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Alligator Weed Attacks Maize Field
Nov 24, 2005
Author: Environment Bay of Plenty

A Tauranga maize field is now officially off-limits to visitors after Environment Bay of Plenty staff found a potentially devastating noxious weed growing in it.

Alligator weed (Alternanthera philoxeroides), an aggressive aquatic plant that also grows on land, was discovered during a routine pest plant inspection of a property in Welcome Bay. The main area of infestation covered about 12 square metres, with scattered fragments elsewhere in the paddock.

Pest plant coordinator John Mather told the council’s operational services committee meeting today (Thursday) that staff immediately started work to destroy the weed. The site was also declared a restricted zone under the Biosecurity Act 1993, which gives Environment Bay of Plenty the legal right to control movement on and off the property. “Alligator weed can grow easily from a small stem or fragment and, because of this, is often spread within and between ploughed fields. It may well have been brought to the site on machinery.”

Mr Mather says that, if alligator weed established in the Bay of Plenty, it could seriously impact on farming activities and block waterways. “So we have to be very very careful.”

After the discovery, staff launched an investigation to trace the origin of the alligator weed. They interviewed contractors and former lessees of the block and inspected all cultivated maize locations in the Welcome Bay, Papamoa and Te Puke areas. However, they did not find any more sites, Mr Mather says. “We talked to all the contractors, and they’ve been very helpful and will be keeping a lookout for it in all the paddocks they have cropped in maize.”

A native of South America, alligator weed is easily recognised by its white, papery, clover-like flowers. Its leaves are shaped like teardrops and grow in tiers opposite each other along hollow stems. It is an aquatic perennial that forms dense floating mats on slow moving water bodies. It can hinder access to water bodies and increase the risk of flooding. As in this case, it may also establish as a terrestrial plant, invading pasture and cropping land. It can form a dense tangle of roots up to 1m deep.

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Alligator weed is already widespread in Northland and has been discovered in Auckland and the Waikato. The Bay of Plenty hosts a handful of land-based sites, including one near Edgecumbe, all of which are under intensive long-term control programmes. Environment Bay of Plenty has already had one alligator weed scare this year, when clumps were discovered growing alongside a canal flowing into the Tarawera River near Edgecumbe.

Mr Mather says alligator weed is very difficult to control because of its extensive and deep root systems. “We will be monitoring and carrying out annual control work on this new site for many years to come,” he says.