Ask Mr. Smarty Plants

We are certain that Opuntia engelmannii var. lindheimeri (Texas pricklypear) occurs in Williamson County and pretty certain that  Opuntia macrorhiza (plains prickly-pear) also occurs there.  Opuntia phaeacantha (brownspine prickly pear) and Opuntia humifusa (low prickly pear) have also been reported in Williamson County.

So, why aren't we absolutely sure what's in Williamson County?  Because not all Texas counties have been completely surveyed, reported and verified by a botanical authority.  When we are asked which species occurs in a particular location, we have several sources we can check for species in Texas counties:  the USDA Plants Database, the University of Texas Plant Resources Center's Flora of Texas Database, and the Atlas of the Plants of Texas by B. L. Turner, H. Nichols, G. Denny and O. Doron.  

Generally, the first one we check is the USDA Plants Database. By selecting to do an Advanced Search in the Search column, we can choose 'Texas: Williamson' under "County Distribution" in the first section of choices and then scroll down to section 2 (Taxonomy) and type in 'Opuntia' in the "Genus" slot. Next we scroll to the bottom of the page and click on "Display Results" to learn that Opuntia engelmanii is the only species of Opuntia recorded in the USDA Plants Database in Williamson County, Texas.  If you click on Texas on the distribution map, you will see that Williamson is one of the counties highlighted.  You will also see that there are 4 different varieties shown for Texas. If you click on each of the distribution maps for those 4 varieties, you won't find Williamson County highlighted; but you will find that the distribution of  Opuntia engelmannii var. lindheimeri (Texas pricklypear) comes closest to Williamson County.  You can also see a statement underneath the USDA Plants Database that says "Our county data are based primarily on the literature, herbarium specimens, and confirmed observations. However, not all populations have been documented, so some gaps in the distribution shown above may not be real."

When we check Opuntia macrorhiza (plains prickly-pear) on the USDA Plants Database, we see that the distribution map shows it occurring in Travis and Burnet counties, but not in Williamson.

When we look at our other two sources, we find that no Opuntia species are reported in the "Flora of Texas Database" for Williamson County.  In the "Atlas of the Plants of Texas" vol. 1, pp. 218-219, both O. phaeacantha and O. humifusa are shown to occur in Williamson County; however, neither O. engelmanii nor O. macrorhiza are shown in Williamson County, but both are shown in Travis County and other counties adjacent to Williamson.

So, this is a very long answer to your question, but an explanation for why we aren't absolutely certain about the prickly pear species found in Williamson County.  Hopefully, someone is doing a botanical survey of the area and will report their findings.