Gillyweed is a magical plant that, when eaten, allows a human to breathe underwater. In its raw form, it appears as a “bundle of what looked like slimy, greyish-green rat tails.” It is described as having a rubbery and chewy texture and an unpleasant taste. When consumed, it instigates a temporary, partial Transfiguration in the user.
The primary magical property of Gillyweed is to grant a human the ability to process oxygen from water. Upon ingestion, the consumer undergoes several physiological changes:
The effects of Gillyweed last for approximately one hour when used in fresh water, as demonstrated by Harry Potter in the Great Lake. The duration of its effects in saltwater is unknown. To be effective, the plant must be consumed just before entering the water. In *Harry Potter and the Half-Blood Prince*, Junior Minister Herbert Chorley is affected by a faulty potion suspected to be derived from Gillyweed, causing him to behave like a duck and attempt to strangle colleagues. This suggests that improperly prepared or modified Gillyweed can have dangerous and unpredictable side effects.
While Gillyweed had likely been known in Herbology and Potions for some time, its most prominent role occurs during the 1994-1995 school year. Severus Snape kept a stock of it in his private stores in the Hogwarts dungeons. During the Triwizard Tournament, Barty Crouch Jr., disguised as Professor Alastor Moody, planned for Neville Longbottom to suggest Gillyweed to Harry Potter for the Second Task. However, when Neville failed to relay the information, Dobby the house-elf, having overheard Crouch discussing the solution with Professor McGonagall, stole the Gillyweed from Snape's office. He gave it to Harry moments before the task began, ensuring Harry could participate. This act later caused friction between Harry and Snape, as Snape accused Harry of the theft.
Gillyweed is a crucial plot device in *Harry Potter and the Goblet of Fire*. It is the sole reason Harry Potter is able to survive underwater for an hour during the Second Task and rescue not only his own “hostage,” Ron Weasley, but also Fleur Delacour's sister, Gabrielle Delacour. Harry's use of Gillyweed contrasts with the methods of the other champions; Cedric Diggory and Fleur Delacour used the Bubble-Head Charm, while Viktor Krum used a partial Transfiguration to transfigure his own head into that of a shark. Harry's success in the task, enabled by Gillyweed, was essential for his continuation in the Triwizard Tournament. The story of how he obtained it also highlights Dobby's unwavering loyalty and the subtle manipulations of Barty Crouch Jr..