We propose an approach, based on GPU Shape Grammars [Marvie et al. 2012], to interactively control grammar growths on the GPU using external contexts (Figure 1(a)). Our work uses textures as GPU-suitable representation encoding external contexts, and adds a texture accessor to the grammar for constraining any shape grammar rule with any external context. In addition, we introduce a texture marching method to follow the underlying surface during the grammar development. For instance, with our system one could paint restricted growth areas on the mesh, and the grammar will adapt itself to the new environment