GroIMP is described as '(Growth Grammar-related Interactive Modelling Platform) is a 3D-modelling platform' and is a 3D Modeler in the Photos & Graphics category. There are nine alternatives to GroIMP for a variety of platforms, including Windows, Linux, Mac, BSD and Cinema 4D. The best alternative is SpeedTree. It's not free, so if you're looking for a free alternative, you could try Arbaro or powerPlant. Other great apps like GroIMP are Tree Studio (Paid), The Grove 3D (Paid), ngPlant (Free, Open Source) and Plant Factory (Paid).
SpeedTree is a group of vegetation programming and modeling software products developed and sold by Interactive Data Visualization, Inc. (IDV) that generates virtual foliage for animations, architecture and in real time for video games and demanding real time simulations.
Tree Studio is a product born from TwistedBrush Pro Studio but with a focus on 2D tree creation with a simpler, streamlined interface. Tree Studio has pairs of brushes for each tree type - one for the frame and one for the leaves.
powerPlant is a plant modelling tool based on parametric plant descriptions, producing highly detailed models that can be exported to 3ds. Deriving similar plants is a matter of 1 click! Complete scenes can be composited with the integrated scene editor.
ngPlant is a plant modeling software package. ngPlant interactive tool can be used to create 3D models of different plants and trees. ngPlant software libraries can be used by developers in their 3D applications, or plant modeling plugins.
Xfrog is a procedural organic 3D modeller that allows you to create and animate 3d trees, flowers, nature based special effects or architectural forms. Xfrog is available as a Mac OS, Windows, and Linux plug-in for Maya, Cinema 4D and as a standalone application
TreeGen consists in a platform indipendent Python script which should work as a plugin for Blender . Thanks to the graphical interface, it aims to create random trees and forests in a simple and alternative way to L-System.