Abstract: We present a sketch-based interface for parameter control of rule-based models. It allows intuitive specification and creation of plant structures with L-systems. Construction lines inspired by concept sketches are employed as a way to define and manipulate global-to-local characteristics of L-system models. The overall structure, posture and proportions of the plant are initially sketched by the user as construction line arrangements. They are automatically encoded as a set of positional functions controlling internode lengths, branching angles, organ sizes, and stem shape. These positional functions are then used to parameterize pre-defined L-system templates representing phyllotactic patterns for positioning lateral organ surfaces such as leaves and petals. Results are presented for single monopodial plant structures, all generated from simple input construction line sketches.