Local Focus: Alligator Weed threatens Manawatū and Whanganui

Horizons Regional Council is concerned after Alligator Weed, a noxious plant with the reputation for being one of the world's worst pest plants, was discovered in an urban waterway in Palmerston North during lockdown.

The South American native thrives in temperate climates and Horizons Biosecurity Co-ordinator for Plants, Craig Davey is worried it will take over parts of New Zealand.

"We've got to go hard, we've got to go early to beat this thing which means full knowledge, full containment and then removal," he said.

Horizons Regional Council recently visited the Waikato where the plant has been a problem for over 20 years. Waikato Regional Council spend over half million dollars each year to contain it, because Alligator Weed poses a particular threat to dairy cows.

"Stock will eat it, they get phyto-synthesis. They want to go away and hide in the shade. If they get into the sun they get blisters, they do not eat they don't produce milk. It's a big impact on them. Stock health disappears and you might even get death of young stock," he said.

Alligator Weed grows on land and in water and is easily spread by flood events. Large matts of weed break up and get carried downstream where they self-propagate. Davey says the effects could be devastating.

"We've got 10 k's we know about, that was about 70 places that have alligator weed and then we've got 70 km of the river from the confluence of the Mangaone down the Manawatū to Foxton, and there are thousands of hectares of floodplain," he said.

Horizons is concerned the weed may have already made its way to the Manawatū and if it takes hold, it's not just pasture and livestock that will be affected.

"If we have Alligator Weed in the Manawatū and it gets to Foxton, we've got a RAMSAR site there with significant wetlands for breeding birds including godwits, spoonbills ... Alligator Weed will take away that habitat."

Much like Covid, stopping the spread of Alligator Weed requires education sanitising and contact tracing

"It's spread by people who don't know they're spreading, Davey said. "Just a fragment of a stem that has root material attached to it that can hide away in pasture grass that's put into hay, it's taken away on dirt and machinery, cultivation machinery or diggers."

Horizons say farmers and gardeners can help by being aware of where things like stock feed and garden manure comes from.

"Find out what the weed profile is. We want people in the lower Manawatū to be really concerned about what this plant can do."

Anecdotal evidence from the Waikato Regional Council shows that co-operation with local authorities is key according to their biodiversity officer Ben Elliot.

"Two neighbours that had similar problems, one has still got a big problem and the other one doesn't because he followed the simple rules with machine hygiene and understood it," Elliot said.

Horizons ask anyone who suspects they've seen Alligator Weed to leave it alone and call them, even if they're not sure. Council staff will know what to do. The 24hour free calling number is 0508 800 800.

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