Elladora Ketteridge first discovered the uses of Gillyweed, although Beaumont Marjoribanks is credited with re-discovering it about one century later. It has been said that Ketteridge once ate Gillyweed, causing her to nearly suffocate. She found that when she stuck her head into a bucket of water, she recovered. Thus the uses and properties of Gillyweed were discovered.
At first glance, harvested Gillyweed may appear to be nothing more than a ball of slimy, greenish-gray, densely packed rat tails. Before its harvest, however, Gillyweed tends to grow as a weed on top of other plants near the edge of large bodies of water. Gillyweed is a parasitic, sentient plant, which will burrow its roots into a host plant and steal its moisture and nutrients. In a case where Gillyweed is denied the copious amounts of fresh water that it needs to survive, it will go into a period of dormancy, shriveling up and appearing as a small, prickly and hairlike plant. In this dormant state, we refer to Gillyweed as Cuscuta, also known as dodder or hairweed. Dry, dormant Cuscuta cannot be harvested and then rehydrated back into Gillyweed. Once it is detached from its host plant in a dormant state, it will never wake.
Cuscuta oxygenium is the Latin name for Gillyweed. It is the only one of the 170-odd types of cuscuta that is sentient and carries magical properties.
When consumed, Gillyweed gives the user gills to breathe underwater and gives them webbed hands and feet for swimming. Its effects last for approximately one hour.
Dormant cuscuta oxygenium is useful for some potions, primarily love potions. Active Cuscuta (Gillyweed) also has many uses in potions, mainly antidotes, although unlike Cuscuta in its dormant state, it is a standalone transfigurational botanical. In other words, we can use both dormant Cuscuta and active Gillyweed in many potions, but active Gillyweed is unique because it can be used all by itself to grow the trademark gills and webbing that make it useful for those who need to spend time underwater.If you were to harvest Gillyweed in its active state and then deprive it of water after harvest, it would dry out and act as a dessicating agent. Enough of it would thicken a potion. The key thing to remember is that the state you harvest it in is the state it will remain in. Think of it as a switch: on or off, active or dormant. If harvested while in active, 'Gillyweed' form, the magical mechanisms in the plant that cause it to seek moisture would still be in effect turned 'on' and it would continue to seek moisture wherever it could find it. Even if you dehydrate it, that switch is still 'on' if it was harvested in an active state.
Thus, active gillyweed can be a helpful ingredient in antidotes, for in combination with the right ingredients, it can seek out the poison and help absorb it, much like a bezoar does. This mechanism in active gillyweed is also what gives the beverage gillywater a slightly thickened consistency.
The single most famous usage of Gillyweed occurred during Task Two of the Triwizard Tournament, when Harry Potter used Gillyweed to swim in the Hogwarts lake and rescue both Ronald Weasley and Gabrielle Delacour.
Live, fresh gillyweed should be allowed to thrive off a host plant, such as an inexpensive shrub or bush. Harvested gillyweed should be kept in a cool, damp burlap sack, well out of light.