I don't think Duane cast on the Wandering Root directly. He started a fire, and once it spread, it affected it normally. But he wouldn't be able to take or modify any of its Aspects. But he's cast directly on Sette before. It even comes up directly that when he casts on her, he uses Old Tainish for human.
It's in keeping with his character, certainly. I was just making an observation.