Sometimes I'm trying to discourage thorns, and sometimes I'm trying to get them to grow for quests. Any idea what causes thorns to grow?
My gf informs me that they grow underneath trees. Apparently if you move all your animals and buildings away from your pine and oak trees they'll grow. After a few days you cut down the trees and the thorns will appear. If you don't want thorns, keep animals or buildings near the trees and they shouldn't appear.