plant is a noun but leggy is not a noun.

plant is not an adjective while leggy is an adjective.

plant is a verb but leggy is not a verb.

plant (noun)	leggy (noun)
buildings for carrying on industrial labor
(botany) a living organism lacking the power of locomotion
an actor situated in the audience whose acting is rehearsed but seems spontaneous to the audience
something planted secretly for discovery by another

plant (adjective)	leggy (adjective)
	(of plants) having tall spindly stems
	having long legs

plant (verb)	leggy (verb)
put or set (seeds, seedlings, or plants) into the ground
fix or set securely or deeply
set up or lay the groundwork for
place into a river
place something or someone in a certain position in order to secretly observe or deceive
put firmly in the mind