The root of the mandrake contains an alkaloid which is a narcotic and a local anesthetic, similar to deadly nightshade. From the old tradition that they have aphrodisical powers, mandrakes were called love apples. Another superstition is that when the mandrake is uprooted it utters a scream, in explanation of which Thomas Newton, in his Herball to the Bible, says, “It is supposed to be a creature having life, engendered under the earth of the seed of some dead person put to death for murder”.
The Bonnefont Cloister is a teaching garden. It has the only examples in the United States of Medieval dye, cooking and medicinal plants. A special corner is devoted to authentic magical plants. Deirdre Larkin's blog about The Cloisters, The Medieval Garden Enclosed, often has entries about these plants.
Harry Potter and his classmates, of course, were not much enamored of mandrakes.