Herb
The altar herb of half of Europe, and nobody can quite say why.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Vervain is an unremarkable-looking plant with a spectacular pedigree. Roman writers describe it swept over altars and used in treaties; Pliny reports its use by the Gauls in prophecy; medieval Europe called it holy herb and simpler's joy and attached to it a Christian legend about the wounds at Calvary. For a plant with thin, sparse, pale lilac flowers on a wiry stem, that is an extraordinary weight of significance.
Practice inherits all of it. Vervain washes altars and tools, goes into protective bundles, is added to almost every purification recipe printed since the nineteenth century, and is credited with strengthening any working it joins. In druidic revival material it is close to a sacrament, though the connection there runs through Pliny and eighteenth-century antiquarians rather than through anything continuous.
Do not confuse it with lemon verbena, a South American shrub with a lemon scent and no relation to any of this. Vervain itself tastes bitter and unpromising, which perhaps explains why almost everything ever said about it concerns ritual rather than flavour.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Nicholas Culpeper, The Complete Herbal (1653)
Culpeper: "This is an herb of Venus, and excellent for the womb." He then lists a dozen further complaints it supposedly answers, which is his usual habit and a good reminder that a long list is not the same as evidence.
Carried forward
Pliny the Elder, Naturalis Historia (c. 77 CE)
Pliny records that the Gauls used vervain in casting lots and foretelling, and that Roman practice swept altars with it. He is a compiler rather than a witness, so this is early testimony about what people believed, not about what worked.
Credit what you carry. Much of this comes from living traditions, and some of it from traditions that are closed — held by particular peoples and not offered for general use. Where you know whose practice a reading came from, name them; where you don't, say that too.