Answer by Rabbi Chaim Hillel Raskin – a rov in Kfar Chabad Beis
Planting (zoreia), whether in earth or in water (i.e. sprouting seeds or pits), is a melacha. Likewise, complete flowers with roots at the bottom can continue growing, and may not be placed into water on Shabbos.
However, regular vase-ready flowers which are cut at the bottom cannot continue to grow, and do not pose a problem of zoreia.
Chazal rules that one may not fill a flower vase with water or add water to a vase under the decree of tircha yeseira, extra labor. If the vase contains water from before Shabbos, the Alter Rebbe holds that one may insert new flowers inside. Adding water is permissible on yom tov, but not on Shabbos.
Bulbs which will open from the water may not be inserted into water (even if they were in the water previously), since the blooming of the bulbs is an extension of zoreia, causing something to grow.
Although ordinary twigs and leaves are muktza, nonetheless if they were designated before Shabbos for use, they are not muktza. Thus, flowers which have been designated for decorative use may be moved on Shabbos.
However, if they contain unopened bulbs, they should be moved softly so that they are not moved in or out of the water.
(Sources)
doesn’t putting it in water keep it from dying?
And? The world is Hashem’s “courtyard,” so He can do what He wants in it on Shabbos (Bereishis Rabbah 11:5), whereas we cannot.
Nature itself doesn’t stop on Shabbos, does it? The sun rises to give us light, rain is formed in the clouds and falls from the sky and waters the plants, etc which enables them to grow.
There is an halocho in alter rebbe’s shluchan aruch to prepare two הדסים before שבת, (one for שמור and for זכור, from the story in גמרא etc.) and to smell them on שבת, that where in water vase set before שבת. I’m yet to find out a good reason why it’s not done by חב”ד , since it’s not a מנהג but an הלכה.