Related על סוכה 24:8

If one puts a [schach] covering of ropes, or with bundles of sheaves, it is kosher; a covering of flax in its natural state is kosher; a covering of soaked flax is invalid, with reeds and with forked reeds, even if they are stuck together, it is kosher. A covering of ears of grain, if the husk is more than the grain, it is kosher; if not, it is invalid. R. Yosi bar R. Yehudah says: A covering of torn pieces of garments, it is kosher.

If it is covered with flax stalks, then it is qualified. with bundles of flax, then it is disqualified.