Other shuls receive some adornment but to a far lesser extent with only flowers and grasses. And then there are shuls where no special Shavuos decorating is done at all. There is quite a varied spectrum of minhagim with each kehillah following its own traditions. We will explore the main prevailing minhagim in Klal Yisrael along with their halachic background.

Replicating Matan Torah

The Rema in Hilchos Shavuos (Orach Chayim 494:3) writes: “V’nohagin lishtoach asavim b’Shavuos b’veis haknesses v’habatim zeicher l’simchas Matan Torah [The custom is to spread grass(es) on Shavuos in the shuls and in houses as a remembrance to the joy of Matan Torah].” This custom is cited as the practice of several early Ashkenazic authorities including the Maharil and Terumas Hadeshen as well as the kehillah of Worms. 

There are several rationales given to explain the connection between adornment of a shul and Matan Torah: 

1. The Levush explains that the pasuk states (Shemos 19:8) that the cattle and sheep were prohibited to graze in the area at the time of Matan Torah implying that Har Sinai was surrounded by grass and thus as a zeicher l’Matan Torah we replicate that setting. 

2. The Maharil states that the custom is to festoon the shul specifically with fragrant grasses and flowers. Several seforim cite as a source the Gemara (Shabbos 88b) that explains the pasuk in Shir Hashirim (5:13) “Lechayav ka’arugas habosem… sifsosav shoshanim [His cheeks are as a bed of spices… His lips are as lilies]” as a reference to Matan Torah when every dibbur that Hashem spoke filled the world with an ethereal fragrance. 

3. Bnei Yisaschar cites a mashal of the Midrash Rabbah about a king in his orchard to explain that Bnei Yisrael at Har Sinai were “k’shoshanah bein hachochim [like a rose among thorns]” (Shir Hashirim 2:2) and that in the zechus of Bnei Yisrael’s united declaration of “naaseh v’nishma” the world was saved. To symbolize this on Zeman Matan Torah we adorn our shuls with flowers. 

Interestingly the Chida cites an early albeit infamous source for this minhag that also evidences its antiquity. According to the Targum Sheini on Megillas Esther during Haman’s diatribe against Klal Yisrael to King Achashveirosh he disparaged the various “bizarre customs” of the Jews including that of spreading and gathering of apples and flowers on the roofs of their shuls on Shavuos. Although we generally do not learn halachah from agaddah Noda B’Yehudah writes that we are permitted to glean from it minhag Yisrael.

Trees Are Terrific

When discussing this minhag Magen Avraham adds another rationale: to place trees in the shuls to commemorate the fact that Shavuos is Rosh Hashanah for peiros ha’ilan tree fruits (Rosh Hashanah 16a). Several later eminent authorities cite this minhag as well. 

The Minchas Elazar of Munkacs in his sefer Shaar Yissachar cites an allusion to this minhag from the Zohar Hakadosh. On the pesukim in parshas Emor discussing the Yom Tov of Shavuos the Zohar writes that Rabi Shimon expounded on the words of Tehillim (96:12 and 13) “az yeranenu kol atzei yaar lifnei Hashem [then will the trees of the forest rejoice before Hashem] ” and then goes on to explain the kabbalistic secrets of trees (“sod ha’ilan v’atzei chayim”). Hence we see the connection between trees and Shavuos. 

It is reported anecdotally (although the facts are somewhat disputed) that the Chasam Sofer was very makpid on this minhag. One year when the gabbai did not set up the trees in the shul for Shavuos he did not live out the year; another version has it that his house burned down.

The Gra’s Oppostion

On the other hand we find that the Vilna Gaon forcefully opposed this minhag roundly condemning its practice. As recorded by Rav Avraham Danzig ztz”l in both Chayei Adam on Orach Chayim and Chochmas Adam on Yoreh Deiah as well as in the sefer Maaseh Rav which details the Gaon’s personal hanhagos the Gra sought to eradicate this minhag from Klal Yisrael based on the fact that in his day (and nowadays as well) the non-Jews set up trees in their houses of worship as part of their holiday festivities. The Gra maintained that for us to follow suit would violate the Biblical prohibition of chukos hagoyim.

In parshas Acharei Mos the Torah exhorts us not to follow in the ways of the local non-Jewish populace: “uv’chukoseihem lo seleichu.” According to the Rambam and as 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; even if they are not done l’sheim avodah zarah with specific idolatrous intent. 

Other Rishonim however primarily the Ran Mahari Kolon and Rivash 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. 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. 

The Vilna Gaon rejects their understanding of the prohibition and Gilyon Maharsha seems to follow Tosafos but the Rema explicitly rules like the Maharik and Ran as does the Beis Yosef that a custom that is secular with no connection to avodah zarah may be observed. 

Most later authorities including the Mahari Kastro the Imrei Eish Shoel U’meishiv (Rav Yosef Shaul Nathanson) the Ksav Sofer the Maharam Schick the Maharsham (Rav Sholom Mordechai Schwadron) the Maharatz Chiyus and more contemporarily Seridei Eish (Rav Yechiel Yaakov Weinberg) and Rav Ovadiah Yosef zichronam livrachah all rule in accordance with the Rema.

Footnotes 

1. For example in some kehillos aromatic plants are passed around while other shuls decorate the sifrei Torah. Some put branches on the wall others on the bimah and yet others layer the floor in a “carpet” of grass. Some place pictures of flowers on windows and others hang flowers in glasses from the ceiling. Others do some sort of flower decorating at home. Some even throw flowers as the Torah passes by. Many of these diverse minhagim are mentioned by the Kaf HaChaim (Orach Chayim 494:53–59). 

2. See for example Minhagei Maharil (Hilchos Shavuos 20 pg. 160) the hagahos on Sefer Haminhagim by Rav Yitzchak Isaac of Tirna [Tyrnau] (Minhagei Chodesh Sivan 49) Leket Yosher (minhagim of the Terumas Hadeshen; pg. 103) Minhagos Varmiza [Worms] (pg. 255) Minhagim D’K”K Varmisha [Worms] (vol. 1:102 pg.110) Rav Yosef Yuspa Haan of Frankfurt’s Yosef Ometz (851) Rav Yehuda Aryeh of Modena’s Shulchan Aruch (pg. 67) and Shnei Luchos Habris (Ner Mitzvah 7). 

3. Levush Malchus (Orach Chayim 494:1). This reason is also given by Rav Yaakov Emden in his Siddur Beis Yaakov (vol. 2 pg. 148:2) and cited briefly by Mishnah Berurah (ad loc. 10). 

4. See Shevet Mussar’s Midrash Talpiyos (vol. 2 pg. 457 erech “dudaim”) Lekutei Chaver Ben Chaim (vol. 2 pg. 34 Lekutei Orach Chayim 494) Rav Chaim Palagi’s Ruach Chaim (Orach Chayim 494:4) and Moed l’Kol Chai (8:33) as well as the controversial Chemdas Hayamim (Hilchos Chag HaShavuos Ch. 3 pg. 105 3rd column). Midrash Talpiyos adds an additional interesting reason based on the Alshich’s explanation (Parshas Vayetzei Ch. 30:14) that Reuven found the “dudaim bimei ketzir chittim” to be referring to Erev Shavuos. The Ramban (ad loc.) cites several definitions of what the dudaim might be including a fragrant flower and fertility drug. Accordingly this means that Leah conceived Yissachar the greatest talmid chacham of the Shevatim on Shavuos night. To allude to this we festoon the shuls with fragrant flowers on Shavuos Zeman Matan Torah. An alternative but similar approach is given in Pardes Yosef (Shemos 2:3) citing the Maharam Bennet. Since we know Moshe Rabbeinu was born on the 7th of Adar and was hidden for three months before he was hidden in the reeds (a type of grass) on the banks of the Nile River this means that he was actually saved by Basya on Shavuos. As Moshe was the leader through whom we received the Torah we commemorate this by spreading grass on Zeman Matan Torah (although according to this understanding it does not necessarily need to be fragrant). Some make that same cheshbon in reverse that if Moshe Rabbeinu was born on 7 Adar then nine months prior he must have been conceived on Shavuos. Rav Yitzchak Nissim Palagi (Yefeh L’lev vol. 2 494:7) adds that Rashi explains the pasuk in Tehillim (45:1) “Lamnatzeach al shoshanim ” to be referring to talmidei chachamim. As such on Shavuos the Yom Tov of talmidei chachamim (see Pesachim 68b as to Rav Yosi’s feast on Shavuos “ie lo hai yoma d’ka garam kama Yosef ika b’shuka”) we honor talmidei chachamim in this manner. 

5. Bnei Yisaschar (Maamrei Chodesh Sivan Maamar 4:7 & 8) citing Midrash Rabbah (Vayikra parshas Acharei Mos 23:3 s.v. Rav Azarya). 

6. Birkei Yosef (Orach Chayim 494:6). He cites it as aggadah.. 

7. Targum Sheini on Megillas Esther (3:8) according to the Tashbatz’s Pas’shegen Haksav translation (ad loc.). 

8. See Yerushalmi Peah (2:4) Rabbeinu Tam’s Sefer Hayashar (Teshuvos 45:3) Rashbam (Bava Basra 130b end s.v. ad sheyomru) Shu”t HaRashba (335) Shach (Choshen Mishpat 81:56) Pri Chadash (Orach Chayim 128:20) Tosafos Yom Tov (Berachos Ch. 5 Mishnah 4) Shu”t Noda B’Yehudah (Tinyana Yoreh Deiah 161) Shu”t Be’er Yaakov (Even Ha’ezer 199) Shu”t Chaim Sha’al (vol. 1:92) Machazik Brachah (Orach Chayim Kuntres Acharon 51) Shu”t Maharsham (vol. 1:163 s.v. hinei) Sdei Chemed (Maareches Alef klal 95-96 and Pe’as Hasadeh klal 39) Shu”t Emek Hateshuvah (vol. 2:1 s.v. v’od) and Shu”t Mishnah Halachos (vol. 9:319 s.v. v’hanlfa”d and u’ma”sh) and Shu”t Yechaveh Daas vol. 4:33). Most authorities maintain that one may learn a halachah from aggadah if it does not contradict any other gemara or halachah. See next footnote. 

9. Shu”t Noda B’Yehudah (ibid. s.v. u’mah sheratzah). 

10.Magen Avraham (Orach Chayim 494:5 s.v. nohagin). 

11.Elyah Rabbah (Orach Chayim 494:12) Shulchan Aruch Harav (ad loc. 14 and 15) Chok Yaakov (ad loc. 7) Be’er Heitiv (ad loc. 7) and Rav Yaakov Emden in his Siddur Beis Yaakov (vol. 2 pg. 148:3). Interestingly many of them do not cite the last words of Magen Avraham (“vayispallelu aleihem”) that we should also daven for the trees. 

12. Shaar Yissachar (Maamar Chag Habikkurim 48) based on the Zohar (Parshas Emor vol. 3 pg. 96a). Interestingly the Munkacser minhag is not to festoon with grass or trees even though their rebbe defends the minhag. See also Darkei Chayim V’shalom (737). 

13. As reported in Sefer Chut Hameshulash (pg. 128). See also sefer Tzena Malei Sifra (pg. 184) who records the alternate ending of the story. The Chasam Sofer himself expounded on the importance of this minhag as well — see Drashos Chasam Sofer (vol. 2 pg. 285 3rd column; newer version vol. 2 pg. 576 Drashos L’Shavuos 5562 s.v. v’asisah). 

14. Chayei Adam (vol. 2 131:13) Chochmas Adam (89:1) and Maaseh Rav (195). 

15. Vayikra (18:3). 

16. Rambam (Hilchos Avodah Zarah 11:1–3) Tur and Shulchan Aruch (Yoreh Deiah 17:1) based on the Sifra (Parshas Acharei Mos parshata 9 13:8). 

17. Tosafos (Avodah Zarah 13a s.v. v’ee); answering the seeming contradiction between the Gemara in Avodah Zarah ad loc. and Sanhedrin 52b). 

18. Ran (Avodah Zarah 2b s.v. Yisrael) Chiddushei HaRan (Sanhedrin 52b) Shu”t Maharik (Mahari Kolon Shoresh 88 anaf 1) and Shu”t Rivash (158). 

19. Biur HaGra (Yoreh Deiah 178:7) and Gilyon Maharsha (ad loc. 1). The Gra 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 Eish Yoreh Deiah 55). However there are those who do resolve the Gra’s difficulty such as the Maharam Shick (Shu”t Yoreh Deiah 165). 

20. Darchei Moshe and Rema (Yoreh Deiah 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 Raavad was arguing on and ruling akin to the Maharik) nonetheless from the lashon of many other authorities including the Maharik himself (ibid.) Sefer Hachinuch (mitzvah 262) Mabit (Kiryas Sefer on the Rambam ibid.) Meiri (Sanhedrin 52b) Bach (Yoreh Deiah 178) and Divrei Chaim (Shu”t Yoreh Deiah 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 Eish (old print vol. 3:93; new print Yoreh Deiah 39 anaf 1:5-14) who explains this at length. See also Shu”t Melamed L’hoyil (Orach Chayim 16) Shu”t Igros Moshe (Yoreh Deiah vol. 4:11) and Minchas Asher (vol. 3 Vayikra Parshas Emor 33 pg. 197–205) who discuss the parameters of the prohibition of “uv’chukoseihem lo seleichu” and its nuances at length. 

21. 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 Paneiach (vol. 1:74) Shu”t Minchas Elazar (vol. 4:61 3) Rav Dovid Tzvi Hoffman’s Shu”t Melamed L’hoyil (Yoreh Deiah 109; also citing the opinions of Rav Samson Raphael Hirsch and Rav Ezriel Hildeseimer) Shu”t Minchas Yitzchak (vol. 1:31) Shu”t Seridei Eish (new print Yoreh Deiah 108) Shu”t Yaskil Avdi (vol. 4 Yoreh Deiah 25) and Shu”t Yabia Omer (vol. 3 Yoreh Deiah 24). 

22. Erech Lechem L’Maharikash (glosses to Yoreh Deiah 178:1; he adds that in his opinion we may not categorize instances not mentioned by Chazal as potential chukos hagoyim) Shu”t Imrei Eish (Yoreh Deiah 55) Yosef Daas (Yoreh Deiah 348 s.v. v’hinei) Shu”t Ksav Sofer (Yoreh Deiah 175) Shu”t Maharam Schick (Yoreh Deiah 351) Daas Torah (Orach Chayim 494 s.v. v’nohagin and glosses to Orchos Chayim ad loc. 8) Shu”t Maharatz Chiyus (6) Shu”t Seridei Eish (old print vol. 3:93; new print Yoreh Deiah 39 Anaf 2) and Shu”t Yabia Omer (vol. 3 Yoreh Deiah 24:5) 

23. Furthermore it must be noted that the Seridei Eish (Shu”t old print vol. 3:93; new print Yoreh Deiah 39 Anaf 2) at length proves that the Gra’s shittah is actually against 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 Gra’s psakim that is only when it is a machlokes Acharonim. Yet he posits when the Gra 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 Gra’s opinion into account (in his specific case) and seems to side with him.