The key below may or may not work. It is my current best guess as to how to deal with Mertensia in the state and I offer it not because I have any particular confidence that it is correct, but because I think the other available treatments of Mertensia in New Mexico (and Colorado, for that matter) are worse!
Back in 1937, Louis Otho Williams published “A monograph of the genus Mertensia in North America” (Annals of the Missouri Botanical Garden 24(1): 17-159). There are problems with his treatment, but so far as I can tell it basically works. Nearing 80 years later, it appears still to be the best (and only synoptic) treatment of the genus in the U.S. Subsequently, names have been synonymized without any coherent rationale, characters have been misinterpreted, the distributions of various species have been expanded based on dubiously identified specimens, and so on and so forth. As a result, our understanding of Mertensia has been slowly and steadily mangled. The situation with Mertensia abundantly illustrates a general rule in taxonomy: After any monographic treatment of a genus, our understanding of its taxonomy doesn’t just stagnate, it declines. Whatever level of clarity and insight was achieved is slowly lost and buried under the compounded, piecemeal modifications (some well-justified, some not) of subsequent authors. This is not an absolute rule by any means and monographers can’t possibly get everything right. It is, however, a good guess that they got more right than anyone else. The problem, of course, is that the later dilettantes (myself included!) tend to assume they know better and, through being more recent if nothing else, tend to be believed. One way or another, a coherent synoptic view eventually disintegrates into a jumble of smaller taxonomic works, varying regional treatments, etc.
This brings me to the present. After a very long wait, someone has finally taken up the challenge of Mertensia. Mare Nazaire, presently of Rancho Santa Ana Botanic Garden, has completed a PhD on the genus and is working on the Flora of North America treatment for Mertensia. Based on what she has published thus far, it appears that Williams sometimes included distinct and relatively distantly related lineages either as varieties within a species or even lumped together within a single variety. Hopefully we will soon have a solid new synoptic treatment of Mertensia. In the interim, the key below is based primarily on Williams’ monograph, with adjustments in nomenclature based on the work of Nazaire. It may at least reduce some of the confusion. Or it may not!
1. Cauline leaves with prominent lateral veins; stems usually more than 4 dm tall; flowering late spring and summer
2. Leaves minutely strigose on the adaxial surface, glabrous or with spreading pubescence on the abaxial surface; sepals 2.5–5 mm long, acute – Mertensia franciscana
2. Leaves glabrous on both surfaces (minutely ciliate on the margins, often papillate on the adaxial surface); sepals 1.5–3 mm long, obtuse – Mertensia ciliata
1. Cauline leaves without lateral veins, or lateral veins very inconspicuous; stems usually less than 4 dm. tall; normally flowering in early spring, as soon as temperatures permit
3. Stamens entirely within the corolla tube (filaments attached in the corolla tube and anthers not projecting beyond the throat) – Mertensia alpina
3. Stamens mostly beyond the corolla tube (filaments attached near the throat of the corolla tube, anthers well beyond the throat)
4. Cauline leaves glabrous abaxially, pubescent or glabrous adaxially
5. Adaxial cauline leaf surfaces strigose, the hairs pointing towards the leaf margin (at a 45–90º angle to the central vein); sepals definitely strigose on the abaxial surface
6. Sepals connate half or more of their lengths – Mertensia fendleri var. fendleri
6. Sepals free to the base or nearly so (look carefully–they may be overlapping in the lower third and appear connate without close inspection) – Mertensia ovata var. ovata
5. Adaxial cauline leaf surfaces strigose, pustulate, or glabrous (if strigose, the hairs directed toward the apex of the leaf, ± parallel to the central vein); sepals glabrous or very nearly so on the abaxial surface (but ciliate on the margins and, rarely, with a few very sparse trichomes on the abaxial surface)
7. Corolla tube 3–6.5 mm long, the corolla limb equal in length or slightly longer; cauline leaves usually strigose adaxially – Mertensia lanceolata
7. Corolla tube 7–9 mm long, the corolla limb slightly shorter; cauline leaves glabrous adaxially – Mertensia ovata var. caelestina
4. Cauline leaves pubescent on both surfaces
8. Sepals connate one half or more of their lengths – Mertensia fendleri var. pubens
8. Sepals connate only at the base, free most of their lengths – Mertensia bakeri
Of the species known in New Mexico, Mertensia franciscana and Mertensia fendleri are by far the most abundant. Three others, Mertensia brevistyla, Mertensia fusiformis, and Mertensia oblongifolia, have been erroneously reported in the state. With the exception of Mertensia franciscana, misidentifications, misapplied names, and baffling synonymies abound in herbarium collections and southwestern regional treatments of Mertensia.