, call us.

Waikato Regional Council is responsible for controlling alligator weed – do not attempt to remove it yourself. However, landowners/occupiers are encouraged to report alligator weed on their properties and liaise with the council in areas where control programmes are in place. Alligator weed is also banned from being sold, propagated, distributed or included in commercial displays.

How to control alligator weed

Do not attempt to control alligator weed yourself.

Waikato Regional Council is responsible for controlling alligator weed and it is illegal for anyone else to remove or disturb alligator weed.

If you see this weed on your property, do not cut or treat it. Call 0800 BIOSEC (0800 246 732) to report it to your local biosecurity pest plant officer.

See our tips below for ideas on how you can help us prevent alligator weed from spreading.

Tips – stopping the spread

Alligator weed can spread by water movement such as floods or tides, soil movement, and equipment such as diggers, farm machinery, eel nets and boats. When disturbed, alligator weed breaks up easily into small fragments which can easily regrow. Take special care not to disturb it or transport it to new sites.

  • Check boats, trailers, vehicles and equipment for fragments before heading home.
  • Check drains regularly.

Farmers should protect their properties from alligator weed and other serious pest plants by:

  • insisting all contractors practice good weed hygiene, cleaning their equipment before entering the farm
  • ensuring supplementary feed brought onto the farm is weed free
  • ensuring aggregates, soil and sand brought onto the farm is weed free.

Similar plants

Similar plants which are often mistaken for alligator weed are primrose willow and willow weed.

Similar plants

More information


  • For advice and additional information on control methods, call our pest plant staff on freephone 0800 BIOSEC (0800 246 732).
  • Chemical company representatives, farm supply stores and garden centres can also be good sources for advice.


View, download or order the following publications or call our freephone 0800 800 401.

  • National Pest Plant Accord (Manual of plants banned from sale, propagation and distribution) ($10.00 plus GST)
  • Plant Me Instead! (Plants to use in place of common pest plants) (free)
  • Waikato Regional Pest Management Plan (RPMP) (free) (Section 5.3, page 46)
  • Waikato Regional Council pest guide (free)

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