Buckthorn

Buckthorn is seen once in the Dragons: Defenders of Berk episode, "The Eel Effect".

Description
Buckthorn most often refers to plants of the Genus Rhamnus in the Family Rhamnaceae, other members of the aforementioned Family, or possibly plants from the Genus Hippophae commonly known as Sea Buckthorns. Common Sea Buckthorn (Hippophae rhamnoides), [ https://en.wikipedia.org/wiki/Frangula_alnus Alder Buckthorn] (Frangula alnus), and Common Buckthorn (Rhamnus cathartica) all occur in areas of Scandinavia and may have been encountered by historical Vikings.