Q. Why do we have flowers and greenery in the synagogue on Shavu’ot?
A. One view is that it recalls Mount Sinai, a little, nondescript mountain which was so excited at being chosen for the giving of the Torah that it sprouted greenery. Likewise, in every generation the synagogue and the Jewish people are proud and excited to be receiving the Torah anew.
(Note that the Gerer Rebbe points out that Shavu’ot is not called the Time of the Receiving of the Torah, but the Time of the Giving of the Torah; the Torah was given once, but it is up to us to receive it in every generation of history.)
Another opinion says that this is the time of year when God judges the world in relation to the fruit of the trees (Rosh HaShanah 16a), and the custom therefore reminds us of our dependence on the Divine bounty.
The Vilna Gaon opposed the custom as an imitation of non-Jewish usage (Christians bring flowers and greenery into churches), but other authorities disagree and say that the Jewish custom is older and has its own logic.