Blue Oleander

The Blue Oleander is a type of flowering plant first seen in the Dragons: Riders of Berk television series.

Description
In the How to Train Your Dragon Franchise, Blue oleander is a short bushy plant with stunning blue flowers with five separated petals of equal size. The leaves are ovate and end in an acute tip.

Biology
Blue Oleander appear to prefer full sun to grow.

In the Franchise, Blue Oleander are poisonous to reptiles (according to a botany book belonging to Fishlegs, Chapter 8). However, Dragons are also noted as being allergic to these flowers. It is unknown if only the flowers are toxic or the entire plant. The ill effects on Dragons is not instantaneous. In "Dragon Flower", the plant's mere presence on Berk affects them, indicating that perhaps pollen is the culprit of illness. However in "When Darkness Falls" and "Defenders of the Wing, Part 1", Dragons appear to be alright while in the vicinity of Blue Oleander. Despite being affected by the flower, Terrible Terrors appear to be attracted to them.

Scauldrons are the only dragon species that are unaffected by Blue Oleander and appear to love eating them. After eating Blue Oleander, Scauldron venom can cure other Dragons being afflicted with Blue Oleander. However, it was later discovered that the Blue Oleander also has an effect on Scauldrons. When Scauldrons eat too much Blue Oleander at one time, they can change it's green color to blue color. Though this effect has been confirmed, it is unknown if the effect is only temporary or permanent. The most likely explanation is that the effect wears off as Blue Oleander-free time goes by.

In actuality, a plant called "Oleander" (Nerium oleander) belonging to the Dogbane Family does exist. It has been used as an ornamental plant for over a thousand years. There is no blue variant, however; flowers are various shades of pinks and also white. They are a small shrubs with lance-shaped leaves.

All parts of Nerium oleander are toxic to humans and most other animals, though its effects on reptiles is not described. Birds and rodents are somewhat tolerant of its effects. Nerium oleander toxicity can affect skin, nervous system, heart, and gastrointestinal tract.

Dragons: Riders of Berk
In the episode, "Dragon Flower", the villagers board Trader Johann's ship and start trading with him. Even Mildew is seen receiving a mysterious item in exchange for some cabbage. Later on, when the dragons begin falling mysteriously ill, the gang discover a bush of flowers, that was never seen before. Recalling from his botany book, Fishlegs identifies them as the Blue Oleander, a flower that is poisonous to reptiles, including dragons. Hiccup orders Fishlegs to retrieve the buried botany book from the pit to find an antidote. Later, it is found out that the dragons can be cured by the venom of a Scauldron. When a Scauldron eats the Blue Oleander, the venom that it produces can be used as an antidote to cure the sick dragons. After a few events, the dragons are cured and they become their usual self.

Dragons: Race to the Edge, Season 1
In "When Darkness Falls", Blue Oleander reappears on one of the islands the Dragon Riders search for a base camp. They quickly leave before the flower takes affect on the dragons, though Barf & Belch sneeze once.

Dragons: Race to the Edge, Season 3
The island, Botany Blight, had the Blue Oleander growing abundantly, which was why Viggo chose the location to discuss a truce with Hiccup in the episode, "Defenders of the Wing, Part 1"

Trivia

 * The blue oleander shares a resemblance to the Rock Blossom needed for the Eel Pox cure from "The Eel Effect".
 * The blue oleander's cousin, the purple oleander, is not poisonous to reptiles, and in fact can be used as a cure for dragon root infection.