Dragonberry

Dragonberry is a plant that appears minimally in How to Train Your Dragon Franchise.

Description
Dragonberry is a simple black plant with lots of green spikes and no flowers. As its names suggests, it produces berries. Though they are not seen, it is assumed that they are purple, from the juice that they produce.

They are mentioned to be poisonous, as much as the Threadtails, however not affecting them.

Function
The only function the Dragonberry has is for the dragon species Threadtail. Due to their allergy to their poison, the Threadtails trail through Dragonberry spiny bushes to relieve the itching.

Dragons: Rise of Berk
The Dragonberry is first mentioned in this game in Threadtail's description.

"Don't touch! This dragon can secrete a poison through its skin when it senses danger. The only problem is the Threadtail is allergic to it too! It spends most of the day trailing around in Dragonberry bushes to relieve the itching."

- Dragons: Rise of Berk

The Dragonberry later appeared surrounding Mildew's Strain's roost.

Dragonberry is mentioned again (though not by name) in Foliander's description.

"This Dragon spends most of its days desperately trying to use bushes to relieve its constant itching, especially during the autumn... however the local fauna is as poisonous as it is."