Abstract

Tropical signalgrass, Urochloa subquadripara, (syn. Brachiaria subquadripara, Bayer code BRASU, also called smallflowered Alexandergrass), is the most serious weed in south Florida St. Augustinegrass turf, according to sod producers. It is the second most serious weed in bermudagrass golf turf, based on survey of 236 south Florida superintendents. Though confused with crabgrasses (Digitaria spp.), tropical signalgrass leaves are more shiny and less flexuous. In new sod fields, tropical signalgrass germinates more rapidly than crabgrasses, remaining appressed to the ground while stolons form. Stolons interweave with cultivated St. Augustinegrass, then grow on top, creating an unsightly appearance. The plant is killed by frost. Tropical signalgrass fills voids left by other weeds. In a preemergence herbicide experiment, there were more (P < 0.05) tropical signalgrass seedlings in plots treated with atrazine at 1.1 kg ha-1 (90.1 seedlings m-2) than in untreated plots (62.6 seedlings m-2). The most effective preemergence treatments were oxadiazon at 2.3 kg ha-1 and pendimethalin at 3.4 kg ha-1 applied 8 days after plug planting, providing early postemergence control of 2-leaf seedlings. The most effective management of tropical signalgrass was preemergence herbicide and early, 25-30 days postplanting, applications of asulam at 1.2 kg ha-1. Seedling numbers were reduced by 90-95%, but hand hoeing and spot treatment were still required.