Growth of aquatic alligator weed (Alternanthera philoxeroides) over 5 years in south-east Australia

The largest known infestation of alligator weed Alternanthera philoxeroides (Mart.) Griseb. in Victoria, Australia, was reported in January 2009 in an urban pond. To determine how long the infestation had been present, high quality digital aerial images (orthophotos) were gathered for the site. Since all infestations are subject to ongoing eradication programs in Victoria, historical orthophoto records provide a unique opportunity for retrospective analysis, to calculate and report on the uncontrolled growth of aquatic alligator weed. Using geographic information system software (GIS), orthophotos were visually assessed to delineate the area of infestation for each year, from which annual increases in area were calculated. The infestation increased in area from ca. 0.029 ha in December 2004 to ca. 0.73 ha in December 2009, to cover 33% of the 2.2 ha water body. The annual area expansion was 200% for the first year of record. This reduced each year, to 22% at the end of the five year period. The mean lateral rate of expansion for floating alligator weed over the five years was 4.3 m (SD 2.2) annually. The average biomass of alligator weed at the site in summer 2010 was 4.9 kg dry weight m. Using the area of infestation from the December 2009 orthophoto the total estimated biomass in the pond equated to 35.6 tonnes dry weight.