GEMARA

Rav Acha bar Yakov: Any Mechitzah that moves in the wind is not a (valid) Mechitzah.

Question (Mishnah): If one made a Sukah between trees and the trees are its walls, it is Kosher.

The trees move in the wind!

Answer: The Mishnah discusses hard trees that do not move.

Question: The foliage moves in the wind!

Answer: It is tied down with branches.

22b - Mishnah: A Sukah on the head of a ship is Kosher.

23a: Our Mishnah is like R. Akiva:

Beraisa - R. Gamliel: A Sukah on the head of a ship is Pasul;

R. Akiva says, it is Kosher.

Abaye: All agree that if it cannot stand in a normal dry-land wind, it is Pasul. If it can stand in an abnormally strong land wind, it is Kosher. They argue about if it can stand in a normal land wind, but not in an abnormal dry-land wind (some texts - in a normal sea wind). R. Gamliel requires Diras Keva. R. Akiva requires Diras Arai. Since it can stand in a normal land wind, it is Kosher.

RISHONIM

The Rif and Rosh (2:5) bring the Mishnah and Rav Acha's law.

Rambam (Hilchos Sukah 4:5): If one made a Sukah between trees and the trees are its walls:

If they are hard trees that do not move constantly in a normal wind, or if he tied them down so they will not move in the wind, and filled between the branches with straw and tied it down so the wind will not move them, it is Kosher.

Kesef Mishneh: It suffices to tie down the foliage to the height of 10 Tefachim.

Rambam (ibid.): Any Mechitzah that does not stand in a normal land wind is not a Mechitzah.

Magid Mishnah: Since we require Diras Arai, it suffices if it can stand in a normal land wind.

R. Mano'ach: A wall that can stand in a normal land wind but not in a normal sea wind is a Mechitzah, but one may not rest the Sechach on it.

Note: If the wind will topple the wall, in any case the Sukah will be Pasul! Perhaps he discusses walls that move in the wind (but will not fall); if the Sechach is on them, it will fall.

Rambam (Hilchos Shabbos 16:15): Any Mechitzah that does not stand in a normal wind is not a Mechitzah.

POSKIM

Shulchan Aruch (OC 630:10): If one made a Sukah between trees and the trees are its walls:

If they are hard trees that do not move constantly in a normal wind, or if he tied them down so they will not move constantly in the wind, and filled the gaps with straw and tied it down so the wind will not move them, it is Kosher.

Magen Avraham (15): It suffices if the majority of the wall is hard (and does not move).

Magen Avraham (16): Even if the Sukah is inside a house, we require walls that would not move in a normal wind. (Bi'ur Halachah (362:1 DH she'Einah) says that the same regarding Shabbos.)

Chazon Ish (Shabbos 52:14 DH v'Hinei): A Mechitzah in a house need not be secured at the bottom so it would not move in the wind. Looseness does not disqualify, rather, movement, and there is no wind in the house. R. Akiva is lenient to permit a Mechitzah which (would not move in a land wind but) moves in a sea wind. However, there is no since to be stringent to disqualify a Mechitzah which does not move (even though it would move in a normal wind). Putting walls around a curtain to prevent wind is like securing it! However, it is possible that it is not normal to make such a loose Mechitzah, and it would not be considered a Mechitzah. The Magen Avraham (315:1 and 630:16) disqualifies this. The Be'er Heitev (630:10) says that some allow it.

Shulchan Aruch (ibid.): Therefore, one should not make all the walls from linen sheets without sticks, even if they are tied well, for sometimes they get untied and the person does not know that the Mechitzah cannot stand in a normal wind. If one wants to use sheets it is good to weave sticks into the sheets less than three Tefachim (apart from each other).

Mishnah Berurah (50): The Taz allows one of the three walls to be from sheets without sticks. Many Acharonim disagree. Surely, the fourth wall may be a curtain, since it is not essential.

Kaf ha'Chayim (70): The Magen Avraham allows sheets with sticks only if there are four walls, just like walls consisting solely of sticks. Many Acharonim allow sheets with sticks even if there are only three walls. This is because secured sheets are proper walls; the sticks are merely 'insurance' in case the sheets become untied. Further, some permit walls consisting solely of sticks even when there are only three walls.

Note: The Magen Avraham (17, citing the Bach) connotes that we always rely on the sticks (and Lavud) because the sticks do not stop the sheets from moving in the wind. The lenient Poskim hold that the sticks are needed in case the sheets become untied. It seems that they discuss different cases. Alternatively, all agree that the sheets move less than three Tefachim, for there is only three Tefachim between places where they are enforced. The lenient Poskim say that this is not called a Mechitzah that moves in the wind, and the Magen Avraham disagrees

Kaf ha'Chayim (73): Many people use sticks less than three Tefachim apart that are not tied securely. This is wrong, for the sticks get moved by people. The ends often become more than three Tefachim from each other. The sticks must be tied firmly, even if sheets are tied to them.

Shulchan Aruch (OC 362:1): If the branches of a tree hang down and are less than three Tefachim from the ground, and they are 10 Tefachim tall where they are attached to the trunk, they are considered a Mechitzah. One must fill the air between the branches with wood or straw and tie the branches so they will not move in the wind, for a Mechitzah which cannot stand in the wind is not a Mechitzah.

Magen Avraham (315:3): A Mechitzah that would be blown in the wind is not a Mechitzah to separate between Seforim and urinating.

Kaf ha'Chayim (12): Similarly, a Lechi, Korah or Tzuras ha'Pesach that moves in the wind is invalid.

Kaf ha'Chayim (13): A Mechitzah that can stand in a normal land wind is valid even at sea, even if it cannot stand in a normal sea wind.