https://docs.unrealengine.com/latest/INT/Engine/Foliage/index.html
The part about foliage culling is wrong, it suggest to use the “vertexcolor” alpha, which is not working, instead a “PerrInstanceFadeAmount” is needed.
I hope i got it right 
Hello DominicBauer,
Thank you for the heads up. I have reported the documentation issue and it will be taken care of when the resources are available.
Cheers