Blackthorn

Removing Blackthorn requires the amount of energy you can see when you move your cursor over it (e.g. 2 energy for total removal at the goblins' camp).

By removing 1 Blackthorn you'll receive the following:

3 Gold, 13 XP (goblins' camp)

Usually, you don't have to remove all these blackthorns to reach something behind.