In this study, we explore root development in two common and three declining Potamogeton species in Danish streams applying an experimental approach to investigate if differences in root traits of importance for colonisation success may contribute to explain why some species within the Potamogeton genus are less likely to maintain populations compared to other Potamogeton species. App. 20 cm long plant shoots from the new growth of all five Potamogeton species were transplanted into 12 perpendicular transects placed with a distance of 1.5 m in the experimental stream reach in Skibby Stream, Jutland, Denmark. The plant shoots were anchored by burying the lower 10 cm of the shoots into the stream sediment. To avoid uprooting, a stone was placed on top. Specifically, number of roots, root growth (g Dry Weight per day), total root length per individual (cm) and number of roots per node were measured on the lower 10 cm of the transplanted plant shoots. Root traits were measured four weeks (T1-T4) from 10 August to 7 September 2020.