How to Label Sex of Monoecious or Hermaphroditic Plant?

You can’t presently label those with annotations, so probably leave it blank. See previous discussions:
(the second one is very long - search for “bisexual” or “hermaphrodite” to find those discussions).
There is, however, an observation field named “PlantSex” that allows female, male, or monoecious as values. If you are doing a special study, you could add that field to any observations you are looking at.