Control Techniques and Management Strategies for the Problematic Navua sedge (Cyperus aromaticus)

Abstract

AbstractNavua sedge, a member of the Cyperaceae family, is an aggressive weed of pastures in Fiji, Sri Lanka, the Malay Peninsula, Vanuatu, Samoa, the Solomon Islands, and Tahiti and is now a weed of pastures and roadsides in north Queensland, Australia. Primarily restricted to areas with an annual rainfall exceeding 2,500 mm (98.4 in), Navua sedge is capable of forming dense stands, smothering many tropical pasture species. Seventeen herbicides were field tested at three sites in north Queensland, with glyphosate, halosulfuron, hexazinone, imazapic, imazapyr, or MSMA as the most effective for Navua sedge control. Environmental problems, such as persistence in soil, lack of selectivity, and movement off-site, may occur using some herbicides at the predicted 90% lethal concentration (LC90) control level rates. A seasonality trial using halosulfuron (97.5 g ai ha−1[1.4 oz ac−1]) gave better Navua sedge control (84%) by spraying in March to September than by spraying at other times (50%). In a frequency trial, sequential glyphosate applications (2,160 g ae ha−1) every 2 mo was more effective for continued Navua sedge control (67%) than a single application of glyphosate (36%), although loss of ground cover would occur. In a management trial, single applications of glyphosate (2,160 to 3,570 g ae ha−1) using either a rope wick, ground foliar spraying, or a rotary rope wick gave 59 to 73% control, whereas other treatments (rotary hoe, 3%; slashing, −13%; crushing, −30%) were less effective. In a second management trial, four monthly rotary wick applications were much more effective (98%) than four monthly crushing applications (42%). An effective management plan must include the application of regular herbicide treatments to eliminate Navua sedge seed being added to the soil seed bank. Treatments that result in seed burial, for example, discing, are likely to prolong seed persistence and should be avoided. The sprouting activity of vegetative propagules and root fragmentation also needs to be considered when selecting control options.