Q. Are they (plant decorations placed at home or shul) muktza on Shabbos or Yom Tov?

A. On question 33 in this forum we wrote that; decorative branches that were placed on the Shul before Yom Tov are not Muktze, see Mishnah Berurah (494 – 9) in the name of the Mogen Avrohom. Mishnas Yaakov (ibid.) writes that he observed Torah sages that moved those branches as they went up to the Bimah.
Others argue that they may become muktze lemitvoso similar to the decorations of the Suka. (Erech Shai O.H. 494). However, Betzel Hachochmo (4: 87- 5: 119) and Binyan Shlomo (20) disagree.

Horav Shlomo Miller Shlit”a is of the opinion that if they were only fixed temporarily and can easily be removed, they are not Muktze.
He maintains that the same would apply when Shabbos precedes Yom Tov, if the branches were collocated before Shabbos with the intent of being a decoration.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.