Mishnah/Seder Moed/Tractate Sukkah/Chapter 1/4
If one lifted onto the succah a grapevine, a gourd, or ivy, and coverd it with valid s'chah, the succah is invalid. But if the valid s'chach exceeded them, i.e. there was more calid s'chach than grapevine etc. atop the succah, or if he detached the vines from the ground, -the succah is valid.
This is the rule: Whatever is susceptible to tumah or whatver does not grow from the ground, we may not use for s'chah. But whatever is not susceptible to tumah, and grows from the ground, we may use as s'chah.