field-pea plant vs pea plant

field-pea plant and pea plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
field-pea plant	Yes	No	No	No
pea plant	Yes	No	No	No

As nouns, pea plant is a hypernym of field-pea plant; that is, pea plant is a word with a broader meaning than field-pea plant:

field-pea plant: variety of pea plant native to the Mediterranean region and North Africa and widely grown especially for forage
pea plant: a leguminous plant of the genus Pisum with small white flowers and long green pods containing edible green seeds

Other hypernyms of field-pea plant include pea.

field-pea plant (noun)	pea plant (noun)
variety of pea plant native to the Mediterranean region and North Africa and widely grown especially for forage	a leguminous plant of the genus Pisum with small white flowers and long green pods containing edible green seeds