This is a work in progress. Not all North American/European taxa are covered here.
This is not so much a dichotomous key as a series of notes/observations about various species. I've tried to sort species within each genus by general subgeneric taxonomic groupings when necessary, though hopefully they are also intuitively ordered by general appearance.
Calypso (monotypic genus)
C. bulbosa
North American varieties:
var. americana: Bright yellow beard, usually with small dark spots in beard region. Lip otherwise unpatterned. Yellow nectary "horns" shorter than labellum in length. Despite being called an eastern variety, it extends from Alaska down the Rocky Mountains into New Mexico (as well as Arizonan sky islands), as well as across Canada rarely into the Atlantic Provinces. It is the only variety that occurs in the American Southwest.
var. occidentalis: Lacks yellow beard. Lip patterned with light red blotches. Whitish pink nectary "horns" longer than labellum in length. A western variety that extends from southeast Alaska down the Pacific coastline as well as into the Montana Rockies. It is the only variety that occurs in California/Oregon.
nothovar. × kostiukiae: A hybrid between var. americana and var. occidentalis. Very variable in appearance but generally with both a pale yellow beard and a lip patterned with reddish pink spots. These spots are usually smaller and more numerous than in var. occidentalis. Mostly occurs in Montana and southern Alberta.
Eurasian varieties:
var. bulbosa: Faintly yellow beard, large, pale brown/maroon blotches in beard region. Lip otherwise unpatterned. Yellowish nectary "horns" shorter than labellum in length. Whereas the rim of the labellum pouch of both var. americana and var. occidentalis tends to be patterned with tiny dark dots, var. bulbosa has an unpatterned, pure pink-white rim. Occurs in Scandinavia, the Baltic States, Russia, and Korea. I would not be surprised if var. bulbosa is ancestral to var. occidentalis and var. americana and diverged upon crossing the Bering Strait.
var. speciosa: Faintly yellow beard, large, pale brown/maroon splotches in beard region; lip otherwise unpatterened. Nectary "horns" much longer than labellum in length (protruding noticeably). Pouch "rim" unpatterned. Occurs in China and northern Japan.
Cleistesiopsis
C. bifaria: Generally pale, shorter lip. Lip ridges spaced more closely, bumpy/warty/uneven in texture. Not strongly associated with coastal habitat. Formerly included within C. divaricata.
C. divaricata: Generally pink, longer lip. Lip ridges spaced further apart, thin and linear. Coastal plain habitat.
C. oricamporum: Generally pale lip, coastal plain habitat.
Corallorhiza
C. maculata: Spotted lip (spotless forms exist). 3 veins on lateral sepals. Flowers have a short yellow mentum (spurlike nub at the base of buds). This mentum can help indicate C. maculata even when the plant is in bud. A white streak on the underside of the buds, which is the labellum, is another useful feature. Floral remnants point in the same direction as the seed capsule. Though flowers are usually pink/bronze in color, pure yellow forms rarely exist (usually with a spotless lip).
var. occidentalis: expanded middle lobe on lip, trapezoidal in shape.
var. maculata: short, narrow lip with parallel edges, often with a pointed tip. Many spotless forms of C. maculata are var. maculata, at least in the western U.S.
(other varieties further south near/in Mexico exist, but not much literature exists on them.)
C. mertensiana: Somewhat similar to C. maculata, but much more elongated sepals/petals/labella. the labellum is streaked/blotchy with purple, often purple throughout (as opposed to spotted). Whereas the petals on C. maculata spread outwards diagonally, they are held nearly vertically/parallel to the dorsal sepal on C. mertensiana, giving the appearance of a perpendicular (⊥) symbol. Buds and flowers have a very prominent mentum, often pink in color. Stems are often vivid magenta, but this species is extremely variable in color with combinations of purple, salmon, and yellow; they may even rarely be pale yellow and white. Floral remnants nearly perpendicular to seed capsule.
C. striata: Striped petals/sepals/labella that are proportionally much broader than in C. maculata/C. mertensiana. Buds are broadened, flattened, and usually duller in color compared to budding C. maculata/C. mertensiana. C. striata also completely lacks a mentum and has an unlobed, cupped/boat-shaped lip.
var. striata: normal pink/red coloration; large, open flowers
var. vreelandii: Mostly yellow coloration; small flowers only ever partially open. Most verifiable observations in American Southwest.
C. bentleyi: Similar to C. striata in general flower shape with cupped tepals and boat-shaped labella, but flowers are extremely small, unstriped, and cleistogamous (never open fully and self-pollinate). Extremely rare; only found on the Virginia/West Virginia border in the Appalachians. Part of the C. striata species complex, which includes other cleistogamous varieties found in Mexico.
C. odontorhiza: Flowers often cleistogamous, ie., never open and self-pollinate (chasmogamous forms exist). Seed capsules usually greenish. Dorsal sepal short, 1-veined. Galea (hood) present, usually. Appears in fall (August-October)
C. wisteriana: Flowers chasmogamous. Dorsal sepal long, 3-veined. Galea present, usually. Appears in spring/summer (January-July). Distinguished from co-occurring C. maculata by the smaller size, hooded flowers, slender, oval-shaped, unlobed labella, and lack of noticeable menta.
C. trifida: Greenish-yellow (only Corallorhiza species with chlorophyll). Lateral sepals thin, 1 veined, and curve forwards and downwards (vs. 3-veined and spreading horizontally outwards in yellow forms of C. maculata). C. trifida is also extremely diminutive and much smaller than C. maculata. Menta not very visible, labella slender. Circumpolar distribution; plants at higher latitudes may display bronze coloration especially towards the tips of the tepals.
Cypripedium
C. acaule: Highly recognizable pink labellum pouch. Only Cypripedium whose pouch opens with a vertical slit. Brown/green/yellow slender tepals. Plants has two oval-shaped, ground-hugging leaves, as well as only ever one flower with a large leaflike bract at its base, making it easily identifiable even out of bloom. Extremely widespread in eastern U.S. and Canada.
fma. acaule: pink lip (immature buds are still yellowish white)
fma. albiflorum: white lip, yellow-green perianth parts (mostly found in New England)
C. reginae: Large flowers with pink labellum pouch opening upwards and broad, white tepals. Often multi-flowered (1-4). Multiple cauline, highly pubescent (hairy) leaves. Occurs across much of New England and the eastern Midwest.
C. passerinum: Small flowers with spherical white labellum pouch and short white tepals (dorsal sepal greenish). Few-flowered (1-2). Like C. reginae, its close relative, it has cauline, highly pubescent leaves. A northern species; occurs across the northern half of the Prairie Provinces and Ontario; follows the Canadian Rockies up into Alaska.
C. californicum: Numerous (reaching 20+) small flowers with short, golden-yellow tepals and spherical white labellum pouches, often with two pink markings at the base. Klamath/Northern Coast Ranges, upper Sierra Nevadas.
C. guttatum: Large pink blotches on white, bucketlike pouch. Petals are thin/lanceolate and hug the pouch. Northwest/Yukon/Alaska into Russia.
C. yatabeanum: Yellow/brown/green blotches on white, bucketlike pouch. Petals are pandurate (violin-shaped) and spread outwards; they also appear to be held further from the pouch. Aleutian Islands into Russia/Asia.
C. parviflorum: Shoe-like labellum pouch is yellow; tepals yellow-green but usually heavily suffused with brown/maroon, creating an overall brown appearance. Along with its relatives, it has +/- alternately spaced cauline leaves. Widespread in eastern U.S.; extends through Prairie Provinces, scattered relictual populations in the Rockies.
C. kentuckiense: Labellum pouch is yellow (though often paler and more ivory-colored), but opening is much larger and pouch appears deeper. Whereas C. parviflorum appears like a shoe (more horizontally elongated), C. kentuckiense appears more like a bucket. Leaves often broader/more oval-shaped than C. parviflorum. Occurs mostly in portions of the southern U.S.
C. candidum: labellum pouch similar to C. parviflorum in shape (shoe-like, elongated), but pouch is white and tepals are yellow-green. occurs mainly in the northern Midwest.
C. montanum: similar to C. candidum in its pouch shape and white coloration, but tepals deep brown/maroon and occurs in the western cordillera of North America (Rockies, Cascades, and northern Sierra Nevadas)
C. arietinum: labellum pouch is pointed/cone-shaped. cauline leaves are more slender/spiraling (compared to broader and +/- alternate in C. reginae and C. parviflorum group). occurs in New England, northern Great Lakes region, and parts of the Prairie Provinces.
Goodyera
G. oblongifolia: Noticeable main white stripe down the center of the leaf. Other white patterning appears to radiate from the middle stripe and is thickest near the center; it tapers down the further it is from the center and often does not reach the leaf's edges. Leaves usually longer and pointed. More or less secund inflorescence (flowers arranged on one side). Highly spreading, long, brownish-green lateral sepals. Mostly occurs in western U.S. but occurs in northern Great Lakes region with scattered populations in Atlantic Provinces.
G. pubescens: Main white stripe down the center of the leaf. Other white patterning is netlike in appearance and is of a uniform thin width. The leaf is often entirely patterned with thinner but denser veining than G. oblongifolia. Leaves usually shorter and more oval-shaped. Extremely dense, cylindrical inflorescence with highly pouch-like lip. Widespread across eastern U.S.
G. repens: Lacks middle white stripe. Patterning seems to originate from the edges of the leaves instead. Patterning consists of thick, silvery veins. Extremely diminutive with short leaves and small, crystalline white flowers. Occurs across much of Canada and New England; extends down the Appalachians. Relictual populations in the Front Range of the Colorado Rockies.
G. tesselata: Similar to G. repens but somewhat larger and with much more numerous, thinner and paler veining (whitish green vs. silvery). Flowers may be tinted green/brown (similar to G. oblongifolia, but to a lesser extent). Hypothesized to be of hybrid origin between G. oblongifolia and G. repens. occurs in upper Great Lakes region, New England, and Atlantic Provinces.
Platanthera (needs more work)
Section Piperia: Small, green-flowered plants that mostly occur west of the Rocky Mountains. Unlike most Platantheras, they do not grow in boggy/perennially moist environments. They have slender, basal leaves that wither before anthesis (flowering), probably to conserve water. They reach their greatest diversity in California's seasonally dry chaparral communities.
Some of the most important characteristics distinguishing different species in section Piperia include (1) overall flower color, (2) shape/positioning of the petals and labellum, and (3) length and orientation of the nectary spurs. Keep these characteristics in mind when reading the following notes!
P. elegans: White flowers with straight, spreading sepals and petals (often greenish in the center with a white border). The labellum is slender and acute, meaning it is pointed at the tip but not triangular. The nectaries are slender, green, downturned, and usually longer than the ovaries in length. Tall, often dense-flowered cylindrical inflorescences. Inland specimens are often more lax-flowered (inflorescences less densely packed). Occurs along much of the U.S. Pacific coast to Los Angeles. Further north, it extends into the Montana Rockies.
ssp. elegans: The most widespread subspecies; occurs across the entire range of the species. Nectaries longer than the ovaries in length. Said to have a pleasant, musky daytime fragrance.
ssp. decurtata: Extremely rare; occurs only in Point Reyes (where ssp. elegans also occurs). Nectaries shorter than the ovaries in length. Inflorescences are said to be extremely stout and compact, and the lateral sepals seem to be very broad. Said to have a spicy cinnamon fragrance strongest at night.
P. yadonii: Initially appears similar to P. elegans; white flowers with falcate (curved/sickle-shaped) petals strongly curved upwards (usually with deep green center and white border). The labellum is slender and acute, and the nectaries are shorter than the ovaries. Rare; occurs only in the Monterey region.
P. transversa: White flowers with straight/very gently curved sepals and petals (often greenish in the center with a white border). The labellum is slender and acute. The nectaries, which extend past the ovaries, are characteristically held horizontally, perpendicular to the vertical axis of the inflorescence. They are whitish green and proportionally thicker relative to their length compared to P. elegans. This last characteristic is especially useful when trying to distinguish few-flowered inland P. elegans from P. transversa, which sometimes may have slightly downturned nectaries. P. transversa is usually less dense-flowered and smaller than P. elegans that has access to coastal fog or other sources of moisture.
P. elongata: Uniformly green flowers with falcate (curved/sickle-shaped) petals that curve upwards. Like the white-flowered P. elegans, the nectaries are downturned and longer than the ovaries in length, but the labellum is triangular (vs. merely slender and acute), and the lateral sepals are often strongly reflexed (bent backwards). This species has a similar distribution to P. elegans but extends all the way to San Diego and occurs along the Sierra Nevadas. It is one of the most widespread Piperia species in California.
P. michaelii: Uniformly creamy green flowers with straight sepals and petals. Most similar to P. elongata and also has downturned, long nectaries, but the labellum is proportionally much broader or even spade-shaped (vs. longer, thinner triangular). This species occurs in the San Francisco Bay Area and further south along the Central Coast. It also occurs in parts of the Sierra Nevadas.
European P. bifolia/P. chlorantha complex:
P. bifolia: anther sacs parallel and held close to each other
P. chlorantha: anther stalks further apart and diagonal to each other.