Vervain, Verbena officinalis, is unusual in this collection: an unremarkable-looking plant with a genuinely documented sacred role in Roman religion, attested by Romans.
The Latin verbenae denotes sacred boughs or herbs carried and used in ritual, and vervain is the plant most consistently identified with the term. Pliny the Elder devotes attention to it in the Natural History, describing its use in purifying altars and houses, in treaties and embassies, the verbenarius was the herald who carried it, and in casting lots. He also reports, with visible scepticism, the elaborate conditions attached to gathering it: at a particular season, with the hand, a circle traced round the plant, honey and honeycomb offered to the earth in payment. Pliny records these things as what the magi say, and his tone is that of a man reporting a superstition rather than endorsing one.
He also connects it to the Gauls, which is the origin of the standard claim that vervain is a druidic herb. That claim rests on Pliny, as the oak and mistletoe claim does, and carries the same warning: it is a Roman writing about a subject people, at second hand.
What is solid is that vervain was a functioning ritual plant in Roman practice with a technical vocabulary attached to it. That is more than can be said for the overwhelming majority of herbs in the magical literature, whose sacred pedigrees are asserted rather than sourced.
The plant itself is undistinguished (thin, wiry, sparse spikes of tiny pale lilac flowers), which makes the reverence harder to explain by appearance and slightly more interesting.