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The objectives of this research were to evaluate temporal and spatial variability in the impact of strip tillage and oat cover crop residue on Powell amaranth emergence and to determine the role of rainfall in mediating these effects. In field experiments conducted in 2010, 2011, and 2012, Powell amaranth seeds were sown in a fully factorial combination of two tillage types (strip tillage [ST] vs. full-width tillage [FWT]) and cover crop residue (oats vs. none) at either 0 d after tillage (DAT) or 7 to 13 DAT to monitor emergence at two timings. In ST plots, seeds were sown both in the tilled zone (“in-row,” IR), and between these tilled zones (“between-row,” BR). In 2011 and 2012, three levels of rainfall were simulated in subplots by either excluding rainfall, allowing natural rainfall, or supplementing rainfall with irrigation. In most cases, ST and oats residue either had no effect on or suppressed emergence of Powell amaranth sown at the early planting date. In contrast, the emergence response to ST and residue at the later planting date was generally smaller and more variable, with increases in emergence observed in several cases. Differences between tillage systems in emergence were most pronounced in the BR zone but also occurred IR in some cases, suggesting that interzonal effects on biotic or abiotic factors influenced emergence. Oat residue effects—but rarely tillage effects—were often mediated by simulated rainfall, with increases in emergence occurring mostly in dry conditions and decreases occurring more commonly in wetter conditions. These results demonstrate that the suppressive effects of cover crops and ST on weed emergence are inconsistent, temporally and spatially variable, and dependent on complex interactions with factors including rainfall.