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With population growth, demand for agricultural products has increased, which affects agriculture, fertiliser use and land management. Due to nutrient inputs from arable areas, the water quality of rivers and lakes near agricultural lands has deteriorated. Constructed wetlands (CWs) are one of the measures used to improve water quality through natural water treatment processes. We studied phosphorus removal in relation to environmental characteristics in five in-stream free surface flow CWs in Finland, Estonia and Latvia. Wetland/catchment area ratios varied from 0.1% to 5%. Three CWs were covered in dense vegetation. On average, the Vända CW and Hovi CW TP removal efficiency throughout the study period were highest (32.1 ± 3.6% and 34.9 ± 4.4%, respectively) among all studied CWs. Nummela Gateway CW, Rantamo-Seitteli CW and Mezaciruli CW showed lower TP removal efficiency and on many occasions the outflow concentration exceeded the inflow concentration in Rantamo-Seitteli and Mezaciruli CW. Vegetation cover emerged as the most important factor in the treatment efficiency of in-stream CWs, reducing flow rate, increasing water retention time and decreasing probability of the CW becoming a source of phosphorus during flood events. • Flow rate controls phosphorus removal efficiency in in-stream constructed wetlands. • High flow values often resulted in significant reduction in phosphorus removal. • Wetlands with dense vegetation cover had higher and more stable phosphorus removal efficiency. • Shallow meandering wetlands benefit vegetation spread, water retention time, and phosphorus removal. [ABSTRACT FROM AUTHOR]