Alligator weed

Alternanthera philoxeroides

On this page:

What does it look like? •
Why is it a pest? •
Responsibility for control •
Stopping the spread of alligator weed •
Restricted places •
More information

Management programme

Progressive containment

Objectives

To contain and where practicable progressively reduce the geographic distribution or extent of alligator weed within the Waikato region to pre-2022 levels.

Impacts

Economic, biodiversity, water quality/quantity, amenity/recreation, animal welfare

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Alligator weed, a low-growing non-woody perennial plant, is an invasive pest plant that is easily spread but very costly to control. Originally from Brazil, it is thought to have arrived accidently in New Zealand in the early 1900s and has now spread to parts of northern New Zealand.

Fast growing, it’s able to grow on both land and on water where it forms dense floating mats. It’s tolerant of brackish sites when growing in flowing waterways. Where it invades it can block waterways, aggravating flooding and impeding access. In pasture, it can out compete more favourable plants and be harmful to animals.

In the Waikato region, alligator weed is found mainly at sites on the margins of and in the Waikato River, on the edges of some harbours (e.g. isolated sites in Kāwhia), on farmland, in wetlands, in market gardens and on urban properties. It’s also present at sites along the margins of other waterways, near Waihi and Waikino, and has become a significant problem in new subdivisions in Hamilton city. New sites have recently appeared along some west coast beaches and on the Coromandel Peninsula.

What does it look like?

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 that can occur in similar habitats: water primrose (Ludwidgia peploides) and willow weed (Persicaria maculosa). Alligator weed has long horizontal stems (stolons), up to 10m long, that are hollow and often reddish in colour. Coverage can be very compact when growing in clumps on land but more expansive when growing as floating mats on water.

Flower

Clusters of papery white clover-like flowers.
Flowers from December to March.

Fruit/seed

No fruit or seeds are produced in New Zealand. Instead, alligator weed easily establishes and spreads aggressively from even the smallest stem fragments.

Leaves/stems

Dark green waxy leaves, in opposite pairs.
Pink-reddish, soft, hollow stems (<10m long) that root at nodes.

Similar plants

Water primrose and willow weed look similar to alligator weed, and are often mistaken for it.


Water primrose (Ludwigia peploides)	Willow weed (Persicaria maculosa)

Why is it a pest?

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.

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.

It can out-compete pastures and crops, affecting farm production and profit. Although stock will eat it, alligator weed is actually toxic and can cause blindness and other health problems.

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.

Responsibility for control

Waikato Regional Council is responsible for the control of alligator weed within the Waikato region. If you think you’ve seen it, call us – don’t try to control or remove it yourself.

Occupiers are encouraged to report suspected sightings of alligator weed (rule ALW-1 of the Waikato Regional Pest Management Plan 2022-2032) and liaise with the Waikato Regional Council in areas where control programmes are in place.

If alligator weed is present on a property that is to be subdivided or developed, there are additional rules and requirements that apply under section 6.6 of the RPMP 2022-2032.

Alligator weed is also banned from being sold, propagated, distributed, or included in commercial displays.

Stopping the spread of alligator weed

Alligator weed is extremely difficult to control and easily spread. In addition, there are restrictions on the use of herbicide near waterways. If you see alligator weed on your property, do not cut, treat or remove it. Instead call 0800 800 401 to report it to your local biosecurity pest plant officer.

You can help prevent alligator weed from spreading

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

Check boats, trailers, vehicles and equipment for fragments before heading home.
Check drains on your property regularly.

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

Insist all contractors practice good weed hygiene and clean their equipment before entering the farm.
Make sure supplementary feed brought onto the farm is weed free.
Make sure any aggregates, soil or sand brought onto the farm is weed free.

Restricted places

A restricted place is an area of land that has a high risk pest plant, such as alligator weed. Waikato Regional Council uses its statutory powers under the Biosecurity Act 1993, to declare an area a “restricted place”. This notice prevents unauthorised removal of soil material. Alligator weed is currently the only pest plant in the Waikato Region that requires a restricted place notices. Restricted place notices remain in force until an authorised person formally notifies the landowner in writing.
Landowners and occupiers must not remove alligator weed or any other material, which may have come in contact with alligator weed from the place to which the notice relates; or introduce any goods of any kind to the place.
A breach of a restricted place notice can have very severe consequences such as fines and/or imprisonment as outlined by the Biosecurity Act 1993.
Alligator weed can be spread by people. For example, machinery operators, cable diggers, utility workers and subcontractors are often not aware of the potential risks their activities pose with pest plants.
Many farmers and other landowners are similarly unaware of how their land could become infested by plant pests such as alligator weed.
Not all alligator weed sties have restricted place notices. Only sites where:
- Some human activities causing a high risk of alligator weed being spread include; moving soil (for example subdivision), making silage, and moving stock feed (risking off-site transfer via agricultural and drainage machinery), and
- where landowners knowingly undertake activities which cause the spread of alligator weed.
Once a restricted place notice is served the council implements a weed management plan with the landowner/s.
a)    No soil is to leave the complex while the restricted place notice remains in force. All earth moving, tracked equipment which is intended to be moved off site must be washed down with a high pressure hose, on the infected site, to help prevent spread of alligator weed to new areas, this must be checked by Waikato Regional Council before the machinery is moved to a new site.
b)    All mown grass is to be removed from the area and taken directly to the Refuse Transfer Station and Recycling Centre in Lincoln Street, Hamilton landfill or a designated site authorised by Waikato Regional Council.  Alligator weed infected material is not to be taken to a composting facility.
c)    Access to the restricted place by a chemical applicator, or any other party approved by Waikato Regional Council, is to be allowed, as becomes necessary, for the purposes of surveying, monitoring and controlling the weed (herbicides are necessary for effective control).
d)    The owner/occupier has a strict obligation to keep all staff and contractors, current and potential residents/tenants and outside contractors who operate in the restricted place, informed of the status of this area as a restricted place. It is the land owner that will be prosecuted the condition of a restricted place is breached.
e)    Any new sites found of alligator weed in this or any other area must be reported to Waikato Regional Council biosecurity pest plant officer immediately.
f)     The owner/occupier is required to display signs (example sign [PDF, 1.7 MB]) provided by Waikato Regional Council at all entrances and exits informing of their status as a Restricted Place.
Restricted place notices for alligator weed have been issued to private land in the region. Please contact Waikato Regional Council for more information about these sites.
- High risk areas - Waikato River Catchment and West Coast [PDF, 6.7 MB]
- High risk areas - Coromandel and Waihou-Piako [PDF, 6.5 MB]

More information

Advice

For additional information and advice on alligator weed, call our pest plant staff on freephone 0800 800 401.

Publications

The following publications are available for download or from Waikato Regional Council. Contact us to request a copy (freephone 0800 800 401).