A paper in 1989 by H.D. Clarke, D.S. Seigler and J.E. Ebinger finally cleared up some of this confusion, synonymising "Acacia smallii" and a number of other taxa under the nominate form of "A. farnesiana", under which they also included all of the plants growing outside of the Americas. In the same paper, they recognised "A. farnesiana" var. "guanacastensis" from herbarium collections made by D.H. Janzen in 1976 in Guanacaste, Costa Rica. This taxon was later elevated to species status as "A. guanacastensis" by the same three in 2000 and later moved to "Vachellia guanacastensis" by Seigler and Ebinger in 2006.