Even trained dragons will try to attack each other, but the dragons' riders can calm them down to a state of wariness, until the dragon root can be removed from their presence.
It seems that most dragons would be affected by Dragon Root. However, for unknown reasons those with a rock-based diet, such as Gronkles, are immune to it.
Whether the root may or may not have an effect on people is unclear. Snotlout has stepped on an arrow coated in dragon root, as well as Astrid being shot in the leg, though both were fine. However, Dagur passed out when an arrow struck him in the back, it might have just been the force of the blow. Hiccup was knocked out when he breathed in a gas for of it. Though the gas was purple, it is possible it had other ingredients which made humans vulnerable to it as well. It took him more time to pass out than Toothless.
As shown in "A Tale of Two Dragons", when the riders were clearing Mildew's cabbage field when Hookfang and Stormfly started to fight. It was later sensed by Meatlug while she and Fishlegs were investigating the spot where the fight started and she uncovered it in the field. When Hookfang and Stormfly started fighting again, Barf and Belch got too close to the root and started to act crazy as well. While Toothless held Barf and Belch and Hookfang knocked them aside in order to calm them down, Astrid and Snotlout calmed down their dragons enough and used Hookfang's wing clap and Stormfly's single spine shot to help Meatlug remove the root and fly out to drop it into the sea.
In "Cast Out, Part 1", after his takeover of the Outcast Tribe, Dagur and Savage did experiments with it on dragons. Dagur then used it as a trap for Hiccup in order for him to surrender Toothless to him. He left it inside the Academy so that the dragons would begin to fight over it, and Hiccup would come to stop them. Fishlegs then used it to lure the Screaming Death to Outcast Island, after doing that he dropped it into the sea just like the last one so the Screaming Death would stop following him.
During the events of "Night of the Hunters, Parts One and Two", the Dragon Hunters used dragon roots at high concentrations and coat them on their arrows. The arrows can effectively knock dragons out of the sky in one shot without fatal injuries. They have been used multiple times as the Dragon Hunter's main weapon. The first time was when Ryker shot down Stormfly.
As shown in "Stryke Out", a group of Hunters use the root in a gas form to knock out dragons for transport. Snotlout unintentionally set it off after Hiccup and Toothless were trapped, and the gas knocked them both out.
Dragon Root looks like a stumpy and somewhat misshapen cactus, with hair-like parts sticking out of different ends. They are green in color with white roots.
- Fire Fern
- Fresh water
- Purple Oleander
- Although Fishlegs claims that Boulder Class dragons are immune to it, the Screaming Death is attracted to it, the Catastrophic Quaken and the Eruptodon are both affected by it, unlike the Gronckle. It is not known if other Boulder class dragons are immune to it like Gronckles.
- It is unknown if Dragon Root affects Tidal Class dragons in the ocean, as the best way to dispose Dragon Root is to drop it into the sea. Tidal Class dragons may have an immunity to it, similar to Gronckles. Quite possible the scent of the dragon root that will be drop in the sea will wash off after amount of time that has no affect on dragons in the sea.
- As seen in "Enemy of My Enemy", Dragon Root appears to have a longer affect on Night Furies, as well as causes them agony.