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Thorn Weeds are enemies found in the Ancient Waterworks in Wario: Master of Disguise. They are Seed Weeds whose heads have been turned metallic and spiky by the Wishstone. This makes them immune to Thief Wario's shoulder bash as well as Cosmic Wario's lasers, so they must be defeated using other means, such as Dragon Wario's fire breath or Genius Wario's boxing glove. Thorn Weeds attack Wario by firing their spikes at him. Although their description says that the spikes are so heavy that they cannot move without firing some of them, this is not true in-game. Unlike Seed Weeds, Thorn Weeds do not reproduce.
The Wishstone turned this plant into a hulking beast, but its spikes are so heavy it can't move without firing off a few of them first.