king fern vs royal fern

king fern and royal fern both are nouns.

Word	Noun	Adjective	Verb	Adverb
king fern	Yes	No	No	No
royal fern	Yes	No	No	No

As nouns, king fern and royal fern are synonyms defined as:

king fern and royal fern: large deeply rooted fern of worldwide distribution with upright bipinnate compound tufted fronds

Other synonyms of king fern include ditch fern, French bracken, Osmunda regalis, royal osmund.

king fern (noun)	royal fern (noun)
large deeply rooted fern of worldwide distribution with upright bipinnate compound tufted fronds	large deeply rooted fern of worldwide distribution with upright bipinnate compound tufted fronds
fern of rain forests of tropical Australia and New Zealand and South Africa