There has been a great deal of taxonomic uncertainty and confusion in this taxon, with some groups categorizing it as a species of Monotropa with no or two subspecies, and others categorizing it as two different species (lanuginosa and monotropa) in the Hypopitys genus. To avoid confusion while still being as accurate as possible and still retaining the two entities as separate, I propose that we turn Monotropa hypopitys ssp. lanuginosa into two forms- Monotropa hypopitys f. lanuginosa (which would be the red form), and Monotropa hypopitys f. monotropa (the yellow form). This proposal is based off of a discussion @jakemccumber, @er1kksen and I have been having in an observation.

After this is done, we would also like the Hypopitys lanuginosa taxon to be made synonymous with Monotropa hypopitys, since both refer to the same plant and are thus redundant. Does that need a separate flag?

This dissertation (link immediately below) contains an exceptionally good and clear overview of the described situation in Chapter 2. The most relevant portions of discussion to this proposal/issue are pages 82 through 89 (phenology, color) with pages 96 to 101 having some useful information titled "Remarks on Nomenclature." https://etd.ohiolink.edu/!etd.send_file?accession=osu1417442819&disposition=inline

There are some appropriate cautions within, including that the widespread northern "forms" (cryptic species) do have some variation in stem color from yellow to pinkish, which is evident with iNaturalist records. However, the proposed "M. hypopitys f. lanuginosa" is well supported and distinct visually and phenologically. If this iNaturalist change is accepted we (proponents) would develop some descriptions and examples on the relevant pages (species level and form levels) guiding the use of each with the species level always available for unclear specimens/photos.

For anyone reviewing, considering this, or in the future making identifications to species or form in iNaturalist, perhaps the most useful single few paragraphs to read in the above link begin at the bottom of page 87 and go through page 88. The red form of the SW US and Mexico is very distinctly different in appearance and season from the eastern red form. The red form with yellowish bracts and flowers in the eastern US and Canada is distinct in appearance and season from the yellow form(s). The others (e.g., northern and midwest) may be a bit trickier, but still workable. Based on this reference it seems warranted to also provide a third form in addition to the other two proposed for the SW US and Mexico, which are quite striking. I would propose for our uses M. hypopitys f. sanguinea.

Finally, the summary paragraph from the color section of Chapter 2 of from Broe (2014, link above):
"In summary, considered as a single species—as it was previously by a number of scholars—or as a single
genus, Hypopitys exhibits a wide variety of color forms. But in the context of the molecular phylogeny, some order at least is introduced to this most polymorphic character: red stems with yellow bracts in the late-blooming
species in the east; pure red stems and bracts in the southwest and Mexico; deep salmon to orange stems with
pale bracts in the North; and monochromatic yellow to pale salmon stems in the Midwest and East s.l. We
suspect that as careful observation of the range of color and its progression through the season is correlated
with the molecular species, even this confusing character may obtain better resolution."

Red and yellow are treated as forms here: https://bsapubs.onlinelibrary.wiley.com/doi/full/10.3732/ajb.0800319 with a detailed description and use of forms here: https://etd.ohiolink.edu/!etd.send_file?accession=osu1417442819&disposition=inline

The taxonomic treatment is still so messy that the literature is often using the term "forms" as a placeholder. There is still much discussion about separating Monotropa into Monotropa and Hypopitys and discussion (i.e., Broe, 2014, linked above) about five different species within M. hypopitys. However, some treat them as subspecies while others still disagree on that. Thus the use of forms. The taxonomy is such a muddle that defaulting to ssp. would be or lead to far more innaccuracies than assigning observations to form.

That is true that the forms are not a formal taxonomic nomenclature being used. The two paper linked the my previous comment describe the ecological (pollination, etc.), phenological, and genetic differences between the forms, which show consistent coloration and timing even where sympatric. Some of this proposal has come from the field experience of myself and @er1kksen , which is also described in the literature. However, one of the big pushes for this is that this is a consistent issue with these species in iNaturalist. It's currently created a fair amount of confusion and we think this will help clarify that confusion by better matching both recent literature and current other taxonomic references (e.g., Flora Nova Angliae: https://gobotany.nativeplanttrust.org/species/hypopitys/lanuginosa/, https://gobotany.nativeplanttrust.org/species/hypopitys/monotropa/). Several states do have H. lanuginosa on a list (e.g., MA has it on its watchlist).

Okay, I have read the thesis more in depth. If anything, I think it supports not distinguishing merely based on color. With "yellow" to pinkish "forms" you would be including Hypopytis americana (= Hypopytis latisquamea, = Hypopytis brevis, = Hypopyti fimbriata), and three undescribed species (one in California/Oregon, one in the Northeast, and the other in the Midwest). In the "red" form, you would be lumping together Hypopytis lanuginosa and Hypopytis sanguinea. Hypopytis monotropa is lectotypified by a specimen from Denmark, so this name can be applied solely to the Eurasian clade without much trouble, I think. Based on this body of evidence (obviously not peer reviewed, unfortunately), I think iNat would have to deviate explicitly from POWO, and do the following:
Hypopytis monotropa/Monotropa hypopytis: Merge Monotropa hypopitys ssp. hypophegea, Monotropa hypophegea, Hypopytis hypophegea, Monotropa hypopytis ssp. hypopytis, and Monotropa hypopytis var. glabra into this taxon, in whichever genus is preferred. A proposal for conservation has been put forward that would allow it to remain Hypopitys, and it is genetically quite distinct from Monotropa.
Monotropa procera: Leave as is, but place under Hypopitys. Not clear to me, but either a name for the unnamed Northeastern clade, or a synonym of Hypopitys americana.
Hypopitys americana: make a record for this species, then swap Monotropa hypopitys ssp. latisquama into it.
Hypopitys lanuginosa: swap Monotropa hypopitys ssp. lanuginosa into this species.
Hypopitys sanguinea: make a record for this species.

I am personally opposed to adding taxa with no formal publication to the database, and I am not sure if we are even allowed to. The more conservative approach more or less in keeping with your suggestion would be to lump everything into two taxa, hypopitys/monotropa and lanuginosa, whether as subspecies or species. This would still be a deviation from POWO, but is sort of supported.

still unresolved (this may be better as a flag on the rank above for M. hypopitys covering all putative subspecies) -- the link to the dissertation is now broken as well

Seeking advice: will I cause headaches for you folks if I go through the New England (current) Monotropa hypopitys and suggest ID Monotropa hypopitys ssp. lanuginosa for the ones that appear to match Haines' Flora Nova Angliae's criteria for Hypopitys lanuginosa? It's as close as I seem able to get with the current taxonomy.

State botanists in New England largely have adopted Haines’ flora and currently treat Hypopitys lanuginosa as its own species. I am currently trying to help assess how rare/common H. lanuginosa is, particularly in New Hampshire. iNat is unusually valuable here, as herbarium specimens lose the color portion of the diagnostic criteria. But right now what we consider two species are all lumped under M. hypopitys on iNat.

From Flora Nova Angliae:
Ericaceae
3a. Inflorescence a solitary flower; stems glabrous, usually white (rarely pink);
sepals similar to the subtending bracts; anthers dehiscing by 2 clefts across the top;
stigmas glabrous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Monotropa
3b. Inflorescence a raceme with 2–16 flowers; stems pubescent, yellow to light brown
or pink to red; sepals not similar to the subtending bracts; anthers dehiscing by a single
cleft; stigmas sparsely to densely villous around the margin . . . . . . . . . . . . . . . Hypopitys
Hypopitys
Stem color, a critical character for identification, is lost in drying. Therefore, it is very
important to note on herbarium labels (less than 1% of collections in regional herbaria note
stem color in life).
1a. Stems pink to red in life; plants flowering during the latter half of August and during September; stigma usually densely retrorsely pubescent . . . . . . . . . . . . . . . . . . . . . H. lanuginosa
1b. Stems light brown to yellow in life; plants flowering during June, July, and the early half of August; stigma usually sparsely pubescent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H. monotropa

Hypopitys lanuginosa (Michx). Nutt. N
hairy pine-sap. Monotropa lanuginosa Michx. • CT, MA, RI. Forests, woodlands, frequently associated with Quercus.
Hypopitys monotropa Crantz N
yellow pine-sap. Hypopitys americana (DC.) Small; Monotropa hypopithys L. • CT, MA, ME, NH, RI, VT; nearly throughout. Forests, woodlands, frequently associated with Pinus and/or Quercus.

It is totally fine. The only reason I did not do it at the time was because I felt like I had spammed everyone when I was following Haines in the first place. I did make a project that collected all of the Hypopitys lanuginosa (per Haines), though I admit I stopped collecting observations into it after the reworking in iNat. I would prefer both subspecies/species be taken to the furthest level while following iNat rules so going through them and giving the subspecific ID at least brings them together.

Thank you both!
@jakemccumber do you only add those that strictly meet Haines' definition? I've seen a bunch in NH that are distinctly pink while blooming but bloom in July or early August. I'd like to add all the potentials to your project to make it easy for me to find them, but I don't want to disrupt your system