Dried thistle is a quest item used in the Eadgar's Ruse quest. It can be made by holding a Troll thistle over a fire to dry it out. This can then be crushed in a pestle and mortar to make ground thistle which, when mixed with an unfinished ranarr potion, is used to make a Troll potion.
If the thistle is dropped, it will disappear and the player receives a message in the chatbox saying "The Troll Thistle withers away as soon as it hits the ground."