Are flowers in a vase Muktzah on Shabbos?
Background: All items that do not contain a use are considered Muktzah Machams Gufo on Shabbos and may not be moved for any purpose in a regular fashion, neither for space, use or to save from damage. Thus, ground products such as plants, flowers, grass, branches and twigs that are laying on the ground are Muktzah on Shabbos, whether they are attached or detached from the ground, plant or tree. However, if one takes any of the above products before Shabbos and designates it for a use, it is no longer considered Muktzah and may be moved on Shabbos like any other non-Muktzah item.
The law: Although flowers that are lying on the ground are Muktzah on Shabbos, flowers that have been gathered and prepared from before Shabbos to be used for decoration of one’s home, are not Muktzah. Accordingly, it is permitted without restriction to move a bouquet or vase of flowers on Shabbos. Likewise, on Shavuos, it is permitted to move the various flowers and plants [that are not in pots with earth] that have been placed around the house or Shul for decorative purposes. For this reason, it is also permitted to lift and move a good scenting branch or leaf [i.e. myrtle, rosemary, mint, etc] that has been designated before Shabbos for smelling, and there is no Muktzah prohibition involved.
Plants in pots with earth: Plants or trees that grow within pots that contain earth do have Muktzah restrictions applicable to them, as will be discussed in a further Halacha!
Entering and removing the flower to and from the water vase: It is permitted to remove flowers from the water vase on Shabbos, however, certain restrictions apply regarding placing the flowers into the water vase, as will be explained in a future Halacha
Flowers in a vase are not Muktzah and may be moved without restriction.
 See Admur 336:18; 494:14; Rama 336:11; M”A 336:13; 494:5
 Admur 308:8; Michaber and Rama 308:7
 Admur 312:9; 336:4; M”A 312:6; See Peri Megadim 312 A”A 6
Other opinions: Some Poskim rule that all grass which is attached to the ground is not Muktzah. [Elya Raba and Tosefes Shabbos brought in Machatzis Hashekel 312:6; so rules also M”B 312:17; Shaar Hatziyon 336:42] According to this opinion Muktzah only applies to those things attached to the ground which had a decree of Shimush Gidulei Karka attached to it, such as fruits of a tree. However, grass and the like is not forbidden due to Gidulei Karka, as explained in 336, and it is therefore also not Muktzah. However, once the grass is detached it becomes Muktzah.
 Admur 308:50
 See Admur 336:18; 494:14; M”A 336:13; 494:5; Piskeiy Teshuvos 336:22
The reason: As they have been designated for a use from before Shabbos and thus have the status of a vessel. [Admur ibid]
 Admur 494:14; M”A 494:5