By Rabbi Yehuda Spitz
In Part 1, we discussed various rationales explaining why in many communities shuls are festooned with foliage for Shavuos. The Magen Avraham, among other authorities, added that this minhag includes using trees in the decorating. Yet, the Vilna Gaon strongly opposed this practice altogether, citing concerns that this festooning might actually be classified as the forbidden Chukos HaGoyim, as gentiles decorate their houses of worship in a similar manner. But first, some background is in order.
In Parshas Acharei Mos, we are exhorted not to follow in the ways of the local non-Jewish populace, “U’Vichukoseihem Lo Seleichu”. According to the Rambam and later codified by the Tur and Shulchan Aruch, this prohibition includes manners of dress, haircuts, and even building styles. Tosafos mentions that this prohibition includes two distinct types of customs: idolatrous ones, and those that are nonsensical; implying even if they are not done l’sheim Avodah Zarah, with specific idolatrous intent, they would still be prohibited to practice.
However, other Rishonim, primarily the Ran, Mahar”i Kolon / Cologne (known as the Maharik), and Rivash, define the prohibition differently. They maintain that a nonsensical custom of the Goyim is only prohibited when it is entirely irrational, with no comprehensible reason for it, or when it has connotations of idolatrous intent. Likewise, following a custom that would lead to a gross breach of modesty (pritzus) would fit the category. On the other hand, they maintain, observing a simple custom of the Goyim that has no reference to Avodah Zarah, especially if there is a valid reason for its performance, such as kavod, giving proper honor or respect, would indeed be permitted.
Although the Vilna Gaon rejects their understanding of the prohibition, and the Gilyon Maharsha seems to follow Tosafos, nevertheless, the Rema explicitly rules like the Maharik and Ran, as does the Beis Yosef. Accordingly, they hold that as long as a custom is secular, with no connection to Avodah Zarah, such a custom may still be observed.
Most later authorities, including the Mahari Kastro, the Imrei Aish, the Shoel U’Meishiv (Rav Yosef Shaul Nathanson), the Ksav Sofer, the Maharam Schick, the Maharsham (Rav Shalom Mordechai Schwadron), the Mahara”tz Chiyus, and more contemporarily, the Seridei Aish (Rav Yechiel Yaakov Weinberg), and Rav Ovadiah Yosef,zichronam levrachah,follow the Rema’s ruling, that as long as one has valid reasons for performing a specific custom, it does not necessarily get classified as the problematic Chukos HaGoyim, unless its origins are rooted in idolatrous practice.
Opposition and Divergence – It’s Not Easy Going Green
Even so, regarding Shavuos, the Gr”a’ s opposition to setting up trees in shuls was so forceful that he was actually mevattel the minhag. He explains that since our putting trees in shuls is simply a minhag, but not an outright Mitzvah, it cannot counteract the potential prohibition of Chukos HaGoyim, since the non-Jews do so for religious purposes as part of their holiday worship.
On the other hand, several Acharonim, including the Shoel U’Meishiv and the Maharsham, argued with his assessment, and defended this custom, even referring to it as ‘Minhag Yisrael’. They maintained that as long as we are performing the minhag for our own reasons, especially to give honor, we do not have to worry about the practices of other religions. They cite precedent from the Rivash, a Rishon, who allowed visiting a cemetery to mourn a niftar every day of Shiva, even though this was a custom most commonly practiced by Arabs; explaining that this does not fit into the category of Chukos HaGoyim. In this vein, they contend, the same should hold true regarding placing trees in our shuls on Shavuos.
An additional reason posited by the Maharsham is that the proscription of Chukos HaGoyim can only apply if we are performing the exact same action as the non-Jews. However, regarding trees, they place trees outside as well, whereas we do so only in shuls. Therefore, he avers, our minhag is still permitted.
A similar rationale is given by Rav Yitzchok Isaac Yehuda Yechiel Safrin zt”l of Kamarna, explaining that since our intent is not the same as theirs, but it just happens to be that we are performing similar actions, it is not considered Chukos HaGoyim. This is similar to the dispensation for one who works in the king’s palace, that he may dress accordingly and not be concerned with potential violations of Chukos HaGoyim.
An added wrinkle to this debate is that it is not entirely clear which minhag the Gr”a sought to abolish. Was he referring exclusively to trees? Or was his intention to argue that even grasses and flowers are now problematic? Authorities differ as to defining his intent.
For example, the Mishnah Berurah understood that the Vilna Gaon only opposed the minhag of placing trees in shuls due to Chukos HaGoyim; ergo, he never objected to festooning with greenery, which is therefore certainly permitted. In contrast, the Aruch Hashulchan maintained that the Vilna Gaon intended to put an end to any sort of Shavuos custom involving plant adornments, not just trees.
Minhagei Yisrael – Seeing Green?
This is why, in practice, this minhag has a wide spectrum of variations in its observance.
Kehillos of German origin (Yekkehs), as well as most Chassidic communities, including Karlin, Belz, Sanz, Spinka, Skver, Chernobyl, Bobov, and Satmar, follow the minhag of the Magen Avraham and his defenders, especially as an allusion to this custom was found in the Zohar Hakadosh, and not only festoon the shul with greenery as per the Rema, but also place trees.
Many others, in deference to the general understanding of the Vilna Gaon’s position, do not employ trees in their adorning, but will still decorate their shuls utilizing flowers and grass, in essence following the Mishnah Berurah’ s conclusion. This is also the opinion of many contemporary Gedolim including the Chazon Ish, the Steipler Gaon, Rav Shlomo Zalman Auerbach, Rav Yosef Shalom Elyashiv, Rav Yisrael Yaakov Fischer, zichronam levrachah, as well as Rav Chaim Kanievsky, contending that the Vilna Gaon only opposed setting up trees, but not grass.
A third custom, based on the strict interpretation of the Gr”a’s ruling, as understood by the Aruch Hashulchan and others, is not to use trees or grass or flowers to bedeck the shuls, as they are viewed as potential violations of Chukos HaGoyim. Rav Moshe Feinstein zt”l ruled this way as well. Curiously, there is no mention of any sort of Shavuos greenery adornment in Rav Yosef Eliyahu Henkin zt”l’s authoritative Ezras Torah Luach, even though he cites the two other main minhagim of Shavuos brought by the Rema, namely staying up all night and eating milchigs; strongly implying his well-known predilection to following the rulings of one of the Gedolim from whom he received Semichah, the Aruch Hashulchan’s, in this matter. Some even refer to this custom of non-decoration as ‘Minhagei HaYeshivos’.
Interestingly, there are several Chassiduses, including Chabad, Kamarna, and Munkacz, whose Rebbes have written in support, even strongly worded defenses, of the minhag to place trees in shuls on Shavuo. Yet, their own community custom is not to do any Shavuos shul decorating.
Regarding Sefardic observance of the Shavuos greenery minhag, it seems from the fact that there is a noticeable lacuna as to its existence in the works of early Sefardic authorities, from the Rambam to the Shulchan Aruch, or even the later Ben Ish Chai, that it is essentially an Ashkenazic minhag. Yet, we do find several later Sefardic poskim, such as the Knesses Hagedolah, Chida, Rav Chaim Pala’gi, and the Kaf Hachaim, discussing the custom’s reasons and merits. In fact, nowadays, there are Sefardic shuls who do observe at least some semblance of the minhag. Perhaps this is due to Rav Ovadiah Yosef zt”l’s staunch support of the custom, referring to it as ‘Minhag Yisrael that is rooted in the words of Chazal’.
In conclusion, whether or not your shul on Shavuos resembles a grassy Har Sinai or some variation thereof, it is important to remember that ‘Minhag Yisrael Torah Hu’; so you can rest assured that by following the Mesorah of your Kehillah, you are properly observing Kabbolas HaTorah.
The author wishes to acknowledge Rabbi Gedalyah Oberlander’s excellent ma’amar on topic in Kovetz Ohr Yisrael (vol. 20; Sivan 5760).
This article was written l’zechus for Shira Yaffa bas Rochel Miriam v’chol yotzei chalatzeha for a yeshua teikif umiyad.
For any questions, comments or for the full Mareh Mekomos / sources, please email the author: firstname.lastname@example.org.
Rabbi Yehuda Spitz serves as the Sho’el U’ Meishiv and Rosh Chabura of the Ohr Lagolah Halacha Kollel at Yeshivas Ohr Somayach in Yerushalayim. He also currently writes a contemporary halacha column for the Ohr Somayach website titled “Insights Into Halacha”. http://ohr.edu/this_week/insights_into_halacha/
 Vayikra (Ch.18: verse 3).
 Rambam (Hilchos Avodah Zara Ch. 11: 1- 3), Tur and Shulchan Aruch (Yoreh Deah 17: 1), based on the Sifra (Parshas Acharei Mos, Parshata 9, Ch. 13: 8).
 Tosafos (Avodah Zarah 11a s.v. v’ee); answering the seeming contradiction between the Gemara in Avodah Zara ad loc. and Sanhedrin 52b).
 Ran (Avoda Zara 2b s.v. Yisrael), Chiddushei HaRan (Sanhedrin 52b), Shu”t Maharik (Mahar”i Kolon/ Cologne, Shoresh 88, Anaf 1), and Shu”t Rivash (vol. 1: 158 s.v. v’yesh and v’im).
 Biur HaGr”a (Yoreh Deah 178: end 7) and Gilyon Maharsha (ad loc. 1). The Gr”a is bothered by the fact that the sugya in Sanhedrin seems to imply differently than the views of the Maharik, Ran, and later, the Rema, that a Chok Goyim, even one that is not a Chok Avodah Zarah should still be prohibited. Others who ask this question and conclude tzarich iyun on the Maharik’s shittah include the Minchas Chinuch (Mitzvah 262: 2) and the Maharam Bennet (Divrei HaBris; cited in Shu”t Imrei Aish, Yoreh Deah 55). However, there are those who do resolve the Gr”a’s difficulty, such as the Maharam Shick (Shu”t Yoreh Deah 165).
 Darchei Moshe and Rema (Yoreh Deah 178: 1). Although he does not cite either side of this machlokes in his Shulchan Aruch, nevertheless, in his Beis Yosef commentary, Rav Yosef Karo elucidates the shittah of the Maharik at great length and does not even cite Tosafos. Although one may infer that the Rambam (and later the Shulchan Aruch who codified his words as halachah) actually meant similar to Tosafos’s understanding, as the implications of the prohibition of not copying actions of the Goyim, is seemingly unrelated to actions smacking of idol worship (and that is what the Ra’avad was arguing on and ruling akin to the Maharik), nonetheless, from the lashon of many other authorities, including the Maharik himself (ibid.), Sefer HaChinuch (Mitzva 262), Mabit (Kiryas Sefer on the Rambam ibid.), Meiri (Sanhedrin 52b), Bach (Yoreh Deah 178), and Divrei Chaim (Shu”t Yoreh Deah vol. 1: 30), it is clear that they understood that the Rambam himself was only referring to actions that had some relation to Avodah Zarah. See Shu”t Seridei Aish (old print vol. 3: 93; new print Yoreh Deah 39, Anaf 1: 5 – 14) who explains this at length. See also Shu”t Melamed L’Hoyeel (Orach Chaim 16), Shu”t Igros Moshe (Yoreh Deah vol. 4: 11), Shu”t Minchas Yitzchak (vol. 1: 29, 3 and 31), Minchas Asher (vol. 3, Vayikra, Parshas Emor, 33, ppg. 197 – 205), and the Aderes’s recently published Ovar Orach (Shema Eliyahu, 275, pg. 271 – 272; 2003), who discuss the parameters of the prohibition of “U’Vichukoseihem Lo Seleichu” and its nuances at length.
 Another interesting contemporary machlokes regarding flowers is whether planting flowers around a grave, ostensibly for kavod hameis, is considered a violation of Chukos HaGoyim. On this topic, see the Rogatchover Gaon’s Shu”t Tzafnas Pane’ach (vol. 1: 74), Shu”t Minchas Elazar (vol. 4: 61, 3), Rav Dovid Tzvi Hoffman’s Shu”t Melamed L’Hoyeel (Yoreh Deah 109; also citing the opinions of Rav Samson Raphael Hirsch and Rav Ezriel Hildeseimer), Shu”t Minchas Yitzchok (vol. 1: 31), Shu”t Seridei Aish (new print, Yoreh Deah 108), Shu”t Yaskil Avdi (vol. 4, Yoreh Deah 25), and Shu”t Yabea Omer (vol. 3, Yoreh Deah 24).
 Erech Lechem L’Maharikash (Glosses to Yoreh Deah 178: 1; he adds that in his opinion we may not categorize instances not mentioned by Chazal as potential ‘Chukos HaGoyim’), Shu”t Imrei Aish (Yoreh Deah 55), Yosef Daas (Yoreh Deah 348 s.v. v’hinei), Shu”t Ksav Sofer (Yoreh Deah 175), Shu”t Maharam Schick (Yoreh Deah 351), Daas Torah (Orach Chaim 494 s.v. v’nohagin and glosses to Orchos Chaim ad loc. 8), Shu”t Mahara”tz Chiyus (6), Shu”t Seridei Aish (old print vol. 3: 93; new print Yoreh Deah 39, Anaf 2), and Shu”t Yabea Omer (vol. 3 Yoreh Deah 24: 5)
 Furthermore, it must be noted that the Seridei Aish (Shu”t old print vol. 3: 93; new print Yoreh Deah 39, Anaf 2) at length proves that the Gr”a’s shittah actually runs contrary to the vast majority of Rishonim who conclude that unless there is at least a ‘shemetz’ of Avodah Zarah in their actions, copying them would not be a violation of Chukos HaGoyim. See also Shu”t Bnei Banim (vol. 2: 30) who writes that the minhag ha’olam is to follow the Rema in this dispute, as even according to those who generally follow the Gr”a’s psakim, that is only when it is a machlokes Acharonim. Yet, he posits, when the Gr”a argues on both Rishonim and Acharonim, then the normative halachah does not follow his shittah. However, see Shu”t Mishnah Halachos (vol. 10: 116) who does take the Gr”a’s opinion into account (in his specific case) and seems to side with him.
 Yosef Daas (ibid.),Daas Torah (ibid.),andShu”t Rivash (ibid.). See also Rav Avrohom Menachem Halevi Steinberg, Av Beis Din of Brody’s Shu”t Machazeh Avraham (Orach Chaim 29), who concludes similarly. Interestingly, the Rivash adds that this visiting the cemetery was performed l’kavod hemeis. He rhetorically concludes that if we would prohibit this due to Chukos HaGoyim simply because non-Jews do so as well, perhaps we should proscribe all hespedim, as non-Jews also give eulogies.
 Daas Torah (ibid.), Glosses to Orchos Chaim (ibid.), andShu”t Maharsham (vol. 7: 55 s.v. u’vazeh).Conversely, Rav Moshe Feinstein zt”l (Shu”t Igros Moshe, Yoreh Deah vol. 4, 11: 5), simply refers to this sevara as ‘aino klum’.
 Otzar HaChaim (Parshas Kedoshim, Mitzvah 263). Interestingly, even so, the Kamarna minhag (Minhagei Kamarna (Shavuos, pg. 80: 359) is not to adorn the shul with trees, flowers, or grass. Several other Chassiduses, including Munkacz and Chabad so as well. Even though they defend the minhag, they personally do not actually observe it.
 Gemara Bava Kamma (83a) and Sota (49b), Rambam (Hilchos Avodah Zarah Ch. 11: 3), Tur and Shulchan Aruch (Yoreh Deah 178: 2).
 Mishnah Berurah (Orach Chaim 494: end 10) and Aruch Hashulchan (ad loc. end 6).
 As per the Rema and the early Ashkenazic authorities who first cited the minhag, as listed in footnotes 2 and 3. See also Minhagei Yeshurun (131), Otzar Kol Minhagei Yeshurun (Ch. 61: 2, Shavuos, pg. 196), and Kovetz Yerushaseinu’s annual Luach Minhagei Beis Knesses L’Bnei Ashkenaz (Sivan, s.v. asavim v’ilanos).
 Aside for the aforementioned Shulchan Aruch HaRav, Bnei Yisaschar, Shaar Yissachar, and hanhagah of the Chasam Sofer, see sefer Zichron Yehuda (hanhagos of the Maharam Ash; pg. 34b), Imrei Pinchas (312), Mei Hashiloach (vol. 1, Likutim, pg. 279; regarding pictures of flowers on the shul windows), Sfas Emes (ibid.), Shu”t Siach Yitzchak (237), Migdal Dovid (Lekutim Yekarim pg. 44), Tiferes Yaakov (Hanhagos of the Maharam Bennet; pg. 24), Shu”t Hisorerus Teshuvah (vol. 4, pg. 74 s.v. sham s.k. 5), She’aris Moshe (ibid.), Lekutei Mahariach (Minhag Erev Chag HaShavuos s.v. v’hinei), Yalkut HaGershuni (Orach Chaim 494), Taamei HaMinhagim (617), Shnos Chaim (Ch. 20: 2), Netei Gavriel (Hilchos Chag HaShavuos Ch. 9: 1 and footnote 2), Piskei Teshuvos (494: 10), Rabbi Gedalyah Oberlander’s ma’amar in Kovetz Ohr Yisrael (Sivan 5760; vol. 20, pg. 148 – 149), and Kovetz M’Beis Levi (on Nissan, Iyar, and Sivan; Nissan 5766; Chodesh Sivan, Erev Yom Tov, pg. 81 and footnote 2 ad loc.; who only adds the Gr”a’s shittah parenthetically and not as the ikar minhag).
 Minhagei HaBeis Aharon M’Karlin (pg. 8).
Belz Devar Yom B’Yomo Luach (5776; pg. 602) and sefer Minhagei Belz (pg. 46).
 Shefa Chaim Kuntress L’Shavuos (cited in Netei Gavriel ibid. footnote 17).
 Spinka Tefillas Yitzchak Siddur (pg. 35, ‘Minhagei Spinka’). See also Toras Hametzaref (Parshas Balak, pg. 208 s.v. nireh) who cites an additional reason for the minhag of grass that he told over to the Spinka Rebbe, and writes that he was mekabel.
 Skver Luach Maagalei Hashanah (5 Sivan, Erev Chag HaShavuos).
 Cited in Netei Gavriel (ibid.) and Kovetz Ohr Yisrael (ibid.).
 Cited in Orchos Rabbeinu (vol. 2, pg. 99; in 5775 edition vol. 2, pg. 118: 8 and 9). The Chazon Ish is cited as personally not doing any green decorating, but nevertheless allowing his shul’s gabbai to use grasses and flowers.
 Cited in V’aleihu Lo Yibol (vol. 1, pg. 184: 289), and Shalmei Moed (Shavuos, pg. 459), and heavily implied in Halichos Shlomo (Moadim vol. 2, Ch. 12: 1, Chag HaShavuos pg. 371).
 Cited in Hilchos Chag B’Chag (Sefiras HaOmer V’Chag HaShavuos, Ch. 8, footnote 24) and Ashrei HaIsh (Orach Chaim vol. 3, Moadim Ch. 66: 1). Rav Elyashiv held that certainly in Eretz Yisrael, with the non-Jews being prominently Muslim, who do not festoon their houses of worship with trees or grass, one would not have to be choshesh for the machmir understanding of the Gr”a’s opinion. Accordingly, one may indeed decorate their shuls with flowers and grasses. Rav Avigdor Nebenzahl (Yerushalayim B’Moadeha on Shavuos, pg. 184) cites similar logic, ruling correspondingly.
 Halichos Even Yisrael (Erev Shavuos, pg. 223: 3 and footnote 3).
 Shoneh Halachos (vol. 3, 494: 4 and 5).
 This author was told similarly by Rav Ephraim Landy, Mara D’Asra of Beis Knesses Aderes Eliyahu in Givat Zev, who is a direct descendent of the Vilna Gaon. Interestingly, the Aderes, in his recently published Ovar Orach (Shema Eliyahu, 275, pg. 272; 2003) regarding the issue of Chukos HaGoyim and smoking on Tisha B’Av, adds as an aside that the Gr”a abolished the minhag of trees on Shavuos.
 This was also the understanding of the Sefer HaLikutim, a talmid of the Gra”s, cited in the Likutei HaGr”a commentary on sefer Maaseh Rav (ibid.), who explicitly writes that the Vilna Gaon’s intention was to abolish the Rema’s minhag. He cites proof to this view from Rashi’s commentary to Parshas Shoftim (Devarim Ch. 16: 22), who explains that setting up Matzeivos is currently prohibited even though the Avos did so, as it was a custom that was ‘hijacked’ for idolatrous practice. He concludes by positing that perhaps in the days of the Rema the non-Jews did not festoon their houses of worship with grasses or trees. Similarly, in the Shivim Temarim commentary on Tzavo’as Rav Yehuda HaChassid (end 4) it mentions that the Vilna Gaon was actively mevattel all greenery minhagim on Shavuos, even though it was practiced by earlier authorities.
 Shu”t Igros Moshe (Yoreh Deah vol. 4, 11: 5).
 See Shu”t Mishnah Halachos (vol. 8: 107 s.v. ha’amnam), Shu”t Bnei Banim (vol. 1: 6 pg. 22, and Ma’amar 1: 10 – 13, pg. 209 and vol. 2: 8), Kovetz Yeshurun (vol. 20, Nissan 5768, ‘Yosef Chein – Mara D’Asra shel America’, pg. 159), and the ‘Ohr Eliyahu’ introduction to the recently published Shu”t Gevuros Eliyahu (vol. 1 – Orach Chaim, pg. 5).
 See the Luach Hahalachos V’Haminhagim shel Kehillos Ashkenazim (Perushim) Uv’Yeshivos B’Eretz Yisrael (5775; pg. 212, footnote 16), as well as Hilchos Chag B’Chag (Sefiras HaOmer V’Chag HaShavuos, Ch. 8, end footnote 24) both of whom, after listing the basic machlokes involved, ending with the Aruch Hashulchan’s understanding of the Gr”a’s proscription, conclude ‘v’chein nahagu b’Yeshivos’. Anecdotally, and quite fascinatingly, in the introduction to Shu”t Igros Moshe vol. 8 (pg. 6, s.v. Rebbi Dovid), it states that Rav Moshe Feinstein zt”l recalled that in his youth the bochurim from the Volozhin Yeshiva would take down the trees placed in shul l’kavod Shavuos, causing an annual brouhaha.
Obviously this is not exclusive, as there are Yeshivos who do decorate their Beis Midrash for Shavuos, for example Torah V’Daas, which uses grasses and flowers, but not trees.
 Although the Shulchan Aruch HaRav (Orach Chaim 494: 14 and 15) supported this minhag, on the other hand, sefer Minhagei Chabad (Chag HaShavuos 7, pg. 293) details at length that it is not commonly practiced. Indeed, both the Netei Gavriel (ibid. footnote 4) and Rabbi Oberlander (Kovetz Ohr Yisrael ibid. footnote 89) cite firsthand testimony that it is not currently done in Chabad circles. On the other hand, noted Chabad Chassid and acclaimed author ofShiurei Torah, Rav Avrohom Chaim Na’eh zt”l (Shnos Chaim Ch. 20: 2), wrote that the prevailing minhag is to decorate with grasses, flowers, and trees.
 See Otzar HaChaim (Parshas Kedoshim, Mitzvah 263) as compared to Minhagei Kamarna (Shavuos, pg. 80: 359).
 See Shaar Yissachar (Maamar Chag HaBikkurim 48) and Darkei Chaim V’Shalom (737), who although strongly defend the practice, nevertheless conclude that it is no longer ‘minhageinu’, nor of “Kehillos Chassidim V’Anshei Maaseh”, due to the opposition of the Gr”a, explaining that festooning is not m’dina and is now considered “chok ha’amim”.
 It is also noticeably absent from the recently published work of the Sefardic Av Beis Din of Yerushalayim before 350 years, Rav Yitzchak Bu’ino’s Shulchan Melachim, even though he mentions (Orach Chaim 494: 3 and 5) the two other famous Shavuos minhagim mentioned by the Rema, eating milchigs and staying up all night learning. In fact, there are several Sefardic authorities, including the Arichas Shulchan (Yalkut Chaim vol. 6, 494: 12) and the Shtilei Zeisim (494 Saviv L’Shilchanecha 3), who expressly write that festooning with greenery is not a Sefardic minhag. The Netei Gavriel (ibid. end footnote 4) writes similarly, ‘V’haSefardim ain nohagim klal l’haamid ilanos v’asavim’, albeit without citing sources.
 See Knesses Hagedolah (Hagahos on Tur, Orach Chaim 494: 2), Shulchan Gavoah (ad loc. 6), Birkei Yosef (ad loc. 6),Moreh B’Eztzba (Avodas Hakodesh, Chodesh Sivan 224, Etzba Ketana), Rav Yitzchak Lapronati of Italy’sPachad Yitzchak (Os Shin, erech Shavuos pg. 37a),Rav Avraham Kalfonof Tripoli’sLeket Hakatzir (Hilchos Shavuos 10), Ruach Chaim (ad loc. 4), Yefeh L’Lev (vol. 2, ad loc. 7), and Kaf Hachaim (ad loc. 53 – 59).
 Shu”t Yechaveh Daas (vol. 4: 33) and Chazon Ovadiah on Hilchos Yom Tov (Hilchos Chag HaShavuos 11, pg. 317 – 318). His son, the current Chief Rabbi of Israel, Rav Yitzchak Yosef, expresses similar sentiments in his Yalkut Yosef (Kitzur Shulchan Aruch, 494: 17). This minhag is also cited by Rav Raphael Baruch Toledano zt”l, in his Kitzur Shulchan Aruch (332: 31 and 32), discussing the greenery minhag but without mention of trees, the same lashon as the earlier Knesses Hagedolah. Similarly, and more recently, Rav Yaakov Moshe Hillel, in his comprehensive Luach Dinim U’Minhagim Ahavat Shalom (5776; Chag HaShavuos, Minhagei HaChag, pg. 197) also writes that ‘nohagim l’shtoach asavim’, implying that nowadays it is a common Sefardic minhag. However, in footnote 155, he adds that the Gr”a abolished the minhag of setting up trees, which is akin to the Mishnah Berurah’s conclusion. It emerges that according to Rav Ovadiah Yosef, Sefardim may even festoon the shul with trees, but according to Rav Raphael Baruch Toledano and Rav Yaakov Hillel, Sefardim may only use grasses and flowers. An interesting dichotomy within a dichotomy.
 For early uses of this and similar dictums, see Tosafos (Menachos 20b s.v. nifsal), Beis Yosef (Orach Chaim 128: 7 and Yoreh Deah 39; citing the Rashba), Rema (Yoreh Deah 376: 4; citing the Maharil), and Matteh Efraim (610: end 11).
Disclaimer: This is not a comprehensive guide, rather a brief summary to raise awareness of the issues. In any real case one should ask a competent Halachic authority.
L’iluy Nishmas the Rosh HaYeshiva – Rav Chonoh Menachem Mendel ben R’ Yechezkel Shraga, Rav Yaakov Yeshaya ben R’ Boruch Yehuda, and l’zchus for Shira Yaffa bas Rochel Miriam and her children for a yeshua teikef u’miyad!