Lay AbstractFree‐floating aquatic plants, or macrophytes, often grow in dense mats, and their feathery, unanchored roots form canopies below the water surface that can affect water flow and water quality. This study examines the physical interactions of an invasive species of free‐floating macrophyte, water hyacinth (Eichhornia crassipes), and the surrounding water to better understand how free‐floating macrophyte root canopies affect hydrodynamics. Experiments in an open‐channel flow chamber were conducted to examine the flow fields through and around root canopies. Acoustic Doppler velocity measurement revealed that water flow was deflected around the root canopy and was reduced within the canopy. Interestingly, the vertical pattern of the velocity in the roots was similar to what has been observed in leaf canopies of terrestrial and submerged aquatic vegetation. In these cases, an unstable velocity profile (one with an inflection point) gives rise to water column turbulence, which was observed at distances >50% of the length of the root canopy, culminating in a large wake region immediately downstream. Although mixing caused by turbulence increased outside the root canopy, it did not increase within the canopy, and so there was limited exchange of water between the root canopy and the open water. Because of this, we expect the time that water resides within the root canopy to be dominated by the slow mean flow velocity, rather than turbulence. This result helps to explain why the root canopies of free‐floating macrophytes can suffer from low levels of dissolved oxygen (hypoxia) and poor water quality, and suggests ways in which to improve the operational efficiency of natural water treatment systems relying on these types of plants.