RNZ reports that AgResearch is teaming up with regional councils and Rural Contractors New Zealand to collate information on alligator weed in a bid to better understand its spread.

Often dubbed ‘the worst weed in the world’, alligator weed has been in New Zealand for more than a 100 years but had been contained to Northland.

In recent years, however, it has been found in Waikato, Manawatū, Auckland, Tauranga and now Hawke’s Bay.

The agricultural sector is increasing risk from alligator weed infestation because of the movement of crops and agricultural contracting equipment.

The Waikato Regional Council website explains that the weed does not set seed in New Zealand but spreads aggressively from even the smallest stem fragments. It can double in area in less than two months.

Alligator weed threatens farms, market gardens and urban properties (often dominating lawns) and clogs waterways and drains, increasing sedimentation and flooding risk.  Access to waterways for recreational purposes (boating, fishing) can be blocked, and plants may affect whitebait spawning areas.

It can out-compete pastures and crops, affecting farm production and profit.

Although stock will eat it, alligator weed is actually toxic and can cause blindness and other health problems.

The Waikato Regional Council urges farm owners to have a weed hygiene plan in place to minimise the risk of invasion from alligator weed and other serious pest plants.

The Bay of Plenty Regional Council says alligator weed originates from Brazil, and was first discovered in New Zealand in Dargaville in 1906 where it is thought to have arrived as a hitchhiker in ship ballast water.

Its website answers the question:  Why is it a pest?

• Alligator weed is regarded as one of the world’s most damaging weeds as it can thrive in both aquatic and terrestrial environments, has a deep root system and is not easily killed by herbicide.
• As a terrestrial weed, alligator weed grows rapidly and can out-compete crops and pastures.
• It is known to be toxic to some livestock.
• As an aquatic weed, it can rapidly spread in streams and drainage canals, forming floating mats that trap sediments, increasing risk of flooding and obstructing water usage.
• Alligator weed is intolerant of frost and will disappear in the winter months in areas where frosts are present, but will grow back each summer from its rhizomes and roots.
• Alligator weed is highly invasive and can reshoot from small fragments of the stem sections. These fragments can be dispersed by water, soil movement, dumped vegetation, eel nets, livestock, boats (or trailers) or agricultural machinery.
• It has also been known to be spread by people where it can easily be mistaken for mukunu-wenna (Althernanthera sessilis), a plant used by some ethnicities as a vegetable.

RNZ says the AgResearch programme is being funded by The Ministry for Primary Industries Sustainable Food and Fibres Fund, with $270,000 over three years.

AgResearch scientist Trevor James said there was a lot of anecdotal evidence about alligator weed and the damage it can do – so it would be good to have hard and fast data.

“What we do know is that it’s causing big problems for farmers and growers, so it will be good to get out and chat to them about it.

“It’s a semi-tropical weed and for a long time it was confined to waterways but it can also invade the riparian areas.

“So it gets into a riparian strip then a fragment is broken off and dragged by a cultivator or something into the middle of the paddock then it grows there.”

James said the weed had caused huge issues for kumara growers and happily grew under kiwifruit vines as well.

“It’s incredibly difficult to kill it because it has huge roots, people often spot spray it with a herbicide which controls it to an extent but doesn’t get rid of the problem.

“Any impacts on stock remain unclear, but the weed can certainly take over pasture and is spread by tiny plant fragments, which may survive for years.”

James said a beetle was introduced to control alligator weed in Northland and Auckland in the 1980s, but this might be proving less effective as the weed spreads to regions with cooler temperatures.

“We don’t know why it’s spreading and we may never know, but if have a better understanding of how it behaves in certain areas it will help in the fight of getting rid of it.”

He said one of the first pieces of research was to test whether alligator weed could survive in baleage, which was often exported to other regions by rural contractors and farmers.

“Human-aided dispersal is still our biggest issue with noxious weeds.”

James said it was vital if people thought they had spotted the weed that they report it to their regional council.

Rural Contractors president Helen Slattery said rural contractors had a major role to play in helping curb the spread of all noxious weeds.

“Our members take machinery from farm to farm and we also often send baleage and hay to other regions. So, we want to do all we can to make our members aware of the risks of spreading anything that can cause harm to our nation’s biosecurity.”

She said that included rural contractors carrying out machinery hygiene practices, especially in areas where there were any alerts about the presence of biosecurity risks such as alligator weed or any other noxious weeds.

Sources:  RNZ, Waikato Regional Council and Bay of Plenty Regional Council