Alligator weed creeps into Manawatū

Alligator weed grows fast and can be easily transferred between different waterways and spread onto farms.
Alligator weed grows fast and can be easily transferred between different waterways and spread onto farms. Photo credit: Supplied / Horizons Regional Council

By Robin Martin for RNZ

The public are being urged to report sightings of one of the world's most invasive pest plants - alligator weed - in the Mangaone Stream and lower Manawatū catchment.

Alligator weed, which has only recently been identified in the region, is known to take over pasture, is toxic to stock, clogs waterways and creates flood risk.

Manawatū Whanganui Regional Council biosecurity programme coordinator for plants Craig Davey said alligator weed spread aggressively and thrived on both water and land.

"Alligator weed can spread by water movement such as floods, eel nets and boats. However, it can also spread via farm machinery, soil movement, and diggers," Davey said.

"Once in a waterway stem fragments can disperse and grow rapidly, destroying the environmental, recreational and aesthetic values we hold dear. It can also double in area size in just two months."

The area of concern includes the Mangaone Stream area north of Colyton through Palmerston North to the Manawatū River, and down to Foxton Beach.

The weed was first spotted in Palmerston North during alert level 4 in late March. The only previously known location of the weed in the region was the Taumarunui effluent ponds.

Davey said the plant could be identified by its low-growing leaves that are generally arranged in opposite pairs along hollow reddish horizontal stems.

"We are asking the public to check boats, trailers, vehicles and equipment for fragments before travelling between water bodies to ensure the fragments are not transported to new sites.

"We also implore farmers to insist that all contractors practise 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."

Davey urged duck shooters, hunters, farmers, landowners and occupiers to report any possible sightings of alligator weed to the pest plant team at Manawatū Whanganui Regional Council, more commonly known as Horizons.

"If you suspect a plant may be alligator weed, do not remove it yourself. Call the pest plant team and we will take care of it.

"Removing the plant yourself may result in further spreading of it. We also need to keep track of where this pest has been seen."

Sightings can be reported by emailing Craig Davey at, by messaging through the Horizons social media pages, or calling freephone 0508 800 800.

For more information about alligator weed, go to the council's website.