Update:
Added an optional version for Ubanga users. It is the same as the main file, but with additional, sparse weed for Ubanga.
If you use this, don't use the main file, it is not needed.
Since Ubanga is a kind of a dry world, the additional weed is also sparse and will only grow where grass or moss is.
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There are some grass and moss ground textures, that have no grass.
With this mod you'll have groundcover even there!
There is only one kind of foliage growing on the no grass textures. It is very low (see image).

There will be no groundcover on rocky grounds, on roads and other cobblestone, on sand and some unique grounds.
The grass stays untouched, this only adds.

I could not get all grass textures to work with groundcover and still try to find out why.
I made this with a jungle Oblivion in mind, so it is a very saturated groundcover I made, but there is a less saturated version available, to overwrite that.

This should work with everything unless it does changes in no grass landscape textures.
The only one I know is Oblivion Full Grass Mod.