pigweed vs greens

pigweed and greens both are nouns.

Word	Noun	Adjective	Verb	Adverb
pigweed	Yes	No	No	No
greens	Yes	No	No	No

As nouns, greens is a hypernym of pigweed; that is, greens is a word with a broader meaning than pigweed:

pigweed: leaves collected from the wild
greens: any of various leafy plants or their leaves and stems eaten as vegetables

Other hypernyms of pigweed include green, leafy vegetable.

pigweed (noun)	greens (noun)
leaves collected from the wild	any of various leafy plants or their leaves and stems eaten as vegetables
common weedy European plant introduced into North America; often used as a potherb
leaves sometimes used as potherbs; seeds used as cereal; southern United States to Central America; India and China