The OpenGL graphics libraries (http://www.opengl.org) provide a powerful environment for programming three-dimensional virtual worlds, and are used in a wide variety of applications. However, the creation of individual 3D models to include within a virtual world is an arduous and time-consuming process. We have designed GenTree, an interactive system that uses genetic algorithms to evolve 3D tree models to be used in virtual environments. The system evolves a set of parameters used to render the trees, with a fitness function provided by user input. The initial system works with a modest set of parameters and uses relatively crude rectangles for rendering, but is able to evolve a variety of plant shapes, including deciduous trees, evergreens, and shrubbery.