• https://www.assetstore.unity3d.com/en/#!/content/3253 A unity asset that provides foliage with displacement shaders (among other things). If you have $20 to spare, this is the simplest way.

  Here's a unity demo of the displacement shader at work: https://dl.dropboxusercontent.com/u/2322017/afs_v2/afs_v2_touchbending/afs_v2_touchbending_202.html

• https://codea.io/talk/discussion/2465/my-grass-simulation-with-vertex-shaders An example of using vertex shaders to make thick grass "look" like it is swaying in wind/has something moving inside it, which could be useful for creating the illusion of the player interacting with grass.

• https://www.youtube.com/watch?v=fjJacIFxEM8 A tutorial on shaders, including some vertex deformation goodness.

• https://www.youtube.com/watch?v=qSGF3a22UmE&feature=iv&src_vid=c-pqEHR1jnw&annotation_id=annotation_3788809159 A tutorial on how to programmatically deform meshes in Unity using the Mesh/MeshFilter/MeshRenderer classes. You could place invisible colliders on the key verticies of a rigged plant mesh.

• http://answers.unity3d.com/questions/274269/mesh-deformation.html Someone else's attempt at using MeshFilter to deform meshes on impact, perhaps something can be gleaned from this.