Prolonged summer flooding switched dominance from the invasive weed Lippia ('Phyla canescens') to native species in one small, ephemeral wetland.

Lippia ('Phyla canescens', Verbenaceae) is an invasive perennial forb from South America that is now widespread throughout the Murray Darling Basin in Australia, threatening the biological values of internationally significant wetlands (Ramsar sites) and the productivity of the grazing industry (Earl 2003). In the Gwydir Wetlands in north western NSW, the spread of Lippia has been associated with large and widespread flooding that occurred in the late 1990s (McCosker 1998). Lippia dominance is believed to be favoured by the altered flood regimes associated with river regulation, namely a reduction in the frequency and duration of flooding. Studies have found Lippia to occur in infrequently flooded sites (Blanch et al. 1999; Mawhinney 2003) and at higher elevations in the floodplain (Blanch et al. 2000). Glasshouse studies have found that Lippia growth is stunted at inundation depths of 20 cm, although no mortality was reported (McCosker 1994; Hobson 1999). This has lead to a widespread belief that restoration of a more 'natural' flood regime may assist in Lippia control. However, few data are available to support this notion.