Flowers, branches, or stems that were picked before Shabbat for their beauty or fragrance, are not muktzeh on Shabbat. Therefore, a vase containing branches that are used for their beauty or fragrance may be moved. Similarly, they may be removed from the water for viewing or smelling. There is no problem of Kotzer, since there are no roots. One may also return stems to the water if they do not have flowers, or if they have fully-developed flowers. There is no problem of Zore’a, since the water will not cause any further growth but will only preserve their freshness so they do not wither.
However, one may not place in water any stems with flowers that are budding or that have not yet fully opened, because placing them in the water causes further growth. Nevertheless, one may remove them from the water. This is not considered Kotzer, because they have not put down roots in the water. Once removed, though, one may not replace them. Therefore, if one receives a bouquet of flowers as a gift on Shabbat and it contains flowers that have not yet fully opened, one may not place them in water, as this will cause the flowers to grow and open. Rather, one should put the bouquet in a vase that does not contain water.
. Rema 336:11. According to Maharikash, one may put flowers in water as well, since even if they open it is not considered new growth. His opinion is not accepted. SHT 336:48 even states that a Torah prohibition may be involved. It is generally agreed that one who removed branches from water may put them back. Aĥaronim disagree, however, whether one may put them in water to begin with. Tosefet Shabbat and Ĥayei Adam prohibit doing so, while SAH and Pri Megadim permit it, as is quoted by MB 336:54. SHT ad loc. 48 states that one may be lenient, as the law in question is rabbinic. This is on condition that he prepared a container of water before Shabbat. However, on Shabbat, one may not fill a container with water in which to place the branches. This is because it involves exerting effort for the sake of the branches, which is prohibited, as explained in Sukka 42a in the context of a lulav. Yeĥaveh Da’at 2:53 is lenient, based on Rashba, and allows one to fill a container with water on Shabbat. The reason is that while we are stringent regarding a lulav because it is muktzeh, this stringency does not apply to other branches. As SA 321:11 states: “One may water a detached plant to prevent it from withering.” This is also the position of Menuĥat Ahava vol. 2 ch. 3 n. 18. It would seem that one may rely on the lenient position, since this is a dispute regarding a rabbinic prohibition.