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

Click on the storymap below to learn more

Mangaone Alligator Weed incursion response progress update

Date Sites and Progress
  Last updated on 18 May 2022
9/05/2022 Mangaone Stream surveyed from Highbury Bridge to the Manawatū River. One plant found, growing approximately 1.5m up the bank.
12/04/2022 Plant removed using a digger and transferred to Bonny Glen.
1/04/2022 Mangaone Stream rafted and one plant found upstream of the Pioneer Highway Bridge.
31/01/2022 Mangaone Stream inspected from Apollo Drain to the Manawatū River. Two plants found. Risk of downstream spread remains moderate. Huge flows in December modified the stream considerably.
23/11/2021 148 properties in close proximity to our original site were inspected for alligator weed. No plants were found, so the hunt for the source continues.
21/10/2021 Mangaone Stream inspected from Apollo Drain to the Manawatu River. No plants found. Historic sites are clear and the stream is looking very good. Due to the large amount of rain recently high flows have cleared vegetation from the lower part of banks. Next inspection in late November for the spring flush.
11/06/2021 Mangaone Stream waded from Apollo Drain to Milson Line to find the source of the fragment found the day prior. One significant site found just above the Apollo/Mangaone confluence. Temporary fencing has been constructed around the site to catch any fragments.
10/06/2021 Mangaone Stream rafted from Apollo Drain to Manawatū/Mangaone confluence. One fragment found just above the Rangitīkei Line Bridge.
2/06/2021 Manawatū River surveyed. Nothing found.
5/05/2021 Mangaone Stream rafted from Apollo Drain to Manawatū/Mangaone confluence. One new site found.
04/03/2021 Excavation of sites at Benmore Ave and Highbury Ave bridges (as seen on TV One News).
3/02/2021 Mangaone Stream rafted from Apollo Drain for Manawatū/Mangaone confluence. Significant regrowth found at a handful of sites.
20/01/2021 Alligator weed dog visited the Apollo Drain, Mangaone Stream and Moutua floodplains. One plain detected in the Apollo Drain.
18/01/2021 eDNA samples taken from the Mangaone Stream and Moutua floodplains. One plant detected in the Apollo Drain.
17/12/2020 Mangaone Stream waded from Apollo Drain to Manawatū/Mangaone confluence. Five sites of regrowth found.
10/11/2020 Horizons team met with Waikato Regional Council to discuss their alligator weed control programme and visit sites of significance.
9/11/2020 Mangaone Stream waded from Apollo Drain to Manawatū/Mangaone confluence. 12 new sites and 13 sites of regrowth (77 sites in total).
20/10/2020 Presentation to DairyNZ Rural Professionals workshop, Manawatū Golf Club.
25/09/2020 Post lower Manawatū surveillance letter sent to 70 land occupiers alerting them to the issue and asking for vigilance and Horizons is here to help.
6/08/2020 Post wading of Mangaone Stream exercise, sites increased to 65. Wading clearly best method of discovery.
29/07/2020 Removal of composting alligator weed post physical removal exercise. Permanently placed at Bonny Glenn, buried 20m deep.
28/07/2020 Inspections begin of 30 likely catch points or accumulation areas below Mangaone confluence along Manawatū River, drains and Moutua spillway inundation zone.
17/07/2020 Small site discovered in thick vegetation in the Mangaone Stream. The new site is downstream from a previously large controlled site.
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.
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.
09/06/2020 Survey of Manawatū river from Mangaone confluence to Foxton estuary completed with no plants found.
22/05/2020 Survey of Manawatū River begins.
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.
30/04/2020 Mangaone St from confluence with Milson drain to Manawatū – survey begins and sites discovered throughout.
30/03/2020 Milson drain above Apollo Pde bridge.