User blog comment:RoyallyBella/Official Season 6 discussion blog/@comment-4776802-20131102223144/@comment-4588346-20131102223905

Okay here is what I am thinking, remember in season three when the trix set the Alfea library on fire and none of the winx could control it because it was dark magic. I think this is the same thing, the treants were summoned by the dark power of the Legendarium,so thus they may be only controlled by the book.