Ferns are mentioned in the How to Train Your Dragon Book series when describing some island environments.
Ferns (Class Polypodiopsida) are plants that have tissue that conduct water and nutrients, but do not produce seeds or flowers. Fern reproduce by spores, which are usually located in clusters of sporangia on the undersides of certain fronds of the fern plant.
Bracken (Genus Pteridium) are a successful group of ferns occurring on most continents, except Antarctica. Bracken is typically found in moist open areas. Common Bracken (Pteridium aquilinum) is the most widespread species and can be found on the British Isles and Scandinavia in moorland habitat. The word "bracken" is derived from an Old Norse word meaning fern.
Ferns act as background plants in the Book series and cover several of the islands in the Barbaric Archipelago, but otherwise are not used.
In reality, ferns are edible, especially the fiddleheads - unfurled fronds - of many species, including bracken. The roots of some species are also eaten. Ferns have also been used historically for some medicinal purposes, as well as bedding for animals and packing material.
The first book describes the Isle of Berk as having a lot of bracken.
|“||You didn't really walk on the island of Berk, you waded - through heather or bracken or mud or snow, which clung to your legs and made them difficult to lift. ... You could be making your way through the ferns and suddenly find yourself fording a river, waist-high and icy cold.||”|
|— How to Train Your Dragon|
Bracken and ferns are mentioned throughout the Book as plants that must be slogged through or to hide in - "boggy bracken" or "They were crouching like giant cats in the ferns" or "shelter of ferns" and "disappeared into the bracken".
|“||What with avoiding the holes, and wading through waist-high gorse and bracken, it took the boys an hours or so even to get up into the hills to practice.||”|
|— Book 2|
The Isle of the Skullions is mentioned as having "high ferns and endless heather". Ferns are mentioned again on the Isle of the Skullions, when they restrict Hiccup from running away faster from the predatory Skullions.
Stoick imagines the wind blowing across the bracken on Berk as whispering something, while he faces a tough decision as chief.
|“|| There will be other sons ..." he said to himself.|
And the wind howling across the ocean and through the wet bracken and blowing open the doors in a flurry of rain seemed to call back to him ...
"... but not like Hiccup.
|— Book 3|