What is the connection between having flowers in the house and Shavuos? When a person sees flowers on his table on Shavuos, what can he think, in order to transform it from a mere externality into a more spiritual experience?
(Also, I noticed that the word for “flower” in Hebrew is perach. Can this be an implication that the flowers on Shavuos are supposed to remind a person that the souls of the Jewish people were porcha, they “flew out” of their bodies, when they stood at Har Sinai?)
The roots of this custom are discussed in Shulchan Aruch Orach Chaim: 494, which cites the custom to hang grass in remembrance of the event of the giving of the Torah, where grass surrounded Har Sinai. The Magen Avraham (ibid) writes that there is also a custom to place trees in the shul and in houses, in remembrance of how Shavuos is the time where the world is judged on its fruit of trees. The Magen Avraham adds that one should pray for the fruit of the trees [to merit a good judgment]. The Gra abolishes this custom, since it is now the custom of gentiles to erect trees on their festivals. Refer to the Mishnah Berurah (ibid, 10) for more on this.
In halachah, only grass and trees are mentioned. But flowers are also included in this. The root of this matter is because at the time when Adam sinned, there were “288 sparks of holiness” which fell to the “side of evil”. The number 288 in Hebrew spells the word repach, an opposite arrangement of the letters of the Hebrew word perach, which means “flower”. At the time of the giving of the Torah, there was a temporary leave from death, for the curse of death was lifted at that time. At that time, the “288 sparks” were raised back to holiness (this is according to one opinion of the Sages in Tractate Avodah Zarah). Since the “288 sparks” were raised back to holiness, the souls of the Jewish people flew out of their bodies – as Chazal state, “parcha nishmasan”, their souls flew away from them. In other words, the “288 sparks of holiness” were raised back to holiness, together with this elevated state.
This is the depth behind hanging grass to remember the giving of the Torah. At the giving of the Torah, they were raised from the level of animal to the level of human being, for on Pesach there is a korbon brought made of barley, which is animal food, and on Shavuos there is an offering brought from wheat, which is food for people [therefore, grass is in remembrance to the food of animals, which we became elevated from, at the time of the giving of the Torah].
The Magen Avraham writes that one should place trees in shuls and homes and pray for the fruit of trees. On a deeper level, this is really a prayer to rectify the sin of eating from the fruit of the forbidden tree, the Eitz HaDaas, the first fruits of the world which were eaten which were an act of sin, and in its place, we pray to reveal the fruit of the Eitz HaChaim, the Tree of Life. This can be accomplished through the study of Torah, for Torah is called the Eitz HaChaim, the “tree of life”. That is also why there is a custom to place the trees near the place in the shul where the Torah is read from.