http://www.thegnomonworkshop.com/tutorials/creating-high-resolution-custom-trees-using-speedtree

Discover how to generate complex-looking trees using SpeedTree and Houdini, with this intermediate-level tutorial by Jean-Michel Bihorel. Starting from simple generating rules before deep-diving into more advanced areas of tree generation in SpeedTree and Houdini, Jean-Michel teaches every step needed to generate a hero tree asset — perfect for use in a cinematic.  Throughout the workshop, Jean-Michel shares how to create a mixture of hand-drawn branches extended by procedural generators, as well as how to use the mesh forces for realistic results. He also reveals how to set up and export a leaf as a point cloud in Houdini in order to manage a more optimized setup, as well as great procedural control over the leaf geometry generation and placement based on the direction of the sun.  For those that don’t want to export their tree assets to Houdini, Jean-Michel also provides guidance for exporting your trees into other software — in this case, Maya — along with advice for how to get your shaders working in other software packages.