Never planted, it grows

Never planted, still it grows.
What's the answer now: who knows?

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Scroll down for answer and notes.

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Of course there are lots of possible answers to this riddle, so to make it really work, there needs to be another bit (also rhyming!) that marks out the answer as hair. Maybe: "It grows from roots, yet not a tree; tell me now, who can it be?"

Hair.