Dioscorea villosa

Common names: wild yam (Kartesz 1999), Atlantic yam, common wild yam, wild yam-root, yellow yam; colic root, rheumatism root.

The similar Dioscorea quaternata has frequently been confused or combined with Dioscorea villosa, but it is "very distinct" and readily distinguishable by its smoothly round and wingless stems, and whorled lower leaves (3-9 at a node). The inflorescences and flowers of Dioscorea floridana differ in several substantial ways from those of both of these species (see Al-Shehbaz and Schubert 1989).

Collectors of wild yams in the U.S. east and south for the medicinal plant trade may obtain several biological (taxonomic) entities which get named "Dioscorea villosa". This is especially so because the taxonomy of the continental U.S. dioscoreas, which probably involves three native species, has been quite confused in botanical literature and is still unsettled (Al-Shehbaz and Schubert 1989; Ward 1977; Weakley 1997; Kartesz 1999). In addition, four exotic species have naturalized to varying extents; one or two of the aliens can also be found in many states of this region, and in Florida three of them occur (Kartesz 1999; Al-Shehbaz and Schubert 1989), but the exotic species are distinctively different and unlikely to be confounded morphologically with the native species. However, in browsing information on the Web via Internet searches, the name Dioscorea villosa is sometimes used very loosely and sometimes more or less as a marketing label, even for wild yams from Mexico and/or Central America.

Kartesz (1999) recognizes three native species occurring in the U.S./Canada region: Dioscorea floridana Bartlett, D. quaternata J.F. Gmelin and D. villosa L.; he placed D. hirticaulis Bartlett [D. villosa var. hirticaulis (Bartlett) Ahles] (hairy wild yam, hairy-stemmed wild yam) in synonymy under D. villosa. This treatment agrees somewhat with Al-Shehbaz and Schubert (1989), which may be the most penetrating analysis thus far, although they did not thoroughly discuss the species as found beyond the Southeast. Other authors of floras and checklists have interpreted these four taxa variously, placing one or more within D. villosa either in synonymy or as distinct taxonomic entities (usually variety); there are details in Al-Shehbaz and Schubert (1989).

Braun (1967) commented on the variable morphology of D. villosa and D. quaternata (considering D. glauca Muhl. ex Bartlett a synonym of the latter). Gleason (1952) noted that plants with glabrous leaves have been distinguished as D. villosa fo. glabrifolia (Bartlett) Fern. [D. villosa var. glabrifolia (Bartlett) Blake], which was recognized by Steyermark (1963) (but not Yatskievych 1999). Kartesz (1999) places this taxon (as a variety attributed to Fernald rather than Blake) in the synonymy of D. quaternata, in agreement with Al-Shehbaz and Schubert (1989); however Yatskievych (1999) treats it as a synonym of D. villosa. Ward (1977) reported (which Weakley [1997] notes) that Shu-Fun Au in an unfinished (and unpublished) study had tentatively recognized D. hirticaulis and D. villosa (as well as D. floridana), but that "all" other entities were considered synonyms of D. villosa.

Al-Shehbaz and Schubert (1989) emphasize that Dioscorea villosa and D. quaternata are both highly variable in stem and leaf pubescence (with a spectrum of leaf variability within some populations and leaf pubescence and glaucesence not of taxonomic value), and that rhizome branching, thickness and surface configuration can be modified by environmental factors. Gleason (1952) had accepted Dioscorea hirticaulis (as occurring on wet alluvial soil from southern Virginia to southern Indiana and southward), although stating it was little known and possibly not deserving of specific rank (Al-Shehbaz and Schubert [1989] believe the Indiana plants are D. quaternata). Fernald (1970) recognized D. hirticaulis, but with a more restricted range of southern New Jersey to Georgia. Gleason and Cronquist (1991) note that some plants of the Coastal Plain with the stem hairy and relatively few-flowered inflorescences have been segregated as D. villosa var. hirticaulis. Radford et al. (1968) accepted this variety; so has Weakley (1997), but with commentary regarding conflicting information and the need for study. Al-Shehbaz and Schubert (1989) furthermore report that the type specimen of D. villosa is morphologically what has been called D. hirticaulis!

A scenario or sketch to understand the taxonomic results of the study by Al-Shehbaz and Schubert (1989) is as follows. In considering the Dioscorea villosa complex (particularly D. hirticaulis, D. quaternata, D. villosa), Al-Shehbaz and Schubert (1989) decided that two usually recognized species were not clearly different and could not be maintained, and moreover that they should be simply combined into one variable species, rather than being considered one species with two subspecies, varieties, or forms. They also decided that there were two good biological entities in the complex, that they were very distinct, and that they should be recognized as species. One is a regional species, the other a very widespread species.

The names for the two good species might have been D. hirticaulis and D. villosa, with D. quaternata just as a synonym of D. villosa, since they were not recognizing for example a D. villosa subsp. villosa and a D. villosa subsp. quaternata (J.F. Gmelin) Knuth. However, it was determined that the type specimen (the standard for the name) D. villosa was a plant of the regional species, not the widespread species. Consequently, they call the Atlantic Coastal Plain species D. villosa (synonym D. hirticaulis). The widespread and quite variable species is therefore called D. quaternata. Bruce MacBryde, Jan. 2000

Dioscorea villosa is a wide ranging perennial herb occurring across much of the eastern United States and southern Canada. With a large range extent, more than 600 occurrences, abundant habitat, broad habitat preferences, and no obvious intrinsic vulnerabilities, this species is considered secure.

Dioscorea villosa is native to the eastern United States and southern Canada from Minnesota to Texas east to Massachusetts and central Florida. Range extent was estimated to be more than 2.8 million square kilometers using herbarium specimens and photo-based observations documented between 1993 and 2024 (GBIF 2024, iNaturalist 2024, SEINet 2024).

By applying a 1 km separation distance to herbarium records and photo-based observations documented between 1993 and 2024, there are estimated to be more than 1000 occurrences rangewide (GBIF 2024, iNaturalist 2024, SEINet 2024).

Although threats are not widely documented, Dioscorea villosa is potentially threatened by development, agriculture, road maintenance, invasive species, and other threats in some places. The dried roots are sold as herbal supplements, and plants are sold in the horticultural trade, though it is unclear how much of this material comes from wild sources versus cultivated sources. However, the overall impact is likely negligible given the species' broad range, large number of occurrences, tolerance of light disturbance, and affinity for typically abundant habitats, as well as no obvious intrinsic vulnerabilities.