GEMARA

R. Aba: Vegetables that are Kosher for Maror on Pesach (can be an Ohel to) bring Tum'ah, but they are not a Chatzitzah to block Tum'ah. (If used for Sechach) they disqualify a Sukah like air.

Question: What is the reason?

Answer: Since when they dry up they crumble and fall, it is as if they are not here.

RISHONIM

Rambam (Hilchos Sukah 5:3): If one used for Sechach vegetables that if they dry up they will wither and nothing substantial remain, even if they are now moist we ignore them and consider air to be in their place.

Magid Mishnah: This is even if they are not specifically for people and are not Mekabel Tum'ah.

Question: The Rambam (5:2) rules that if one used something that wilts for Sechach it is Kosher b'Di'eved!

Answer (Kesef Mishneh): That refers to something in which some of its leaves will wilt, but it will remain mostly shade. It is forbidden lest one leave the Sukah due to the dropping leaves. Here the Rambam discusses something that will give a minority of shade after it wilts.

Rosh (1:26): Since they will dry they are like air. If they are three Tefachim wide they disqualify the Sukah. If they do not crumble they are Pasul Sechach and do not disqualify unless they are four Tefachim in the middle or four Amos from the side.

Ran (7a DH Garsinan): Since they are very thin they fall when they dry up. Similarly, anything prone to dry up within seven days to the point where it will give a minority of shade is Pasul even now (like air).

POSKIM

Shulchan Aruch (OC 629:12): If one used for Sechach vegetables that dry up quickly, even though they are Pasul for Sechach because they are Mekabel Tum'ah, they are not like Pasul Sechach which disqualifies only if it is four Tefachim. Rather, they are like air, and three Tefachim disqualifies.

Bach (DH v'Im): This refers to vegetables that wither and nothing substantial remains. One may use vegetables that dry but something substantial remains. All vegetables dry in the sun (yet R. Aba specified those Kosher for Maror)! Abaye forbids l'Chatchilah to use Heigei (prickly bushes) because their leaves fall down. The Mishnah (10a) forbids a sheet to prevent things dropping from the Sechach. Without the sheet it is Kosher!

Bach (DH u'Mah she'Chosav): The Rosh and Tur say that if they do not crumble they are Pasul Sechach. Perhaps this teaches about things that sometimes dry and nothing substantial remains, but usually they do not dry quickly. The Ran connotes that the Shi'ur of time to dry is seven days from when they were detached, from the beginning of Sukos.

Mishnah Berurah (33): The Shulchan Aruch discusses vegetables fit to eat, which are Mekabel Tum'ah and Pasul for Sechach. One might have thought that another Pesul does not take effect on them. The Shulchan Aruch teaches that this is not so.

R. Akiva Eiger (DH Ela): These vegetables bring Tum'ah, but they are not a Chatzitzah. This shows that it is a stringency to consider them like air. Therefore, if there are two Tefachim of (regular) Pasul Sechach and two Tefachim of these vegetables, they join to the Shi'ur of four Tefachim to disqualify, since really they are Pasul Sechach.

Bikurei Yakov (23): If mid'Oraisa they are like air, like the Panim Me'iros learns from the Rambam and Tur, this is even to be lenient, and they do not join with Pasul Sechach. If they are like air only mid'Rabanan, like the Levush, this is only a stringency. The Ran and Ritva hold that it is only mid'Rabanan.

Kaf ha'Chayim (71): Most hold that it is a decree mid'Rabanan to consider it like air. Therefore, if one cannot make another Sukah he should sit in such a Sukah, but he should not bless because some say that mid'Oraisa it is like air.

Rema: Anything that normally dries up within seven days is considered as if it is already dry. It is like air and disqualifies even from the side.

Mishnah Berurah (35): Since the Sukah cannot last for seven days, Chachamim decree to disqualify the Sechach immediately.

Eshel Avraham (13): The Levush says that we immediately disqualify anything that will dry within seven days until its shade will be a minority. We decree lest someone will be unable to make another Sukah and he will be Batel from the Mitzvah. It seems that this Pesul is mid'Rabanan. However, if it will dry and fall within seven days it is Pasul mid'Oraisa. Perhaps in the former case we do not decree at all, and it is Kosher as long as it is mostly shade (Mishbetzos Zahav 15).

Bi'ur Halachah (DH v'Chol): The Levush is like the Ran. However, the Rema cites the Ran without mentioning that the shade will become the minority. This suggests that he disagrees (and disqualifies only what will dry up and fall).