What's Christmas without a Christmas tree, mistletoe, holly and maybe a poinsettia or two? You've probably been warned that many of the traditional holiday plants contain toxic chemicals. Yet, how dangerous are they, really? While a couple of holly berries might give you enough caffeine to help energize your holiday, the same dose would harm a household pet. Other plants, like poinsettia, might taste bad, but really don't pose much of a threat.