". . . and then I got the bezoar down his throat and his breathing eased up a bit, Slughorn ran for help, McGonagall and Madam Pomfrey turned up, and they brought Ron up here. They reckon he'll be all right. Madam Pomfrey says he'll have to stay here a week or so ... keep taking essence of rue . . ."
-- Harry Potter (HBP19)
An herbal remedy for poisoning, and an ingredient in the potion Felix Felicis.
Powdered rue is an ingredient of liquid luck, Felix Felicis (BoP)
About garden-rue, from Culpeper Complete Herbal:
"The seed thereof taken in wine, is an antidote against all dangerous medicines or deadly poisons. The leaves taken either by themselves, or with figs and walnuts, is called Mithridate's counter-poison against the plague, and causes all venomous things to become harmless; being often taken in meat and drink it abates venery."
Perennial evergreen shrub, late 14c., from Old French rue (13c.), earlier rude, from Latin ruta "rue." The bitter taste of the plant has given the word "rue" the alternate meaning "to feel regret" or "to grieve" as in the old saying "rue the day."
Bartholomaeus Anglicus wrote in the 13th century that rue is the only plant which can withstand the withering stare of the Basilisk (he called it a Cockatrice), and if a weasel ate some rue before being bitten, it could kill the monster. However, in the books the only known antidote for Basilisk venom is phoenix tears (DH6).
Rue is also known as "herb of grace" because it's bitterness is associated with regret and redemption. Many poets and writers made allusions to this fact.
Ophelia in Shakespeare's "Hamlet" says:
"...there's rue for you; and here's some for me: we may call it herb-grace o' Sundays: O you must wear your rue with a difference."
Also from "Richard II" Shakespeare writes of a weeping queen:
"Here did she fall a tear, here in this place I'll set a bank of rue, sour herb of grace."
It's possible that the literary allusion of Ron being given rue after his ordeal might mean that he is sorry for taking the love potion, although it was accidental, and especially sorry for drinking the mead which poisoned him.