Alligator Weed

We need your help locating Alligator Weed in the Mangaone Stream and the lower Manawatū catchment. This area includes the stream area north of Colyton through Palmerston North to the Manawatū River, and down to Foxton Beach.

Alligator Weed or Alternanthera philoxeroides is one of the world’s worst pest plants which originates from South America. It has the ability to dramatically alter wetlands, small lakes, rivers, dams and drains. It can also take up heavy metals from the soil, is known to be toxic to some livestock and can block access to waterways for recreation.

The weed was first spotted in Palmerston North during Alert Level 4 in late March 2020. The only previously known location of the weed in the Horizons Region was the Taumarunui effluent ponds, which makes this new find very concerning.

Horizons urges communities to help eradicate this pest from our region by reporting any sightings to the pest plant team by calling freephone 0508 800 800, email, or social media

Alligator Weed by Carolyn Lewis, Weedbusters

How can I identify the plant? 

You can identify the plant by its low-growing, dark green, waxy leaves that are generally arranged in opposite pairs along reddish hollow horizontal stems. Stem and leaf sizes can vary.

The plant has papery white flowers that pop up from December to March. It does not produce any fruit or seeds.

Mukunu-wenna (Alternanthera sessilis), nahui (Alternanthera denticulate), and Ludwigia species all look similar to Alligator Weed and can be easily mistaken for it. While Primrose Willow and Willow Weed look similar to Alligator Weed, Primrose Willow has dark green stems, whereas Alligator Weed stems are green/brown with a red tinge.

However, please still report the sighting if you are unsure. We can always help identify the plant through photos.

How does it spread? 

Alligator Weed can spread by water movement such as floods or tides, eel nets and boats. It spreads on land from soil movement, and equipment such as diggers and farm machinery.

When disturbed, the weed breaks up into small fragments and when those fragments are transported to new sites, it regrows very quickly. Please make sure to always check for these fragments on all the above-mentioned equipment.


How do I help prevent the spread?

If you think you may have spotted the weed, always contact us either through email, social media or on freephone 0508 800 800 and we will take care of it. If you do happen to contact us through social media or email, please include the location and if possible, some photos.

As previously mentioned, the pest spreads through fragments being transported. Always check boats trailers, vehicles and equipment before travelling between waterbodies.

Check farm drains regularly, especially in the lower Manawatū river for plants.

What should farmers do to prevent the spread?

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

How does Horizons control it?

The method is all dependant on the site itself. However, the standard method is to first use herbicide, then remove the material and dig out the roots. The site is closely monitored afterwards. This process will and can be repeated if need be.

Is the herbicide poisonous to our waterways?

No. If Alligator Weed is found in any water-based sites, Horizons will apply for resource consent to enable to the use of herbicide options cleared for use over water by the Environmental Protection Agency.

Mangaone Alligator Weed incursion response progress update

Date Sites and Progress
30/03/2020 Milson drain above Apollo Pde bridge.
30/04/2020 Mangaone St from confluence with Milson drain to Manawatū – survey begins and sites discovered throughout.
20/05/2020 All known locations of plants removed by digger or hand and securely transported in wrapped bags to site for composting before permanent disposal
22/05/2020 Survey of Manawatū river begins.
09/06/2020 Survey of Manawatū river from Mangaone confluence to Foxton estuary completed with no plants found.
29/06/2020 45,000 leaflets mailed out to residents in the Mangaone catchment from Palmerston North, Shannon, Foxton towns and surrounding rural areas asking for reports of possible sightings of Alligator Weed.
01/07/2020-08/07/2020 Horizons Regional Council staff met with Source2Sea community group representatives, and spoke at the DairyNZ discussion group to seek help with locating the plant within groups and farmer areas of interest.
17/07/2020 Small site discovered in thick vegetation in the Mangaone Stream. The new site is downstream from a previously large controlled site.
Last updated 21 July 2020