The Rama (Rabbi Moshe Isserles, Poland, 1525-1572), in his glosses to the Shulchan Aruch (336), rules that it is forbidden on Shabbat to place flowers in water if this will cause the flowers to bud. Many other authorities take this position, as well, including the Chida (Rabbi Chayim Yosef David Azulai, Israel, 1724-1806) and, more recently, Chacham Ovadia Yosef (Yechaveh Da'at 2:53). Thus, it would be forbidden to place roses in a vase with water on Shabbat, as this causes the leaves to open.
May one ask a gentile to place roses in a vase with water on Shabbat?
Rabbi Shemuel Pinchasi (contemporary) ruled that since the issue of placing flowers oneself in water on Shabbat is subject to controversy, as some authorities permit doing so even if the flowers will open, we may allow asking a gentile to do so on one's behalf. Even though Halacha follows the opinion that forbids placing flowers of this type in water, in light of the difference of opinion that exists in this regard we may be lenient with regard to asking a gentile to perform this act on Shabbat.
Summary: It is forbidden on Shabbat to place in water roses or other flowers that will bud as a result; one may, however, ask a gentile to do so on one's behalf.
