Gurdyroot is a plant resembling a green onion, according to Luna Lovegood it is excellent for warding off Gulping Plimpies. Luna and her father, Xenophilius Lovegood, make a drink with this plant, which they call a Gurdyroot Infusion, which has a bogey-like taste, and it is apparently reddish purple in colour. The Gurdyroot was mentioned in the radio programme Toots, Shoots 'n' Roots.
The gurdyroot possibly made its first appearance in 1992 when Neville Longbottom bought an "evil-smelling green onion" to protect himself from the monster in the Chamber of Secrets (the Basilisk). Though others pointed out that only Muggle-borns were being attacked, Neville felt he still needed this protection as he was "almost a squib".
It also does not have a pleasant taste. When Hermione Granger was transforming into Bellatrix Lestrange in the spring of 1998 using Polyjuice Potion, Hermione stated that the potion tasted "disgusting, worse than Gurdyroots".