With the full release of Unity 5, you can have AAA tree editing tools by subscribing to the SpeedTree Modeler.
Note: You must have Unity 5 to use SpeedTree with the engine. Trees and plants created with the SpeedTree Modeler will work with Unity 5 Beta, but not Unity 4 or Unity 5 alpha.
Highlights of SpeedTree for Unity 5:
*Scalable wind effects, including Wind Wizard
*Seamless LOD tuning
*Unlimited variations with one click
*Branch seam blending
*Sub-division surface modeling
*Collision primitives added automatically or manually
*Modeling with mesh forces (e.g. grow trees around or through 3D meshes)
*Much more! See our online Modeler documentation for complete details.