Abstract

Ludwigia peruviana is an aquatic shrubby species introduced from the Americas and spreading to wetlands in coastal south-eastem Australia. It has become dominant, replacing much of the former vegetation in the Botany Wetlands, a series of shallow urban swamps and lakes near Sydney, Australia. Studies of L. peruviana were essential to the development of a management plan for the Botany Wetlands. Ecological studies were conducted in situ and supplemented by a series of germination and growth experiments in controlled environments. Flowering peaked in early autumn. Seed production for 1990-91 was ~ 450000 seeds m^-2 and there was also ~ 65000 seeds m-2 in the soil seed bank and ~ 300000 seeds m^-2 in old fruits that remained on the stems over winter. The seeds are ~ 0.8 mm long, weigh ~ 0.05 mg, are hydrophobic, usually germinate while afloat, and are easily dispersed by water, machinery and birds. Viability of fresh seed was 99% (tetrazolium dye test), and - 20% of seeds were dormant. Dormancy appeared to break down within two years. Propagation and dispersal may also occur by means of stem pieces that produce new shoots. New plants from seed, seedlings or stem fragments may become established on strand lines or form floating islands that may eventually become stranded. Management needs to focus on control of the seedlings and soil seed bank.