Is a marijuana plant illegal if it has no THC? Is it a plant if it has no roots?

Yes it is illegal to possess both marijuana and THC or any synthetic equivalents of the substances contained in the plant, or in the resinous extractives of Cannabis, sp. and/or synthetic substances, derivatives, and their isomers with similar chemical structure and pharmacological activity. See Tenn. Code Ann. § 39-17-415. As to the second question of whether it is a plant if it has no roots, I am a lawyer not a botanist.