The Leek Jinx results in leeks growing out of the target’s ears. A leek is a vegetable in the onion family.
References from the canon
culinary herb, Old English læc (Mercian), leac (West Saxon) "leek, onion, garlic,"
A leek is a vegetable related to onions and garlic. According to Welsh legend, King Cadwaladr of Gwynedd ordered his soldiers to wear them on their helmets after fighting a battle with the Saxons in a leek field, and they appear on the helmets of the Welsh Guard in the British Army.
The leek looks something like the description of the magical plant Gurdyroot which Luna Lovegood wanted to share with Ron when he was in the hospital - a large green onion (HBP20).