Dear Rabbi Simon,
I just was wondering if I’m allowed to use holly leaves as my sechach?
Yes you may, if they are real, not the decorative artificial kind popular in December (!).
Best to use the leaves that are still attached to the branches or tendrils, as the leaves on their own may dry fairly quickly which could have implications for the extent of the shade they provide. So use enough cover to provide reliable shade throughout the seven days of the festival.
Rabbi Rashi Simon