No, if only a few degrees of frost are expected, it's unlikely your hardy perennials (geraniums, bleeding hearts) will suffer any damage at all. If it goes to -10 or -15 C, then there may be foliage damage on some, but even then it's very unlikely they'd suffer extreme damage; the damaged leaf edges would soon be replaced by new leaves. This kind of thing happens all the time in colder zones - it's nothing to get all excited about.