The willows are plants from the genus Salix, which consists of approximately 400 species of deciduous trees and shrubs. Willows occur mostly in the Northern Hemisphere in Europe, Asia, and North America and is naturalized elsewhere. Willows prefer wet areas in temperate to Arctic climates.
The roots of a willow are very tough, big and have a fast tenacity to live. Their wood is tough and they have slender branches, which tend to bend over the water and touch it. The leaves of a willow are normally elongated, but can also be round or oval, usually with serrated edges.
Some willow species include: Long-leaved Violet Willow (Salix acutifolia), Arctic Willow (Salix arctica), and White Willow. The latter (Salix alba) is the species many people are most familiar with. It is a tree with stems that resemble whips in a way, and can be called 'withies'. It is deciduous, meaning the white willow sheds its lance-shaped leaves for the winter months.
In the franchise, willow bark is used as medicine, as mentioned in "No Bark, All Bite".
In real life, willow bark is also used for medicine, along with willow leaves for thousands of years in many cultures. Primarily willow bark has been used as an analgesic and antipyretic. The main active ingredient - Salicin - has been studied extensively and is the precursor to aspirin. Its study led to the understanding and development of the Non-Steroidal Anti-Inflammatory (NSAID) drug class.
Historically there have been other uses for Willows. The withies have been used to make baskets, fishing nets, fencing, and other useful items. The wood of the willow tree is also used, usually to make small items as the wood does not hold up well for uses such as building like other woods. Willows are also used as food for bees, in order to produce honey. It is also used for biomass and biofuel because of its fast growth, catkins can be eaten (usually in times of famine), charcoal used in art, and may also be used as a decorative tree in parks and landscaping. Willow trees also figure into many culture's religion and folklore.
In the episode "No Bark, All Bite", Stoick gets rid of all the willow bark from Gothi's medicine, because it was brought by Johann, who presumably poisoned it. While Gobber is left to select one of three new traders, Stoick and Hiccup leave to find some more willow bark on their own. Together, they gather some bark, when they encounter Dragon Flyers, setting islands on fire, making the pair realize they have been tricked. Stoick and Hiccup figure out that the new traders did not have willow bark amongst their goods because Johann got the traders first and used it as a ruse to lure Hiccup and Stoick away, leaving Berk vulnerable. After defending Berk, Dagur sends two boxes full of bark to his allies.