We present a method to generate procedural models with global structures, such as growth plants, on existing surfaces at interactive time. More generally, our approach extends shape grammars to en- able context-sensitive procedural generation on the GPU. Central to our framework is the unified representation of external contexts as texture maps. These generic contexts can be spatially varying parameters controlling the grammar expansion through very fast texture fetches (e.g., a density map). External contexts also include the shape of the underlying surface itself that we represent as a texture atlas of geometry images. Extrusion along the surface is then performed by a marching rule working in texture space using indirection pointers. We also introduce a lightweight deformation mechanism of the generated geometry maintaining a C1 continuity between the terminal primitives while taking account for the shape and trajectory variations. Our method is entirely implemented on the GPU and it allows to dynamically generate highly detailed mod- els on surfaces at interactive time. Finally, by combining marching rules and generic contexts, users can easily guide the growing process by directly painting on the surface with a live feedback of the generated model. This provides friendly editing in production environments.