If you spread a sheet over the sichach because of the sun or under the sichach because of falling leaves or you spread a cover over a four poled bed the succa is posul. A spread over a two poled bed is kosher.

Succa 1:4

If you placed a grapevine, gourd or ivy which is still attached to the ground as sechach and put kosher sichach upon them, it is still posul. If you detached them or had mostly kosher sichach then it is good.

A general rule is anything that can become tamai or doesn’t grow from the ground is posul for secach. If it cannot accept tumah and grows from the ground it is kosher.