Gemara

34b (Mishnah): If he put the Eruv on top of a reed, if it was initially detached and later inserted into the ground, it is valid.

(Rav Ada bar Masnah) Inference: If it were attached, it would be invalid!

(Rava): This is unlike Rebbi, who permits Shevus (e.g. using something attached) during Bein ha'Shemashos.

(Ravina): Even Rebbi decrees to forbid [an attached reed] lest one cut off the top when taking his Eruv.

Rav Nachman told his Talmidim 'go bend reeds. Tomorrow (Shabbos) we will sit on them.'

Question (Rami bar Chama): The Mishnah permits only if it was detached and inserted, but not if it is attached!

Answer (Rav Nachman): That refers to Uzradin (reeds that became hard like a tree);

(Beraisa #1): Reeds are trees. They are not Kil'ayim in a vineyard;

Contradiction (Beraisa #2): Reeds are Yerek (vegetables). They are Kil'ayim in a vineyard.

Resolution: Beraisa #1 discusses Uzradin, and Beraisa #2 discusses regular reeds.

100b (Beraisa #1): The same [Isur] applies to a moist or dry (dead) tree.

Contradiction (Beraisa #2): A moist tree is forbidden, but a dry tree is permitted.

Resolution: It is forbidden in winter, and permitted in summer.

Question: If we permit using a dry tree, fruits or twigs will fall off, and people will come to detach from living trees!

Answer: We permit a barren stump without fruits or twigs.

Question: Rav visited Apastiya and forbade such a tree!

Answer: Rav saw a need to be stringent. (The people there were unlearned.)

(Rami bar Aba): One may not walk on grass on Shabbos - "v'Atz (one who tramples) b'Raglayim Chotei."

(Beraisa #1): One may walk on grass on Shabbos.

Contradiction (Beraisa #2): One may not walk on grass on Shabbos. (The Gemara gives five resolutions of the contradiction.)

The Halachah follows R. Shimon, who permits in every case [for it is Davar she'Eino Miskaven].

Shabbos 125b (Mishnah): If a vine branch is tied to a flask, one may draw water with it on Shabbos.

Inference: If it is not tied, one may not use it.

Suggestion: This is unlike R. Shimon ben Gamliel, who permits detached Lulav branches!

Rejection #1 (Rav Sheshes): All agree to the Mishnah. It discusses attached vine branches.

Question: One may not use something attached!

Answer: He uses them within three Tefachim of the ground.

Rejection #2 (Rav Ashi): The vine branches are detached. If they are not tied, it is forbidden lest one cut them to size.

Rishonim

Rambam (Hilchos Shabbos 21:6): One may not alight on a tree, whether moist or dry. One may not use anything connected on the ground at all. This is a decree, lest he detach.

Rosh (10:12): We conclude like R. Shimon, who permits in every case, i.e. walking on grass. We decree about a dry tree, for parts are more prone to break off. There is no source from here to permit it. It seems that the Rif equates trees and grass. Therefore he did not bring the law of a dry tree.

Beis Yosef (OC 336 DH Ein): Also Tosfos (100b DH v'Ha'idna) says that there is more reason to decree about a tree, for it is evident. How can the Rosh say that the Rif equates trees to grass? If so, why does the Rif forbid alighting on a tree?! Also, the Rif brought the resolutions to the contradiction about grass, and then the ruling like R. Shimon. Why didn't he do similarly for trees? Rather, the Rif holds that the Isur to walk on grass is only because it is uprooted through this. One may not alight in a tree, lest he break off a branch.

Hagahos Ashri (3:4, citing Or Zaru'a): One may use reeds that are soft like Yerek, even though they are attached to the ground, for Chachamim did not forbid Yerek.

Rashi (Shabbos 82a DH v'Eino): One may not clean himself [after eliminating] with pebbles, for they are Muktzeh. Therefore, one uses attached grass, without moving it. One may use something attached. We forbid only trees and Uzdarin reeds, which are hard (Eruvin 34b).

Poskim

Shulchan Aruch (OC 336:1): One may not alight on a tree, whether it is moist or dry.

Beis Yosef (DH Ein): We decree dry trees due moist, like the Rambam says. Even though the Gemara permits dry trees, the Magid Mishneh says that we find that Rav saw a need to be stringent. We rely on this to forbid all trees. Also the Rif forbids trees without distinction. Even though the Tur rules like the Rosh, we follow the Rif and Rambam.

Taz (1): One may walk on grass (Sa'if 3) because even if one detaches while walking, we permit Davar she'Eino Miskaven. All forbid a tree, lest one break off a branch. The Rosh says that the Rif does not distinguish, i.e. he permits even a moist tree.

Mishnah Berurah (1): Some permit in summer, when all know that it is dry.

Shulchan Aruch (ibid.): One may not use anything Mechubar at all. This is a decree, lest he alight and detach.

Taz (2): This connotes that even Yerek is forbidden.

Mishnah Berurah (4): It is forbidden only [at least] three Tefachim above the ground.

Mishnah Berurah (5): Perhaps he will intentionally detach leaves, branches or Peros, and be Chayav Chatas.

Kaf ha'Chayim (6): Mechubar refers to trees and similar things, i.e. Yerek with hard, tall stalks. This is why the Mechaber says 'lest one alight...' One may use Yerek, as long as he does not move it.

Rema: We forbid trees. One may use reeds that are soft like Yerek, even though they are attached, for there is no Isur to use Yerek.

Beis Yosef (DH Kasav): Hagahos Ashri learned from the episode with Rav Nachman. He told his Talmidim to tie reeds together to make like chairs, to sit on them on Shabbos. The Gemara asked a contradiction, and concludes that an Eruv on a soft reed is Kosher (for then one may take the Eruv Bein ha'Shemashos). The Poskim did not mention this. Perhaps they hold that it sufficed to teach that one may not use trees. We infer that Yerek is permitted. If not, they would have written 'one may not use Mechubar'!

Taz (4): This requires investigation. This is from Hagahos Ashri, who permits Yerek even mid'Rabanan. The Beis Yosef says that he learned from the episode with Rav Nachman. The Beis Yosef concluded that the Poskim permit Yerek, since they did not write 'one may not use Mechubar.' If so, since the Shulchan Aruch forbids Mechubar, Yerek is included, like the Rambam says. Why does the Rema permit soft reeds? According to the Beis Yosef, why did the Poskim rule unlike the Stam Gemara, which distinguishes between hard and soft reeds? The Gemara is Machshir an Eruv on a tree, like Rebbi, who permits Shevus Bein ha'Shemashos. The Mishnah permits an Eruv on a reed only if it was previously detached. This is unlike Rebbi. Ravina says that even Rebbi forbids an attached reed lest one cut it when taking his Eruv, for it is soft. This is not Shevus. One is liable for this for Kotzer (reaping)! We do not decree lest one detach from a tree Bein ha'Shemashos, since it is hard.

Taz (4): Rav Nachman permits soft reeds, like Yerek. He did not decree lest one cut them. He forbids hard reeds, like trees. Ravina holds oppositely. He is more stringent about soft reeds, lest one cut them! The Rema rules like Rav Nachman. The Rosh rules like Ravina. If so, all the more so he should forbid Yerek, which is softer, lest one cut it! This is unlike the Heter to walk on grass (Sa'if 3). There, he does not use it, and his hands are far from it. Here, he uses it. Perhaps he will cut it! Chachamim forbade using trees lest one detach, since his hands are close! This is why the Rambam forbids using Mechubar, like Ravina. It seems that it is forbidden also due to Muktzeh. Why didn't the Gemara forbid due to Muktzeh? It seems that the Gemara explained why even the opinion that permits Muktzeh, forbids here. We do not hold like the Rema, who permits reeds. Why didn't the Rema say that the Mechaber is like the Rambam [and he disagrees]?!

Gra (DH v'Chol): This is like it says on 34b. This is like Rashi (Shabbos 82a, who permits using Mechubar, except for trees and hard reeds). The Rambam and Shulchan Aruch forbid Mechubar. They learn from Shabbos 125b.

Mishnah Berurah (13): This refers to hard reeds.

Mishnah Berurah (14): Many Poskim forbid reeds that are prone to harden even while they are soft. We permit only swamp reeds that never harden, even if they are taller than three Tefachim.

Mishnah Berurah (15): Yerek is included in grass, which Chachamim did not decree about (Sa'if 3, Ritva). The Poskim forbid using cabbage and pumpkins. Even though Chachamim did not forbid Yerek, these are hard. Also, they eventually harden and become like wood (Chulin 127b). The Bach and Taz forbid reeds and Yerek, like the Rambam and Mechaber. Eliyahu Rabah brought several Rishonim who hold like the Rema. (There is no proof from the Rambam. He discusses a tree, which he mentioned just above.) However, he permits only if it is soft like Yerek. He forbids if it is hard, even if it is not as hard as trees. One may not use it or put anything on it, for it breaks easily. Tosefes Shabbos and Chayei Adam agree.

Kaf ha'Chayim (15): The Eliyahu Rabah supported the Rema from 312:6 [which forbids trees and permits Yerek]. Many others say that one may rely on the Rema.

Kaf ha'Chayim (312.35): Perhaps one may clean himself [after eliminating] even with a tree. Due to Kevod ha'Briyos, Chachamim did not decree. Even if one will detach leaves, it is Melachah she'Einah Tzerichah l'Gufah. Also, he does not intend, and it is not a Pesik Reishei. Levush and Eliyahu Rabah say so. R. Zalman permits a tree only if he does not have rocks or pebbles.