Bitterweed is a small flowering plant with a purple five-petalled bloom at maturity. In the show, no leaves of any kind are seen, only a flower stuck in the dirt. It is not clear how this plant photosynthesizes.
In "Enemy of My Enemy", a green variation is also seen. Dagur the Deranged indicates that the Bitterweed looks like that green flower, "only darker". It is possible the green variation is an immature flower that must still turn purple.
In actuality, there are a number of plants generically referred to as "bitterweed". Most commonly though, bitterweed refers to Ambrosia artemisiifolia (also called Common Ragweed) and Helenium amarum (also called Bitter Sneezeweed). Both of these species are native to the Americas and would not have been encountered by Vikings. Ragweed does have medicinal uses to some Native American Tribes. Ragweed is implicated in allergies in humans, while Sneezeweed is mildly toxic and has a noxious odor when crushed. Neither have purple flowers, but rather yellow ones.
In the How to Train Your Dragon Franchise, bitterweed is part of an herbal concoction for Dragons to combat the effects of Dragon Root. Other components include: fresh water, Fire Fern, and Purple Oleander. The ingredients are heated together to make a tea. The final product is purple in color and is fed directly to the affected dragon. In "Enemy of My Enemy", Toothless was hit by an arrow tipped with Dragon Root. It is not clear if this antidote is useful only on Dragon Root entering the blood stream or also for other means of delivery, such as aerosolized Dragon Root. It is also not discussed if bitterweed has use in human herbal medicine.
- "Bitter weed! That's the first ingredient. Looks like this, but darker."
- ―Dagur in "Enemy of My Enemy"
Bitterweed is seen only in one episode, "Enemy of My Enemy". Toothless is shot with a Dragon Root arrow by Dragon Hunters who set a trap for Hiccup and Toothless. The two barely make it to a nearby Unnamed Island. The Dragon Hunters pursue. Hiccup attempts to save himself and hide Toothless, who is extremely lethargic, but not unconscious from the Dragon Root. A mysterious man who is later revealed to be Dagur tried to help Hiccup. Because of the past, Hiccup has a hard time trusting him. Dagur recalls seeing Ryker Grimborn making an antidote for Dragon Root, and the two humans set out to find the ingredients.
- The flower of the Bitterweed and Purple Oleander are shown as completely identical. It is possible the animators did not spend a large amount of time on the plants, but instead focused on other parts of animation.
- Bitterweed is also nearly identical in shape to the Rock Blossom and Blue Oleander.