Flora of North America species comparison

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.)