Taxonomy of this species is confused and has received little attention. The most recent treatment applicable to our area is that of B.L. Turner (2008; Phytologia 90(1): 36-40), in which he splits Funastrum cynanchoides into two species: Funastrum cynanchoides and Funastrum hartwegii. These have elsewhere been treated as two subspecies, with varying distinctions made between them. However, Turner’s treatment just doesn’t work. His key distinguishing the two is as follows:
1. Leaves broadly lanceolate, 2–4 times as long as broad, cordate at the base; flowers mostly white; corona vesicles widest above the middle — Funastrum cynanchoides
1. Leaves narrowly lanceolate, 5–10 times as long as broad; flowers purple or pinkish; corona vesicles widest below the middle — Funastrum hartwegii
Turner also states that the two differ in flowering times: “F. hartwegii flowering in spring; F. cynanchoides flowering in the late summer and fall”.
Going through these characters in the order in which they appear above, my experience is that: 1) Leaf shape certainly varies substantially in Funastrum cynanchoides sensu lato, but I’m not sure there is a line to be drawn here (the majority of plants fall clearly on one side or the other, but many do not); with regard to the bases of the leaves, Turner omits any description of the shape of the base of the leaf for Funastrum hartwegii but the plants I have seen with narrow leaves typically have hastate leaf bases but may have cordate bases or even cuneate bases as leaves are reduced towards the stem apex; 2) Flower color also varies, but here I am quite certain there is no line to be drawn and, worse, variation is not correlated with that in leaf shape (plants with broadly lanceolate leaves may have either pale or purplish flowers; plants with narrowly lanceolate leaves may have either pale or purplish flowers); 3) I do not know at all what distinction is to be made regarding the shape of the corona vesicles as I have not seen any Funastrum cynanchoides s.l. in which these might be described as “widest below the middle”–even in plants that definitely have both narrowly lanceolate leaves and purplish flowers and therefore would be presumably assignable to Funastrum hartegii! 4) I have never seen Funastrum cynanchoides flowering in spring in southern New Mexico, but only in late summer or fall.
So what are we to make of this? We have one character with, at best, marginally discrete variation (leaf shape), another with substantial variation uncorrelated to leaf shape that does not appear to be at all discrete (flower color), and two characters for which Turner mentions two character states where I have seen only one (corona vesicle shape and flowering time). One possibility is that we only have Funstrum cynanchoides in southern New Mexico (despite the map Turner provides showing both in the area) and that I am seeing some “abnormal” variants of that species. This would explain why I have seen Funastrum cynanchoides neither flowering in the spring nor with corona vesicles wider at the base (if I have not seen “proper” Funastrum hartwegii, it would not be surprising that I have not seen these character states). A second possibility is that the plants I have seen are in a hybrid zone between the two, and Turner does mention that he has seen specimens intermediate between the two purported species. A third possibility is that there is a single species with seasonal variation in flower and leaf morphology–i.e., that in Texas, the area of Turner’s primary focus, we have a single species that flowers in both spring and late summer / fall, and that the leaf and flower morphology typically differ between seasons. A fourth possibility is that this is simply one variable species, in which several characters show substantial but independent variation that Turner has shoe-horned into two species regardless. A fifth possibility is that Turner is actually distinguishing between Funastrum cynanchoides and Funastrum crispum. Both of the character states that he attributes to Funastrum hartwegii, but which I have not seen in Funastrum cynanchoides s.l., are found in Funastrum crispum–which, moreover, always has narrowly lanceolate leaves and purplish flowers! I do not have any particular basis on which to prefer one of these possibilities over the others. I also do not have any reason to believe his treatment of Funastrum cynanchoides s.l. as two distinct species is correct. The most conservative approach is to take the fourth possibility to be provisionally correct–this is just one variable species. Barring further insight into the taxonomy of Funastrum cynanchoides s.l., this is probably the best option. We might want to recognize two rather dubious subspecies, differing in leaf shape only, but from what I have seen that is about as far as we could go. Given that Turner’s key does not work, if we recognize two species… what characters are we to use to distinguish them and on what basis? In answering this question we have no good options and are adrift in taxonomic murk.
Unfortunately, this is a common situation in New Mexico botany. We have what appears to a fairly straightforward treatment, by an unquestionably talented and knowledgable botanist, which nonetheless falls completely apart on closer inspection. Until someone tackles these myriad taxonomic problems with real, quantitative data, we are stuck. We do not know how many taxa there are and we do not know how to distinguish them. The best one can do until the situation is resolved is, as I have attempted above, express the current state of confusion. This is a small example. The entirety of Cactaceae is in a similar–but dramatically worse–situation, for example. We need more taxonomists.