"You has to eat this, sir! ... Right before you go into the lake, sir - gillyweed! ... It will make Harry Potter breathe underwater, sir!"
-- Dobby the Hogwarts House Elf (GF26)
Native to the Mediterranean, this water plant looks like a bundle of slimy, greyish-green rat tails.
- When eaten, gives a person gills to breathe underwater and gives them webbed hands and feet for swimming. The duration of the gillyweed effect is approximately one hour.
- Snape keeps gillyweed in his private stores; it is not available to the students. Snape believed Harry had stolen the gillyweed, but actually it was Dobby (GF26, GF27)
- Barty Crouch Jr. as Fake Moody tried to give Harry a hint about gillyweed by giving Neville Longbottom the book Magical Water Plants of the Mediterranean. When that didn't work, he let Dobby overhear a "loud conversation" about it with McGonagall in the Staff Room (GF35).
- The effects of gillyweed were first dicovered by Elladora Ketteridge (FW). About a century later, gillyweed was apparently re-discovered by Beaumont Marjoribanks (FW).
- In one of the possible timelines following the use of the Experimental Time Turner, Albus Potter and Scorpius Malfoy take gillyweed when they re-visit the second task of the Triwizard Tournament (CC2.19).
"gill" = a breathing organ found in many aquatic organisms