witch broom vs witches' broom

witch broom and witches' broom both are nouns.

Word	Noun	Adjective	Verb	Adverb
witch broom	Yes	No	No	No
witches' broom	Yes	No	No	No

As nouns, witch broom and witches' broom are synonyms defined as:

witch broom and witches' broom: an abnormal tufted growth of small branches on a tree or shrub caused by fungi or insects or other physiological disturbance

Other synonyms of witch broom include hexenbesen, staghead.

witch broom (noun)	witches' broom (noun)
an abnormal tufted growth of small branches on a tree or shrub caused by fungi or insects or other physiological disturbance	an abnormal tufted growth of small branches on a tree or shrub caused by fungi or insects or other physiological disturbance