© 2015, Springer International Publishing Switzerland. Background and aims: Barnyard grass (Echinochloa crus-galli (L.) P.Beauv) is a major C4 weed in rice fields. We aimed to explore a management strategy to reduce its competition with rice (Oryza sativa L.) (C3) in low-phosphorus (P) soil with varying soil moisture availability. Therefore, we investigated the effect of P placement at different depths. Methods: The main experiment was conducted in pots (15 cm diameter, 70 cm height) filled with P-deficient soil as a replacement series with different plant combinations (rice and barnyard grass monocultures; four plants per pot, and a mixture of rice and barnyard grass; two plants of each species per pot) with two P placement depths (0–5 and 15–20 cm from the soil surface) and three moisture treatments (continuous flooding - CF, alternate wetting and drying - AWD, and top soil drying - TSD from flowering). The pots were maintained until crop maturity. A rhizobox experiment supplemented the main experiment to study the root system plasticity during the initial growth stages (i.e., after 14 and 28 days of growth). Key results: Placement of P in a deeper soil layer (15-20 cm) reduced the growth and P uptake of barnyard grass by over 70 and 80 %, respectively, irrespective of the plant combination, and moisture treatments, while such reductions were not observed in rice. Reduced growth and P uptake of barnyard grass were associated with the arrested root elongation even from the very early growth stages failing to search for P applied to 15–20 cm soil layer. In contrast, root elongation of rice increased from early growth stages when P was applied at 15–20 cm soil depth and reached the P-supplied soil layer to ensure P uptake. Average root diameter decreased by 12 %, and the percentage fine root production (i.e., <0.4 mm diameter) increased by 8 % when P was applied at 15–20 cm deep compared with those at 0–5 cm depth. Conclusions: Phosphorous placement at 15–20 cm soil depth reduced the competitive ability of barnyard grass over rice in P-deficient soils, irrespective of the soil moisture management method.