STUDIES ON THE CONTROL OF WATER-DROPWORT (OENANTHE PIMPINELLOIDES) INSOUTH AUSTRALIA

Water-dropwort (Oenanthe pimpinelloides L.), a tuberous perennial herb , is currently known in South Australia from only a single locality in the Mount Lofty Ranges. There is little information on water-dropwort control, and 2 experiments were conducted to assess the effect of sow ing pasture, with or without presowing herbicides, on the control of t his weed. Phalaris (Phalaris aquatica L.) and perennial clovers were s uccessfully introduced into infested pastures by direct drilling in au tumn. Water-dropwort regenerated from seed more densely in unsown plot s than plots of established perennial pasture, suggesting that upgradi ng pastures may be a strategy to reduce the rate of spread by seed of this weed. A range of herbicide treatments applied to water-dropwort a t the stem elongation stage in spring before autumn sowing of pastures provided effective shortterm control. The best short-term control was provided by glyphosate at 1440 g a.i./ha; metsulfuron methyl at 6, 12 , and 36 g a.i./ha; and metsulfuron methyl at 12 g a.i./ha tank-mixed with glyphosate or 2,4-D amine at 720 or 1000 g a.i./ha, respectively. These treatments, and chlorsulfuron at 21 g a.i./ha, also significant ly (P<0.05) reduced water-dropwort abundance (relative to untreated ar eas) for up to 18 months after sowing and initially improved the densi ty of sown pasture species, but these improvements were not evident 14 months after resowing. Although prior season herbicide treatments con trolled water-dropwort in newly sown pastures, 2 separate applications of herbicides, in May and October, gave no better control of water-dr opwort than a single herbicide application in spring. Water-dropwort i nfestations do not appear to prevent successful direct drilling of pha laris and perennial clovers. Although pasture renovation did not provi de long-term suppression of water-dropwort, the maintenance of vigorou s pastures may reduce the rate of population growth from seedlings of this weed. Recropping restrictions may limit the role of chlorsulfuron for water-dropwort control in pasture renovation situations.