Alligator weed in Pauanui storm pond a risk to Coromandel waterways

Benson Lockhart says alligator weed is not a plant anyone would want to spread in the Coromandel Peninsula, due to its invasiveness and impacts on waterways.

The appearance of alligator weed at a storm retention pond in Pauanui highlights what can happen when lawn clippings and garden waste are dumped rather than securely disposed of at an approved landfill or transfer station.

The invasive pest plant, which can clog waterways and spreads aggressively from even the smallest stem fragments, has been found in the pond on Holland Close growing among other hard to control pest plants canna lily, arum lily and yukka.

Waikato Regional Council pest plants biosecurity officer Benson Lockhart says alligator weed is not a plant anyone would want to spread in the Coromandel Peninsula, due to its invasiveness and impacts on waterways, and it likely arrived unintentionally by garden waste dumping or via machinery.

“You can see here we have a load of grass clippings that is being dumped at the side of the pond, and we also we have arum lily, canna lily and yukkas all growing around the pond,” says Benson.

Benson Lockhart sprays the alligator weed every six months to control it.

“Alligator weed is also a pastural plant, so it could have got here in the grass clippings, but the other plants almost certainly established here after being dumped as garden waste.”

In the Waikato region, alligator weed is found mainly in and on the margins of the Waikato River: in wetlands, on farms, market gardens and urban properties. It has become a significant problem in new subdivisions in Hamilton city.

Alligator weed is also present at sites along the margins of other waterways near Waihi and Waikino.

The Pauanui site was discovered by council staff in November 2022 during surveillance for another pest plant, evergreen buckthorn.

“This site is our top priority to control and eradicate since it is so far away from the other sites and being the only one in the Coromandel,” says Benson, who sprays the alligator every six months.

“As this is a storm retention pond, there is a risk that the alligator weed will spread with the overflows into the Tairua River.

Canna lillies, yukkas and lawn clippings around the stormwater retention pond.

“Once it gets in the river, the fragments get spread by the moving water and it can grow rapidly to form floating mats.

“This makes alligator weed very difficult to control, and left unchecked it’s capable of blocking a waterway, so no boating or kayaking or swimming, and it’d affect habitat for native fish species too, like whitebait spawning areas.”

Alligator weed can also out-compete pasture and crops, affecting farm production and profit, and is toxic to stock, causing blindness and other health problems.