I am pretty sure the point cache won't work if the polycount changes. All it does is change the position of existing verts in order to prevent calculating bones, morphs, etc.
I have exported proxies for this but you end up having hundreds of proxies per plant, (one for each frame). This does work. It is probably the way you will need to go if you need proxies. I haven't done it in a very long time, however, I think I remember it being pretty easy.
There may be a better way to convert a growing animation to a mesh. I am just not aware of it ( the last time I did it was 2012 )