Wave action on exposed rocky coasts can be severe, generating large hydrodynamic forces that have been proposed to constrain the size of intertidal animals and plants. In contrast, flows subtidally are more benign, and organisms, particularly seaweeds, may grow quite large. The large dimensions of these flexible macroalgae allow them to move during much or most of a passing wave cycle, reducing relative water velocities and modifying the forces the plants must endure. The consequences of such wave-induced motion are explored for the stipitate understory kelps Eisenia arborea and Pterygophora californica using a numerical model that approximates these seaweeds as vertically oriented cantilever beams subjected to lateral hydrodynamic forces acting at their stipe tips. Bending moments and peak stresses induced in the stipes of these species during the passage of waves are calculated as functions of plant size and shape and of water depth and sea state. Model predictions for a subset of conditions are validated against real-time measurements of bending moments acting on a Pterygophora individual in the field. The results suggest that the allometric patterns of growth exhibited by Eisenia and Pterygophora can greatly reduce the stresses generated in the stipes of these plants relative to isometric growth. Low stipe stiffness acts as a general, particularly effective, stress-lowering mechanism. The dynamic swaying associated with this low stiffness can also modulate the magnitudes of peak stresses induced in the stipes of these kelps. In particular, in shallow water under large waves, dynamic loading can substantially increase induced stress, suggesting that plant motion is an important factor affecting the loading regime encountered by these organisms.