Mistletoe is a parasitic plant with white berries, traditionally used as a Christmas decoration. It was used to decorate the Room of Requirement when Dumbledore's Army had a session on 18 December, 1995. Harry and Cho Chang shared a kiss underneath it. Luna Lovegood believed that it is commonly infested with nargles.
The mistletoe in the Room of Requirement is shown still growing when Cho and Harry kiss, suggesting the room grew it, as they required it.