Chomping away at alligator weed a big mission in Manawat

The arrival in Manawat of what looks like a lovely succulent to put in your garden or terrarium, but is actually a highly invasive weed, has environmental watchdogs on high alert.
They fear the weed could cause widespread harm to farmers, rivers and even one of New Zealands most significant wetlands.

Horizons Regional Council has been patrolling 10 kilometres of the Mangaone Stream regularly since the discovery of alligator weed in and along the water.

High on the worlds hit list of weeds, alligator weed has already taken hold in Waikato and further north.


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It can dramatically alter wetlands, rivers, dams and drains, while it also absorbs heavy metals from soil, making it especially risky to animals which may eat it.

Horizons pest plant team leader Craig Davey said the weed was found in the Horizons region in the 1990s, when contaminated reeds were used as part of a wastewater treatment project in Ruapehu.

It made its way into a maize paddock, but had since been culled back.

The Mangaone Stream discovery, made during the coronavirus lockdown in 2020, was unable to be traced to a specific source, as the DNA of the plant did not match the weed in Waikato or Auckland, he said.

DAVID UNWIN/Stuff
Horizons Regional Council pest plant team leader Craig Davey at one of 70 sites where alligator weed has been found along the Mangaone Stream.

It was used as a herb in some cooking and was known to survive in fishing nets and other water equipment, so may have ended up in the stream through all sorts of ways, he said.

It could have been an over the fence' drop.

Staff found 70 sites where alligator weed was growing after walking 10km along the stream from the Apollo drain to the Manawat River.

The weed was a problem child" as it lived on land and in the water while appearing differently in each habitat.

It also easily dispersed itself if sprayed with weed killer and only needed a small piece of plant to grow.

While it did not spread seed, it would travel far if a lot of water flowed through the stream.

That was the primary concern for Horizons, as the Mangaone Stream flows into the Manawat River south of Palmerston North.

The rivers spillway during floods goes across highly productive farmland, while the river itself flows to the Manawat Estuary, a RAMSAR site and home to a variety of birds including godwits that migrated annually from Alaska and Siberia.

DAVID UNWIN/Stuff
Alligator weed can survive in water such as here, in the Mangaone Stream in Palmerston North and on land, having a different appearance in each habitat.

That was why Horizons recently dug large amounts of gravel and dirt out of the stream, dumping the material in the Bonny Glen landfill near Turakina, Davey said.

Environmental groups were being taught what to look for, but Horizons wanted the general public to be aware too, he said.

We want people to understand and have the knowledge about this, but not to try removal.

Alligator weed was extremely difficult to destroy, with it jumping out of bags when Horizons had removed it and put it inside two plastic bags, Davey said.

We don't want river sites to become compost sites, silage sites or paddock sites.

People can report possible alligator weed sightings to Horizons by calling 0508 800 800.

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