Biology, Ecology and Management of the Invasive Navua Sedge (Cyperus aromaticus)—A Global Review

Navua sedge (Cyperus aromaticus (Ridley) Mattf. & Kukenth) is an invasive perennial sedge, native to tropical Africa, which is threatening many natural ecosystems and agroecosystems, especially in northern Queensland, Australia. Crop and pasture production have been impacted by Navua sedge and it is also directly causing reductions in dairy and beef production in affected regions. This review documents the biology, ecology and potential management options to minimise the spread and impact of Navua sedge. The weed reproduces both sexually (seeds) and vegetatively (via underground rhizomes). Its tiny seeds can be spread easily via wind, water, vehicles, farm machinery and animals, whilst the rhizomes assist with establishment of dense stands. The CLIMEX model (which uses distribution and climate data in native and novel ranges) indicates that in Australia, Navua sedge has the potential to spread further within Queensland and into the Northern Territory, New South Wales and Victoria. Several management strategies, including mechanical, chemical and agronomic methods, and their integration will have to be used to minimise agricultural production losses caused by Navua sedge, but most of these methods are currently either ineffective or uneconomical when used alone. Other management approaches, including biological control and mycoherbicides, are currently being explored. We conclude that a better understanding of the interaction of its physiological processes, ecological patterns and genetic diversity across a range of conditions found in the invaded and native habitats will help to contribute to and provide more effective integrated management approaches for Navua sedge.

Differential germination response of Navua sedge (Cyperus aromaticus) populations to environmental factors

Navua sedge (Cyperus aromaticus) is a hard to control C4 perennial weed species in tropical regions of Australia. Knowledge of its seed biology could help to develop integrated weed management programs for this species. This study was conducted in laboratory and screenhouse conditions to evaluate the effect of alternating day/night temperatures, light, pretreatment high temperatures, burial depth, and flooding depth on the germination and emergence of two populations (Ingham and Tablelands) of C. aromaticus. Both populations germinated at temperatures ranging from 20/10 to 35/25 C; however, the Ingham population germination (76%) was greater than the Tablelands population (42%) at the highest temperature regime (35/25 C). None of the populations germinated at 15/5 C. Darkness completely inhibited germination in both populations, suggesting that the seeds are positively photoblastic. Seeds (dry and wet) of both populations germinated after exposure to pretreatment temperatures of up to 100 C for 5 min. After pretreatment at 150 C, only the Ingham population germinated, and the germination of dry seeds (62%) was greater than wet seeds (1%). None of the populations germinated after the exposure to 200 C. For both populations, maximum germination was observed for seeds at 0 cm, and a burial depth of 0.5 cm completely inhibited emergence of the Tablelands population and 2.0 cm inhibited germination of the Ingham population. A flooding depth of 10 cm greatly reduced emergence in both populations compared with 0 cm (62 and 78%) but 12 to 14% of seedlings still emerged, suggesting the need to integrate flooding with other management tools. The results also suggest that the Ingham population may have a greater potential to spread into new areas or become more invasive than the Tablelands population. Knowledge gained from this study can be used to manage C. aromaticus by fire/burning, tillage, and flooding.

Germination biology of three populations of Navua sedge (Cyperus aromaticus)

Navua sedge [Cyperus aromaticus (Ridley) Mattf. & Kük.] is an aggressive perennial sedge native to equatorial Africa that has become problematic in many Pacific islands and wet, tropical Queensland, Australia. It has had a significant impact on the livestock-grazing industry, sugarcane (Saccharum officinarum L.) and banana (Musa acuminata Colla) plantations, and various other ecosystems. A laboratory-based research investigation was conducted to understand germination and emergence requirements under various environmental conditions of three geographically varied populations sourced from South Johnstone (SJ), Mackay (M) and Nyleta Creek (NC) in Queensland. Germination was identified to be stimulated by light, with no germination recorded under darkness. Populations SJ and NC had optimal germination at alternating temperatures of 25/15, 30/20, and 35/25 C, whereas population M had optimal germination at 25/15 and 30/20 C. All populations recorded greater than 85% germination at all pH levels tested. Seeds of population SJ were more sensitive to salinity compared with populations M and NC, with SJ showing no germination at 100 mM, whereas populations M and NC had 23% and 9% germination, respectively. An inverse relationship was observed between osmotic potential and germination, with no germination recorded at osmotic potentials below −0.8 MPa in any population, indicating moisture availability is a critical requirement for germination. Exposing seeds to 120 C radiant heat completely inhibited germination in populations M and NC, whereas 3% of population SJ germinated following a 180-s exposure at 120 C. Seedling emergence decreased as planting depth increased. Emergence was greatest for seeds on the soil surface or at 0.5-cm burial depth, consistent with germination being stimulated by light. Knowledge of these biological characteristics of C. aromaticus seed germination will assist in investigation of suitable control actions for this species, particularly in the early stage of its invasion into new areas, and will contribute to significant reduction in the soil seedbank.

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

Navua 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.