Bioretention is typically lower maintenance than some other types of BMPs. However, it is a fallacy to approach bioretention as no maintenance. Routine maintenance activities for bioretention include trash removal, weeding, and plant/tree trimming. Less frequent, but typical maintenance activities include mulch replacement, soil aeration, spot replacement of bioretention soil and possibly some plant replacement.
More information and an example bioretention maintenance checklist is available here.