Coreopsis tinctoria (Asteraceae) is a widely-distributed North American prairie flowering C₃ annual. We hypothesized priority effects of planting C. tinctoria before a C₄ grass would decrease interspecific competition. Coreopsis tinctoria was grown in south-central Texas (29°35′N, 98°37′W) with Bouteloua curtipendula (Poaceae), a widespread C₄ prairie grass. The two species were planted in a de Wit replacement series with plant ratios of 12:0, 10:2, 8:4, 6:6, 4:8, 2:10, and 0:12 (C. tinctoriaa: B. curtipendula). There were five planting offsets with C. tinctoria planted 60 d before B. curtipendula (–60), 30 d before (–30), at the same time (0), 30 d after (30), and 60 d after (60). There were significant species by offset, species by frequency, and offset by frequency interactions. Coreopsis tinctoria produced the greatest total dry mass per plant in the 60 d before offset. Coreopsis tinctoria was competitively favored when planted 60 d before B. curtipendula and B. curtipendula was favored when planted 30 and 60 d before C. tinctoria. Results suggest a synergistic benefit from both spatial gaps and priority effects.