Auto landscape painting and foliage

I’ve poked around a few tutorials etc. but can’t seem to find an answer to this and I’m fairly sure it’s something simple.

I want the foliage to spawn on the ‘Plain’ material only. Here is my master material which paints the different textures on to the landscape quite nicely.

If I change the grass param to ‘Slope’ No foliage spawns. as it is, foliage spawns everywhere despite the ‘slope’ texture being applied correctly. I am also wondering how to paint layers on to something that uses the world-Aligned blend.

If anyone can help or point me at the right tutorial, it will be much appreciated.