Field experiments were conducted to determine the effectiveness of herbicides in controllingRottboellia cochinchinensis(Lour.) W. D. Clayton andCyperus rotundusL. in upland rice(Oryza sativaL.) at different moisture regimes as imposed by a line source sprinkler system. Preemergence application of pendimethalin [N‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitrobenzenamine] was highly effective in controllingR. cochinchinensisirrespective of soil moisture after herbicide application. Bentazon [3‐(1‐methylethyl)‐(1H)‐2,1,3‐benzothiadiazin‐4(3H)‐one 2,2‐dioxide] and 2,4‐D [(2,4‐dichlorophenoxy)acetic acid] applied at postemergence effectively controlledC. rotunduswhen moisture supply was well above pan evaporation. These herbicides also had no adverse effect on rice stand and resulted in higher yield over thecontrol.Water application rates above upland pan evaporation for a season‐long period was essential to obtain a high response to weed control either by herbicidesorhand weeding. The data suggest that proper weed control by herbicides or hand weeding will not result in high upland rice grain yields if moisture level from rains fall below the critical level.