Update: this article is out of date. iNaturalist has a taxon framework on Phylum Tracheophyta referencing Plants of the World Online downstream to infraspecies that is described here alongside the set of deviations
iNaturalist used to use several regional floras as vascular plant taxonomic references with The Plant List as the tie-breaker. As discussed here, we are now using Plants of the World Online (POWO) as the primary vascular plant taxonomic reference with two exceptions:
1) POWO isn't yet complete for genera beginning with the letter 'P' (e.g. Phacelia) or Ferns (Class Polypodiopsida).
2) We maintain a list of potential deviations from POWO that we will discuss on a case by case basis. Those we determine to be 'Accepted Deviations' take precedence over POWO.
If you'd like to propose a deviation from POWO, flag the taxon (if its not already flagged) kick off a discussion for why you think the name is valid despite not being in POWO and also add the name to the list of potential deviations.
Here's a decision tree for how to go about helping resolve vascular plant species in iNaturalist that are not in POWO:
Names added to the potential deviation list will be discussed (in their corresponding flag) and potentially discussed with Kew Botanical Garden staff (which maintains POWO). From there we will decide whether to go with POWO or maintain them as 'Accepted Deviations'. As POWO updates, any corresponding rows should be removed from the potential deviation spreadsheet.
For example, Lithospermum parviflorum is an active species in iNaturalist not valid in POWO. It was added to the potential deviation list while @bouteloua led a discussion that seemed to indicate that it should be valid in POWO. This recommendation was brought to the attention of Kew and they agree it should be a valid species. We responded by marking it as an 'Accepted Deviation'. If Kew updates POWO by adding Lithospermum parviflorum, it should be removed from the list (as it will be no longer needed).