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.

This study examined if one grazing strategy (namely seasonal rest) was effective in the control of the invasive forb 'Phyla canescens' (Kunth) Greene (hereafter lippia). We examined if rest from grazing could increase the competitiveness of native palatable species by allowing time to recover from defoliation, thereby altering competitive interactions between native species and lippia. In a field trial, we manipulated cattle grazing to determine its effects on the biomass of lippia and native species. We compared rest from grazing at different times of the year with year-long grazing (low intensity continuous and high intensity short duration) and no grazing (permanently excluding large grazing animals). Experimental plots were stratified into different hydrological areas (approximately annual flooding and flooded less than once every 5 years) to include flood dynamics in the management scenarios. We detected no negative impacts of seasonal rest on lippia, but some positive effects on native species. We found that complete exclusion from grazing in areas that already have substantial lippia invasion (and no flooding) may actually favour the expansion of lippia (at certain times). This study does not suggest that grazing management cannot be used as a tool for lippia control - simply that seasonal resting had no effect over a 3-year period. This is likely due to the dynamics of a boom-and-bust landscape in which if the abiotic conditions are not suitable for growth, then native species will not grow whether grazed or rested.