The game engine has an erosion system for world-creation. The world will remain static post-creation. So as of now - without tweaks - the game world is final and there will be no further 'erosion' (aside from mod landscapes)
On plant growth - i would presume it to be insane if the plants actually 'grew' in real time. That would eat up processor power like no tomorrow (the entire world is covered in flora) and the devs probably (rightfully) figured players wouldn't actually stand there waiting for a plant to grow. It will simply pop up when the player is out of view.
Naturally this is mostly speculation based on logic.
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