stubble vs plant material

stubble and plant material both are nouns.

Word	Noun	Adjective	Verb	Adverb
stubble	Yes	No	No	No
plant material	Yes	No	No	No

As nouns, plant material is a hypernym of stubble; that is, plant material is a word with a broader meaning than stubble:

stubble: material consisting of seed coverings and small pieces of stem or leaves that have been separated from the seeds
plant material: material derived from plants

Other hypernyms of stubble include plant substance.

stubble (noun)	plant material (noun)
short stiff hairs growing on a man's face when he has not shaved for a few days	material derived from plants
material consisting of seed coverings and small pieces of stem or leaves that have been separated from the seeds