About Solanum xanthi :
https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:821564-1
Solanum xanti A.Gray
First published in Proc. Amer. Acad. Arts 11: 90 (1876)
This name is a synonym of Solanum umbelliferum var. xanti
But a deviation has been discussed and approved here: https://www.inaturalist.org/flags/477105
@loarie @silversea_starsong @graysquirrel

About Solanum parishii :
https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:239457-2
Solanum parishii A.Heller
First published in Muhlenbergia 2: 133 (1906)
This name is a synonym of Solanum umbelliferum var. glabrescens
0 observations for Solanum umbelliferum var. glabrescens : https://www.inaturalist.org/taxa/242955-Solanum-umbelliferum-glabrescens
3016 observations for Solanum parishii : https://www.inaturalist.org/taxa/56116-Solanum-parishii
Maybe a discussion could be useful before swapping or deviating?
Main identifiers that may be have an opinion (not-exhaustive list): @leytonjfreid @catchang @oceanf @joergmlpts @jrebman @possum_noises @lagoondon @hikingsandiego @wranglebees @plantperson7654 @wojciech @aguilita @bryanconnolly @mjpapay @leaf0605 @galanhsnu @nathantaylor @sambiology @peakaytea @pioleon

No one will agree on the treatment of these species and this isn't something major taxonomy platforms (like Plantlist, Calflora, etc.) will even resolve. For the time being it seems most sensible to retain the "general" listings (that is, the full species status) so that if definitive research comes out, things can be adjusted. But the reverse, returning all the species back, is a much messier process once everything has been lumped.

David's treatment is a step in the right direction but it's not enough yet to cover these species throughout their full range.

To be frank, I'm not (yet) content with trusting Solanceae Source for the following reason. There's been a push to lump these three variously because they require a lot of legwork and research to unravel. So far, no one has seemingly actually put that work in to treat these species and compared them throughout their whole range. Work has been done for local areas, but this does not show the story or taxonomy of the full "species" and is quite inadequate.

Until this effort has been done to actually prove with certainty they should be synonymized, I'd exercise great caution with doing so regardless of which authority says what. These authorities often take recent papers or those with peer review, but few of them cover this situation adequately and none entirely.

Depending on the treatment you follow. I'm still not sure that subspecies treatment is adequate (as it masks all the diversity within those groups, like S. xanti from the Santa Monicas and San Diego have diverged far more than most standard "species" and are probably not really related), but it is better than lumping it all.

Here's the commentary from A Revision of the Dulcamaroid Clade of Solanum L. (Solanaceae) by Dr. Sandra Knapp, under S. umbelliferum:

"The species I am recognising here as Solanum umbelliferum has normally been treated as several taxa, differing only by pubescence density and type. Parish (1901: 163–164) characterized these plants as being those for which “their satisfactory segregation is a matter of no little difficulty. The exercise of that botanical industry that multiplies “species” by the minute description of individuals might reap here an abundant harvest.” He used variation in pubescence as the key character for distinguishing his taxa but also admitted that this character was continuous. For Parish, however, recognition of taxa was of importance and so he recognised as taxa what he called “certain diverging lines of development…without insisting upon definite cleavages, which do not exist.” He considered the treatment of this complex as a single taxon unacceptable, but given the variability in leaf form and pubescence present in many other taxa in the Dulcamaroid clade, I feel that it is better to recognise the variation as interesting and complicated, rather than suggesting, by the recognition of separate taxa, that it is understood. That some of this variation is environmental is clear; a pair of collections from Ventura County (Abrams & McGregor 154, 160) in open and shaded habitats show striking differences in leaf size and hairiness. To really understand this group more field work with local populations needs to be undertaken, coupled with transplant experiments and more detailed genetic marker work. A preliminary survey from across the species range revealed little (1 base pair) or no variation at several plastid markers (George 2011; George et al. in prep.), but more detailed studies with microsatellites or AFLP markers may reveal differentiation."

And the link: https://phytokeys.pensoft.net/article/1465

I'm not sure what the best course of action would be. I've mostly avoided ID'ing these due to all the confusion surrounding them. It does feel bad though, to leave them sitting at the genus level; it would be nice to have S. parishii and S. xanti sorted into subspecies of S. umbelliferum. However, if adequate research into these plants hasn't yet been done I can understand the argument against changing the taxa here on iNat. They appear to be similar enough to be subspecies, but in-depth genetic research might tell a different story.

Another solution :
create a deviation with a « Complex Solanum umbelliferum » (corresponding to the species Solanum umbelliferum of POWO), with all taxa as species in the complex (corresponding to the subspecies of POWO).
Maybe some subspecies can stay as subspecies ?

To be clear, here is what I have in mind:

Complex Solanum umbelliferum

Solanum umbelliferum

Solanum umbelliferum var. clokeyi
Solanum umbelliferum var. glabrescens
Solanum umbelliferum var. hoffmannii
Solanum umbelliferum var. incanum
Solanum umbelliferum var. intermedium
Solanum umbelliferum var. montanum
Solanum umbelliferum var. obispoense
Solanum umbelliferum var. umbelliferum
Solanum umbelliferum var. wallacei

Solanum parishii
Solanum xanti

What do you think?
@loarie @silversea_starsong @graysquirrel @plantperson7654 @leytonjfreid @catchang @oceanf @joergmlpts @jrebman @possum_noises @lagoondon @hikingsandiego @wranglebees @wojciech @aguilita @bryanconnolly @mjpapay @leaf0605 @galanhsnu @nathantaylor @sambiology @peakaytea @pioleon

I need to remind myself of the research, but there should be no outcome where the island taxa are lumped with umbelliferum.

That sounds like a good idea to me. I think it would be very helpful to have a complex set up like that.

We can't keep both Solanum umbelliferum var. glabrecens and Solanum parishii: I changed here: https://www.inaturalist.org/taxon_changes/135569

Deviation here: https://www.inaturalist.org/taxon_framework_relationships/303203

I am closing this flag, but don't hesitate to comment if there is anything wrong.

"Solanum xanti is synonym of Solanum umbelliferum var. glabrecens."
You mean S. parishii = S. umbelliferum var. glabrescens in POWO.

Note: there was no identification for Solanum umbelliferum var. glabrecens before I changed the taxon.