SignificanceIce templating is the mechanism by which cellular materials with hierarchical architectures are formed when aqueous solutions or slurries are directionally solidified or freeze cast. Characteristic for freeze-cast solids are lamellar cell walls with unilateral surface structures such as regularly spaced “ridges” and other complex subfeatures that do not reflect the underlying hexagonal symmetry of ice. How these unilateral structures form has remained a mystery. Combining experiments and computational modeling, we reveal that, by localizing diffusion-limited morphological instabilities to the atomically rough side of ice lamellae, anisotropic ice-crystal growth templates hierarchical architectures with complex subfeatures. Those results lay the theoretical foundation for understanding the formation of hierarchical architectures and structural subfeatures produced by directionally solidifying aqueous mixtures.