The initial stage of solidification of a free supercooled water drop consists of the formation of a cloud of dendrites surrounded by liquid water. Although the impact of such mushy drops onto solid surfaces can significantly influence the process of ice accretion, neither the dynamics of impact of such mushy drops, nor their material properties have ever been investigated in detail. In this study, the kinematics of spreading and the geometry of the residual splat have been characterized for the impact of dendritically frozen water drops prepared under well-defined conditions. The impact process is then modelled theoretically, assuming plastic flow in the mushy drop. The model provides a description of the flow development near the impact axis and allows estimation of the values of the apparent yield strength of the solid-liquid mixture for different initial temperatures of the water drop, i.e. for varying solid-liquid ratios.