Plants can be generated by L-Systems, or their growth simulated in more detail.
PCG Wiki References
FSPM07 - 5th international workshop on Functional Structural Plant Models.
Fast Simulation of Realistic Trees - An article from volume 28, issue 3 of IEEE Computer Graphics and Applications by Jason Weber.
The Algorithmic Beauty of Plants - Downloadable PDF chapters of a book published in 1991.
XNA Procedural Trees - Library (with source) for procedural trees in Microsoft XNA.