Flora of North America species comparison

Robinia pseudoacacia is native at least in the Appalachian and other mountainous regions of eastern North America; it is widely cultivated and escaped throughout North America and other temperate regions of the world. Black locust can be highly invasive and is considered a threat to native biodiversity in many areas (J. Heim 1990; B. M. Farris 2007; L. Derickx and P. M. Antunes 2013). Populations growing at latitudinal extremes (such as Canada) are sometimes more likely to be shorter (3 m or less).

The report of Robinia pseudoacacia from Prince Edward Island is based on a single old specimen, and the species is here excluded from that province.

The winged and glabrous fruits and flowers with white petals arranged on pendent racemes set Robinia pseudoacacia apart from other species of the genus.

Nothospecies based on putative hybrids involving Robinia pseudoacacia and R. neomexicana (R. × holdtii Beissner, R. × coloradensis Dode) are found in areas where the former has been planted; those involving R. pseudoacacia and R. viscosa (R. × ambigua Poiret) are found in western North Carolina.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

The conspicuously hispid leaf axes and branches distinguish the common forms of Robinia hispida from other pink-petaled species of Robinia. Forms of R. hispida lacking such a hispid indument are distinguished from R. viscosa by leaf axes and branches without glands, and by leaves with 13 or fewer, and usually broadly elliptic, leaflets.

Robinia hispida appears to be native in Alabama, Georgia, North Carolina, South Carolina, Tennessee, and Virginia (D. Isely 1998), and should be considered introduced in other areas.

In addition to the synonyms above, the following are also considered synonymous with Robinia hispida: R. grandiflora Ashe (1922) is an illegitimate later homonym of R. grandiflora Linnaeus (1753); R. rosea Marshall (1785) and R. rosea Elliott (1822) are illegitimate later homonyms of R. rosea Miller (1768); and R. speciosa Ashe (1923) is an illegitimate later homonym of R. speciosa Swartz ex Besser (1816).

Robinia hispida varies in chromosome number: some plants are diploid, 2n = 20, others are triploid, 2n = 30. Only diploids in the mountains set fertile seed on a regular basis. Triploids as well as diploids of the Coastal Plain and adjacent Piedmont are sterile or nearly so, propagating almost exclusively by root-suckering. Morphology varies considerably within the species, particularly in habit and in the amount and distribution of pubescence; due to the predominance of asexual reproduction, populations in many cases are clones and quite uniform in these features (D. Isely and F. J. Peabody 1984; Isely 1998).

To accommodate this variation, Robinia hispida has been divided into five varieties (D. Isely and F. J. Peabody 1984; Isely 1998). The sterile diploids of the Coastal Plain and adjacent Piedmont are perhaps the most distinctive: subshrubs 5–8 cm, with unbranched, zigzag stems that are glabrous or slightly hispid. These have been called R. hispida var. nana. From the Piedmont into the mountains are found shrubs and small trees 1–8 m that resemble the last in their reduced pubescence but which have branched, straight stems and are sterile triploids; these have been called R. hispida var. rosea. Sympatric with both varieties are shrubs 0.6–3 m that differ in their obviously prickly-hispid stems. The widespread, sterile triploids with this morphology are R. hispida var. hispida, while the fertile diploids, endemic to the mountains of western North Carolina and adjacent Tennessee, are R. hispida var. fertilis. The fifth variety, R. hispida var. kelseyi, does not exist in nature but likely represents a horticultural selection of R. hispida var. fertilis with narrower leaflets.

Interspecific hybridization exacerbates the complex variation patterns seen in Robinia hispida. Nothospecies resulting from putative hybridizations involving R. hispida and R. pseudoacacia (R. × margaretta Ashe, R. × oconeensis Ashe) are found in Georgia, North Carolina, and Tennessee; R. hispida and R. viscosa (R. × longiloba Ashe, R. × ashei Schallert) are found in North and South Carolina.

(Discussion copyrighted by Flora of North America; reprinted with permission.)