Shavuot Midrashim: Mishnah Berurah 494:10
“Flowers are a reminder of the happiness of the Giving of the Torah. Because there were plants around Mount Sinai, as the verse states: “No one shall ascend with you (Moses), neither shall anyone be seen anywhere on the mountain, neither shall the sheep and the cattle graze facing that mountain.” (Mishnah Berurah 494:10 on Exodus 34:3)