Alligator Weed (Alternanthera philoxeroides)

While the South Island is trying to cope with ‘Didymo’ algae, up north there’s an equally nasty hitchhiker called Alligator Weed. 

This really nasty weed apparently arrived here in the 1880’s, dropped by ships coming from South America and emptying their ballast while here.  For the next century they coped with it in Northland and parts of Auckland, and it stayed mainly an aquatic plant which formed large dense mats of weed in drainage ditches, canals, rivers, lakes, reservoirs, swamps and wet pastures.

These days it has both moved onto land, hiding out in maize crops in particular, and also found ways of hitching rides on contractors’ machinery and moved its operations further south.  In the last year it has been found in Tauranga, Cambridge, Morrinsville, Welcome Bay, Te Kauwhata, Matamata and other parts of the Waikato.

Not only does it grow fast (it can double its biomass in 50 days) but it does best in nutrient rich waters and can even tolerate salinity of up to 30% in flowing water.  It will out-compete pastures and crops and is toxic to livestock.

Alligator weed is a hairless perennial, which has leaves growing in opposite pairs along a creeping hollow stem which can grow horizontally on water to a length of up to 10 metres, with nodes 5-10cm apart.  Fibrous roots are produced along the stem which reach down into the water or soil, with those in soil thicker and longer, extending over 50cm in depth.  The stems are green/brown but often red-tinged.  Green waxy leaves have a conspicuous midrib, are up to 7cm long and up to 2cm wide.  The white flowers on long stalks appear December to March and resemble clover, but seeds are not viable, with reproduction being entirely vegetative.

On waterways it has proved almost impossible to get rid of.  Mechanical harvesting with scoops or draglines gives only temporary relief and is not recommended.  The stem fragments that break off merrily float away and start growing elsewhere, and what is collected in parts of the machinery can get transported to the next site.  Spraying with herbicides does not work well either, as it only affects the top growth, and the submerged stems simply regrow even faster.  Also the Resource Management Act has a bit to say about which herbicides can be used and where (contact your local Regional Council for details).

Biological control agents such as the Alligator Weed Beetle have been introduced with some success on aquatic infestations, but these effects are seasonal only.  Both larvae and adults feed on the foliage, and with a large enough population they can defoliate the plants entirely.  They are less successful on terrestrial plants or areas where the weed is submerged by regular flooding.

When growing in maize it can hide out until the crop is harvested, and if cut for silage gets entwined with the stalks and can hitch a ride to wherever the crop is transported to.  This has already occurred with a crop transported to Taupo, an area previously free of alligator weed.  If alligator weed is found on a property, any maize grown can only be taken for grain, and restrictions are placed by the Regional Council on what can happen on that property.

It can also be moved into new urban subdivisions when contaminated soil is brought in on truckloads or on contaminated machinery.  It can be controlled with several applications of metsulfuron within a year, but even with regular and persistent treatments eradication is likely to take at least five years and the potential for re-establishment remains even longer.

So if you happen to discover Alligator weed on your property, contact your Regional Council rapidly for advice and possible help in getting rid of it as fast as possible.  In the Waikato the contact number is 0800 800 401 (Biosecurity Group) and in the Bay of Plenty it is 0800 368 267 (Pest Plant Officer)

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