Trans. Linn. Soc. London 1: 77, plate 2, fig. 2. 1791.
Plants erect, 70–700 cm. Flowers: sepals greenish or yellowish (often obscured by darker markings); dorsal sepal suborbiculate or ovate to ovate-lance-acuminate, 19–80 × 7–40 mm; lateral sepals connate; synsepal 11–80 × 5–34 mm; petals horizontal to strongly descending, same color as sepals, commonly spirally twisted or undulate, sometimes flat, linear-lanceolate to lance-ovate or oblong, 24–97 × 3–12 mm; lip rather pale to deep yellow, very rarely white, rarely with reddish spots or suffusion on adaxial external surface, 15–54 mm; orifice basal; staminode cordiform-ovoid, deltoid, lance-ovoid, or ovoid-oblong.
Varieties 3 (3 in the flora).
Cypripedium parviflorum is extremely variable, reflecting individual phenotypic plasticity, infraspecific differentiation, and hybridization with related species. In particular, var. pubescens is difficult to delimit. In exposed situations, especially in calcareous soils, plants are low-growing with ascending, often narrow leaves and rather small flowers; in exposed boreal and arctic sites, plants and flowers may be very small with scarcely spiraled to flat petals. Such plants from Newfoundland were originally described as C. parviflorum var. planipetalum. When transplanted to less severe conditions, those plants often develop into larger plants with larger flowers of more common form; indeed, the holotype sheet of var. planipetalum includes a range in habit and floral morphology and includes a plant rather typical of boreal var. pubescens. The lips of smaller flowers often are markedly compressed laterally, with parallel sides, but when larger flowers are produced by the same plant, the proportions of the lip often change, the lip then being very broad below and tapering toward the adaxial surface (trapezoidal in cross section) or sometimes dorsiventrally compressed and broader than high.
The southeastern var. parviflorum differs from var. pubescens primarily in flower size and color, and the two might be merely forms. Most works dealing with Cypripedium parviflorum have treated the primarily boreal var. makasin as var. parviflorum, either including all small-lipped plants within var. parviflorum, or in some cases restricting the name to the northern variety and excluding the southeastern plants described by Salisbury as C. parviflorum. Fernald’s original publication on C. calceolus var. parviflorum actually treated var. makasin, citing a description of that variety and clearly discussing the northern plant. Additionally, although geographically accommodating Salisbury’s plant, Fernald excluded most of the range of the southeastern var. parviflorum, thereby referring most plants of var. parviflorum to var. pubescens, and further restricted var. pubescens to the east, thereby assigning most plants of that variety to his northern var. parviflorum, i.e., var. makasin. Consequently, most published illustrations of var. parviflorum are in fact var. makasin. Variety parviflorum has been dealt with primarily in publications on the southeastern flora. In the east, var. makasin is quite distinct, but in the west it becomes difficult to separate from very small plants of var. pubescens that are common there; in that area, fragrance is often the least equivocal character. In the northwest it seems to merge with C. ×columbianum, and in fact the northwestern elements of the species are only artificially accommodated within the variety. Very rarely plants apparently referable to var. makasin or var. parviflorum bear white lips. In some cases that may reflect past gene flow, but in others the plants appear to be color forms.
Hybrids of Cypripedium parviflorum with C. candidum are C. ×andrewsii A. M. Fuller, and different varietal parentages are recognized as nothovar. andrewsii [C. candidum × C. parviflorum var. makasin], nothovar. favillianum (J. T. Curtis) B. Boivin [C. candidum × C. parviflorum var. pubescens], and nothovar. landonii (Garay) B. Boivin [C. parviflorum var. parviflorum × C. ×andrewsii nothovar. favillianum]. Hybrids of C. parviflorum with C. montanum are C. ×columbianum Sheviak; the type was evidently derived from a cross with var. pubescens. Hybrids of var. pubescens commonly exhibit small lips and thus obscure varietal limits: in the northwest, C. ×columbianum merges with var. makasin and var. pubescens; in the midwest, C. ×andrewsii nothovar. favillianum may simulate var. parviflorum, and the delimitation of those two entities is unclear. Additionally, many plants of putative C. ×andrewsii nothovar. favillianum suggest C. ×columbianum. In general, those hybrids exhibit vegetative and floral morphology and color intermediate between those of their parents, or combinations of their parental characteristics. In particular, lips are commonly creamy, ivory, or yellow; often lips are yellow when the flower first opens and fade to white over the period of bloom. Consequently, different flowers on the same plant frequently exhibit a range of lip colors. Lip color furthermore sometimes varies from year to year in individual plants. Additionally, C. montanum and C. parviflorum var. makasin commonly contribute the dark coloration of their sepals and petals to hybrids with C. parviflorum and C. candidum, respectively. The apical margin of the orifice in C. candidum and C. montanum is typically acute, forming a sharp angle directed toward the apex of the lip. In C. parviflorum, this is a variable feature, but typically the margin is obtuse. Variation in this feature in yellow-lipped plants may in some circumstances aid the recognition of hybrids.
|1||Abaxial surface of distalmost sheathing bract (and often next) sparsely and inconspicuously pubescent to glabrous when young; flowers small; lip 15–29 mm; sepals and petals usually suffused with dark reddish brown or madder, or in the west often spotted and blotched; scent intense, sweet.||Cypripedium parviflorum var. makasin|
|1||Abaxial surface of distalmost sheathing bract (and often next) densely and conspicuously silvery-pubescent when young (later sometimes glabrescent); flowers large to small, lip 20–54 mm; sepals and petals variably spotted, striped, blotched, and reticulately marked with reddish brown or madder (rarely unmarked); scent moderate to faint, rose or musty.||> 2|
|2||Flowers small, lip 22–34 mm; sepals and petals densely and minutely spotted with dark reddish brown or madder and appearing uniformly dark (rarely coarsely spotted and blotched); s New England to Kansas and southward.||Cypripedium parviflorum var. parviflorum|
|2||Flowers commonly large, lip to 54 mm, but very small in some boreal and northern cordilleran specimens (as small as 20 mm); sepals unmarked to commonly spotted, striped, and reticulately marked with reddish brown or madder, rarely extensively blotched.||Cypripedium parviflorum var. pubescens|