In 'A Midsummer Night's Dream', what flower's juice is used as a love potion?

In “A Midsummer Night’s Dream,” the love-potion flower is love-in-idleness. Oberon tells Puck to find it after Cupid’s arrow changes the flower from white to purple, giving it magical power over love.

The juice is placed on the eyelids of a sleeping person. When that person wakes, they fall in love with the first living thing they see. Oberon intends to use the potion on Titania, the fairy queen, to humiliate her and force her to surrender an Indian changeling boy.

Puck later makes a mistake by applying the juice to Lysander instead of Demetrius. Lysander wakes when he sees Helena and instantly abandons his former love, Hermia. The confusion drives much of the play’s comic middle section.

The flower is also called “heartsease” in traditional botanical contexts, but “love-in-idleness” is Shakespeare’s name for it and the canonical answer to this question. It should not be confused with a generic rose, violet, or daisy: those flowers are not the enchanted ingredient named in the play.