References for Verbenaceae
Lewis, W.H., and R.J. Oliver. 1961. Cytogeography and phylogeny of the North American Verbena. Amer. J. Bot. 48:638-643.
Perry, L.M. 1933. A revision of the North American species of Verbena. Ann. Missouri Bot. Gard. 20:239-358.

Source for the Reference Lists

George W. Douglas, Del Meidinger and Jim Pojar. 1998-2002. Illustrated Flora of British Columbia.