Report: TR05/38

Author: Environment Waikato Biosecurity Group

Abstract

Alligator weed (Alternanthera philoxeroides) has been described as one of the world's worst aquatic and terrestrial weeds. Originally from Brazil, alligator weed is now a major weed problem in parts of the United States, Australia, China, India, Thailand and Indonesia. It was first discovered in New Zealand in the northern Wairoa River, near Dargaville, in 1906. Alligator weed is an accidental introduction to New Zealand and is believed to have arrived with ship ballast. It is now widespread in Northland where it has become a serious weed of waterways and cropping land. It is common in Auckland waterways and there are one or two sites in the Bay of Plenty, Christchurch and Taumaranui. There are several infestations in a range of habitats in the Waikato.

Aquatic alligator weed takes root in shallow water and on stream banks, or floats as 'rafts' of vegetation. Stems can grow up to 10 metres out over water, and up to one metre in height. Aquatic alligator weed grows very quickly and can rapidly overtake streams and drainage canals, clogging them with vegetation and trapping sediments, thus increasing the risk of flooding. In natural wetland areas alligator weed is a threat to native flora and fauna, and would be extremely difficult to eradicate in such a habitat once established. On rivers and lakes it can form large mats, altering natural vegetation, interfering with recreational activities and poses a threat to hydro power.

As a terrestrial weed, alligator weed is capable of rapid growth, particularly under warm, wet conditions and can out-compete crops and pastures. On land it tends to have a different growth form from that of the aquatic form, being more prostrate and developing a very deep root system. Alligator weed will take up heavy metals from the soil and is believed to be toxic to livestock.

Alligator weed is a subtropical plant, flourishing in warm, damp conditions with growth slowing considerably over winter months. Reasonable frosts will knock back all vegetation, however underground stems and the root system persist and vigorous re-growth occurs in spring.

Alligator weed's 'weediness' can be attributed to its tolerance of a wide range of habitats, very rapid growth rate under favorable conditions, extremely efficient vegetative reproduction and its ability to quickly bounce back from conventional weed control techniques. It does not set seed in New Zealand, but the plant stems break easily and rapidly form root systems. Alligator weed is subsequently spread through the transport of stem fragments naturally via water movement, and through human activities such as boating, fishing, drainage works, agricultural contracting and soil movement - particularly soil movement associated with subdivision development.

Few herbicides are effective against alligator weed and most do not translocate very effectively into the root system so that, even though above ground vegetation dies off, underground parts remain viable and vegetation very quickly reestablishes. This is the situation with both glyphosate (Roundup) and diquat, the only herbicides registered for use over water in New Zealand. While glyphosate knocks back alligator weed, where it is associated with other vegetation treatment with glyphosate appears to give it a competitive advantage as it recovers much more quickly than the other vegetation.

There have been many observations by Environment Waikato Biosecurity Plant Pest Contractors (BPPCs) and Biosecurity staff of alligator weed, post herbicide treatment, where the plant has been defoliated but the stems remain turgid and appear to be viable. Without follow-up treatment these stems quickly produce re-growth. There have been reports of treatment with glyphosate increasing dispersal of alligator weed due to increasing fragmentation of stems.

Alligator Weed control 2004/2005 [PDF, 628 KB]