The presence of that poison in our protected garden has nothing to do with me, since, my Poisons skill being merely Respected +55%, I can't plant it. And I doubt if any other Animist is higher in that skill, since the rest all have more important skills to learn. Indeed, I doubt if I shall ever be able to plant it, the guild limit being GrandMaster.

There is supposed to be something in the game to cause absent herbs and poisons to come into existence somewhere (though it doesn't seem to have worked for jitsu...), and I can only suppose that it is responsible for the presence of the blackroot.