The green-flowered Platantheras with the exception of the tiny and rare Platanthera chorisiana (Chamiso's Orchid) are very difficult to tell apart and most experts consider them to be a series of species many of which hybridize with one another and thus intergrade. This is our experience as well. We have found plants that very closely fit the description of each species, but have also found endless numbers of plants that are very difficult to identify and seem to have characteristics of more than one species.
The species are Platanthera aquilonis, huronensis, sparsiflora and stricta and a number of hybrids, Platanthera x correllii, x estesii and x lassenii, the first a hybrid of Platanthera aquilonis and stricta and the both the latter hybrids of the white-flowered Platanthera dilatata (with P. stricta and P. sparsiflora). The white-flowered Platanthera dilatata with its three varieties cannot be confused with any of the green-flowered species, but because it hybridizes with them must be considered here also.
Of these species, Platanthera sparsiflora, the Few-Flowered Rein Orchis, is the easiest to identify and the most clearly defined as a species in our experience, though our experience of this species is somewhat limited since it is at the extreme northern limit of its range in Washington. The narrow flowers, usually widely spaced on tall spikes are the identifying features. It is found in Oregon, California, Nevada, Utah, Colorado, Arizona and New Mexico and in southern Oregon and northern California often grows in serpentine soils.
Next is Platanthera aquilonis, the Northern Green Bog Orchis, distinguished by its green flowers with a yellowish lip, a lip that is more or less straight-sided, a club-shaped spur that is curved forward and shorter than the lip, an absence of scent and anther sacs that are low in the flower, widely separated at the base and close together at the top. The pictures below represent close approximations to the description of this species, but as we shall see there are plants that only partly fit this description and have also characteristics of other species.
Platanthera huronensis, the Green Bog Orchis, can be difficult to distinguish from the previous species. Though it is ordinarily a whitish-green, the color varies and the two species cannot be distinguished by color alone. Along with the color, its features are a lip that is not yellowish, a lip that has a dilated or narrowed base, a spur that is not as distinctly thickened as the previous species and that is equal in length to the lip and does not curve forward, with anther sacs toward the top of the flower and almost parallel.
Very similar to this species is the natural hybrid of Platanthera dilatata and stricta, P. x estesii. In our experience the slender curved spur, and the widened base of the lip, both very much like the spur and lip of the Platanthera dilatata parent are distinctive as well as the whitish color. This hybrid seems quite common - we have seen it in the Olympics, in Yellowstone National Park and in the Canadian Rockies - and if we have correctly identified it would indicate that the two parents rather freely hybridize with one another. In all these locations the two parents grow and flower together.
Platanthera stricta, the Slender Bog Orchis, is distinguished by rather small green flowers, a straight-sided lip and a spur described as "scrotiform," i.e. with an inflated tip, a spur that is shorter than the length of the flower. This species is very common and blooms along-side of and concurrently with the other green-flowered Platantheras. At first meeting it seems rather easy to identify until one begins to find plants that do not match the description of the species exactly and have characteristics of the other species.
Finally, here are a couple of pictures of flowers that do not clearly fit any of the descriptions. The first has every characteristic of Platanthera stricta, spur shape and lip shape especially, but has a lip that is very yellow, a characteristic of Platanthera aquilonis and the reason I identified it in the field as the latter species. The second has all the characteristics of Platanthera stricta, too, except the spur in length and shape is much more like that of Platanthera huronensis. This would probably be classified as Platanthera stricta but is not completely characteristic of that species.