Vegetated ditches are widely used to treat agricultural wastewater, but effective nitrogen removal at low temperatures remains a challenge because plants wilt in the winter. In this study, three simulated drainage ditches vegetated with Myriophyllum aquaticum were operated with low, medium, and high water levels to study ammonium nitrogen (NH-N) removal under cold temperatures. The M. aquaticum ditches had a mean NH-N removal efficiency of 75.8-86.8% throughout cold period. Based on nitrogen mass balance, plant uptake, sediment adsorption, and microbial removal accounted for 12.4-21.5%, 0.0-8.1%, and 38.9-54.6% of the influent total nitrogen loading, respectively. The accumulation of nitrate confirmed that intense microbial nitrification occurred in M. aquaticum ditches even at low temperature. These results suggest that M. aquaticum is appropriate as a cold-tolerant plant for NH-N removal in drainage ditches.