When used without prefixes, cажать and садить differ only stylistically. But when used with prefixes, they actually differ in their meanings:

пересадить - here пере- means trans-, inter-
1) to transplant
2) to replant
imperfective: пересаживать

пересажать - here пере- means "over and over",
1) to plant (several objects) one after other in sequence; to plant all of (several objects)
2) to jail (several persons) in sequence; to jail all of (several persons)
no imperfective

пересажать can be further supplemented with the emphasis prefix по- (попересажать), having the meaning of an
instant, sudden and/or inevitable action.