Board Thread:Legitimate Canon Questions/@comment-186.155.91.172-20140316041800/@comment-4091815-20140316045943

We don't know how the are created. They are a thing that exists outside Beacon Hills as well.

We know they can be summoned up by the breaking of Noshiko Yukimura's tails.

Breaking off the new growth of the Nemeton appeared to be the cause of their first appearance but that's not been confirmed.

The bottom line is that we don't know enough about them to give you a definitive answer.