cold frame vs protective cover

cold frame and protective cover both are nouns.

Word	Noun	Adjective	Verb	Adverb
cold frame	Yes	No	No	No
protective cover	Yes	No	No	No

As nouns, protective cover is a hypernym of cold frame; that is, protective cover is a word with a broader meaning than cold frame:

cold frame: protective covering consisting of a wooden frame with a glass top in which small plants are protected from the cold
protective cover: a covering that is intend to protect from damage or injury

Other hypernyms of cold frame include protection, protective covering.

cold frame (noun)	protective cover (noun)
protective covering consisting of a wooden frame with a glass top in which small plants are protected from the cold	a covering that is intend to protect from damage or injury