Succah 1:3

If one spread a sheet over a succah’s roof because of sun, or under the roof to catch falling leaves, or over the four posts of a bed in a succah – in all of these cases, it is invalid. One may, however, hang a sheet over two perpendicular poles at the head and foot of a bed (as this does not form a canopy).

Succah 1:4

If a person trained a vine, a gourd or ivy over his succah and the plant covered it, the succah is invalid. The general principle is that if something is susceptible to ritual impurity and does not grow from the ground, it may not be used for the roof of a succah; if something is not susceptible to ritual impurity and does grow from the ground, it may.