Take a look at a Vegetation Generation Tool for Houdini shared by Marta Feriani.
The tool relies on the model suggested in a paper “Modeling Trees with a Space Colonization Algorithm” by Adam Runions, Brendan Lane, and Przemyslaw Prusinkiewicz:
We extend the open leaf venation model by Runions et al. to three dimensions and show that it generates surprisingly realistic tree structures. Model parameters correspond to visually relevant tree characteristics identified in landscaping, offering convenient control of tree shape and structure.
You can find the full paper here.