Layer mask while painting foliage

I can’t get Foliage to only be painted on one specific layer. I am using 4.8.

http://puu.sh/j4V1F/de2c9d7907.jpg

http://puu.sh/j4Vee/6102615d62.jpg

So, how to I get my foliage to paint just on the grass and not on the black areas?
The new feature using a grass layer in the material works, but I can’t scale my foliage if I use that method.