Question: Is one permitted to place a bouquet of flowers or Hadassim (myrtle branches) into a vase filled with water on Shabbat or is this forbidden?
Answer: In the previous Halacha we have explained that one of the forbidden works on Shabbat is planting. We have also quoted the Gemara which states that water trees or seeds on Shabbat is likewise a transgression of the forbidden work of planting, for just as planting causes fruits and plants to grow, so too, watering trees on Shabbat causes the seeds and fruits to grow.
The Rashba writes in his responsa (Volume 4, Chapter 73) that it is permissible to place tree branches, such as Hadassim and other fragrant objects, into a vessel filled with water on Shabbat, for this does not cause it to sprout at all and only prevents the leaves from drying out. Although it is forbidden to place wheat or barley kernels in water for a prolonged time on Shabbat, for this causes a new occurrence of the kernels cracking open and beginning to take root in the water, nevertheless, this prohibition does not apply to tree branches since no new occurrence is taking place. The Maharil and Rama rule likewise.
Nevertheless, the Rama (in his notation on Chapter 336, Section 11) writes in the name of the Maharil, as follows: “It is permissible to place tree branches in water on Shabbat as long as they do not contain flowers or buds that will open up as a result of the moisture.” Based on this, although the Rama rules that placing tree branches in water is not forbidden on Shabbat, nevertheless, if there are flowers which will open as a result of the water, this is considered a new occurrence of blossoming and constitutes the forbidden work of planting, similar to the law regarding placing wheat or barley kernels into water on Shabbat.
Nevertheless, the great Maharikash (Rabbeinu Yaakov Castro zt”l, who lived in Egypt during the times of Maran Ha’Bet Yosef; when he came to visit the graves of righteous Torah scholars in Tzefat, he was hosted by Maran Ha’Bet Yosef himself who showed him great honor. He then returned to Egypt and passed away in the year 5370/1609) disagrees with the Rama’s opinion, for only with regards to wheat and barley kernels which actually sprout something new as a result of placing them in water is it prohibited to place them in water because of the forbidden work of planting. On the other hand, the opening up of a flower or bud is not considered a new sprouting and there is thus no prohibition to place flowers into water on Shabbat. Nevertheless, since this is still subject to a disagreement among the Poskim, Maran Rabbeinu Ovadia Yosef zt”l writes that, halachically speaking, that if the buds of the flowers are still closed and may open as a result of placing them in water, one should act stringently in accordance with the Rama’s opinion and not place them in water on Shabbat. (It is permissible though to add water to a vase which contains flowers on Shabbat.)
Clearly, all of the above applies only to flowers or tree branches that were prepared on Erev Shabbat for decoration or fragrance on Shabbat. However, just any regular tree branches or flowers may not be moved on Shabbat due to the prohibition of Muktzeh.