Issues with foliage

Hi!
Everytime that i try to add tree's to my scene, some of the planes become dark and i dont know how to fix this. The normals should be correct but i think it has something to do with the material settings?
If someone could point out what im doing wrong, that would be great!




https://imgur.com/a/tboZION
heres the link incase the images dont work(first forum post)
Thanks in advance!