Blue Oleander

The Blue Oleander is a type of flower. They are very beautiful and very soft, as well as poisonous to reptiles (according to a botany book belonging to Fishlegs, Chapter 8). As the name suggests, these flowers are blue in colour and almost resembles a hibiscus. They have a dark blue shade at the edges of their petals, and a lighter shade towards the middle. It is also revealed that Scauldrons love to eat the Blue Oleander.

In DreamWorks Dragons: Riders of Berk
In episode 9-"Dragon Flower", the villagers board Trader Johann's ship and start trading with him. Even Mildew is seen recieving 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 in the episode,(read "Dragon Flower") the dragons are cured and they become their usual self.

Trivia

 * Despite being affected by the flower, Terrible Terrors appear to be attracted to them.
 * Scaldrons are the only dragons so far not only immune to the flower but also eats it to.