Unchecked growth of aquatic alligator weed (Alternanthera philoxeroides) over a 5 year period in a Melbourne pond.

Alligator weed, Alternanthera philoxeroides (Mart.) Griseb.), is a serious weed that has invaded a wide range of habitats in Australia. It poses a significant threat to Australia’s waterways, wetlands, floodplains and irrigation systems and has the potential to become far more widespread. Alligator weed is a stoloniferous and rhizomatous perennial that grows rapidly in both terrestrial and aquatic habitats (Sainty et al. 1998). The largest known infestation of alligator weed in Victoria was reported in January 2009 in a 2.2 ha urban pond used for irrigation purposes. Site characteristics included stable and slow moving water conditions, high nutrient input from urban stormwater runoff (total nitrogen and phosphorus averaged 1.48 mg L 1 and 0.073 mg L 1, respectively, from monthly low flow sampling between 2004–2009) and low interspecific competition with minimal marginal over story vegetation. These provided good growing conditions for alligator weed at the site. To determine how long the infestation had been present, high quality digital aerial images (orthophotos) were gathered for the site. Unchecked growth rates of alligator weed are unknown in Victoria, as all known infestations are subject to control measures. These orthophotos provided a unique opportunity to calculate and report on the uncontrolled growth rate of aquatic alligator weed over consecutive years. Orthophotos were analysed using a GIS application to determine the area occupied by alligator weed Unchecked growth of aquatic alligator weed (Alternanthera philoxeroides) over a 5 year period in a Melbourne pond