I use a very powerful organic software, called Xfrog it is available as a stand-alone version (Xfrog 3.5) and NEW as plugin (Xfrog 4 PlugIn)!
For the new Xfrog 4 see:
It seems very easy to build really nice models of trees, flowers etc. and animate them - they sell over 1000 plant models (tropical, desert, usa, europe, asia, flowers, etc.).
for examples of the stand-alone version see: http://www.xfrogdownloads.com/greenwebNew/gallery/galleryStart.htm
and for examples of the NEW plugin version see: