hush vs hush up

hush is a noun but hush up is not a noun.

Word	Noun	Adjective	Verb	Adverb
hush	Yes	No	Yes	No
hush up	No	No	Yes	No

As verbs, hush and hush up are synonyms defined as:

Other synonyms of hush include quieten, shut up, silence, still.

hush (noun)	hush up (noun)
(poetic) tranquil silence

hush (verb)	hush up (verb)
become quiet or still; fall silent	cause to be quiet or not talk
cause to be quiet or not talk	cover up a misdemeanor, fault, or error
run water over the ground to erode (soil), revealing the underlying strata and valuable minerals
wash by removing particles
become quiet or quieter