How To Add CERTAIN Foliage To Layer?

Hey everyone. I have a shot in which I painted Foliage in the foreground. I intend to have this masked out by opening the Layers window and adding this certain foliage to a new layer so it can be transparently rendered via movie render que. I’m able to select the certain foliage, but I can’t add it to the Foreground Layer. This foliage is placed all around my scene. How can I add CERTAIN Foliage I select to a new layer? Thanks!