Abstract: Modeling realistic looking plants is still a complex problem requiring specification of plant structure, geometry and surface characteristics. Modeling a collection of plants is more problematic especially since each plant is slightly different. Altering the shape of branches and stems is one of the most dramatic and natural methods of creating differing instances of the same plant type. We present a sketch-based interface for modeling plant variations through specification of branch and stem shape. Our system is based on interaction with the 3D Tractus: a new physical interface we developed to support direct 3D sketching. The 3D strokes from the 3D Tractus are used as input to a biologically-based modeling method that mimics natural growth variation factors of real plants.