In a bare channel (without vegetation), the incipient velocity for sediment motion, Ucrit, has historically been related to the mean bed shear stress (t) or friction velocity (U* = (t/ρ)1/2). More recent studies, however, suggest turbulence also plays a role. This paper examines whether the onset of sediment motion in a vegetated channel is correlated with U*, or turbulence (kt). Images collected with a digital camera were interrogated with a particle-tracking code to measure sediment transport for different vegetation density and channel velocity. The trend in sediment transport with channel velocity was used to identify Ucrit for each stem density. The values of U* and kt were estimated at Ucrit. However, none of these parameters produced a constant threshold across all stem density and bare bed. We construct a new metric representing the peak turbulent velocities impinging on the bed that produces a constant threshold value for all cases.