Negative space is not emptiness for its own sake. It is a place for attention to land. One stem, one rock, one lamp at dusk — the room stops competing with you.
Change the stem with the season if you like. Or keep the same dry branch for a month. Constancy can be as calming as novelty.
One stem is enough. The room will do the rest.
Clear the surface around the vase first. The object only works if the desk is not arguing with it.