Endemic to Texas and apparently restricted to the Edwards Plateau, where it has been documented from Bandera, Blanco, Comal, Hays, Kerr, Real and Travis counties; oddly, it has also been reported from Bailey County in the Texas Panhandle (Rowell, 1949). Locally common in Blanco and Hays counties, but relatively rare elsewhere in the state.The map (computer generated) provided by the Digital Flora of Texas Herbarium Specimen Browser closely matches Carr's statement, although it lacks data from Comal Co. I assume the far west Brewster Co. specimen as well as the two vouchered specimens from Frio Co. reflect errors in the TAES database. For reference I have added a yellow-colored Bailey Co. as well as an orange-colored Howard Co. representing the northwest edge of the Edwards Plateau.
Atlas of the Vascular Plants of Texas (Turner et al., 2003) does not include the questionable reports from Bailey and Brewster counties. But does include the doubtful Frio Co. specimens and omits Comal Co. — overlooking E. J. Palmer's specimen in the U. S. National Herbarium (Vines 1960, p. 273). (As noted by Turner, p. 3, the dots on the map do not reflect location below the county level nor is the absolute number of dots significant.)
Two other maps provided by the Texas A&M Bioinformatics Working Group are:
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| Texas Endemics: Annotation status of Berberis swaseyi | Distribution of Mmahonia (sic) swaseyi |
Correll & Johnston (1970) paint a much broader picture:
In limestone ridges and canyon walls of the Edwards Plateau, reported from Bailey Co. in the Panhandle,... endemic.Although the past half century has failed to shed light on Rowell's report of B. swaseyi in the Panhandle, it seems to be passed along uncritically. Perhaps if one considers B. swaseyi to be a plant of the entire Edwards Plateau, then Bailey Co. might not seem so far off.
Other descriptions vary widely in scope:
| Note also the map for B. swaseyi given in FNA: |  |