In East Germany a great amount of agricultural wastewater is derived from dairy farms. It is highly polluted with average concentrations of 3 300 mg/L COD and 460 000 Colony forming units (CFU)/mL fecal coliforms (FC). This study was designed to show how far constructed wetlands may eliminate pathogenic bacteria from dairy farm wastewater under changing climatic conditions. During both summer and winter a constructed wetland was loaded for 10 weeks with dairy farm wastewater with a hydraulic load of 0.013 m3/m2‐d and 0.010 m3/m2‐d, respectively. The removal rates for COD and FC were 89.2 and 99.3% in the summer and 92.0 and 95.8% in the winter, respectively. Survival and transport of FC in the water and the substratum are controlled primarily by temperature and type of substratum. Adsorption on soil particles plays a key role in the retention of FC.