Mistletoe is a plant mentioned briefly in the DreamWorks Dragons Franchise.
Mistletoe is a hemiparasitic plant of the order Santalales. There are over 900 species, but the most common one is the European mistletoe (Viscum album). It has smooth-edged, oval, evergreen leaves borne in pairs along the woody stem, and waxy, white berries that are in clusters of two to six.
Mistletoe grows on trees, having the aspect of a shrub. They absorb water and nutrients from the host plant through a structure called haustorium. They also do perform photosynthesis, but for a short period of time.
It is stated in School of Dragons that mistletoe is used as a decoration for Snoggletog, a celebration similar to Christmas. Like in real life, vikings have the tradition to kiss under a branch of mistletoe, which Astrid uses as inspiration for another tradition.
In Norse mythology, the god Loki uses mistletoe arrow to kill Balder, due to it being his only weakness.
In a deleted scene, Astrid makes up a new tradition similar to the mistletoe, in which she strings up a severed toe to the corner of one of the Berk house's roofs, punching everyone who walks under it.
Mistletoe is briefly mentioned in a stable quest as a decoration for Snoggletog.