Rose, Contributions from the U.S. National Herbarium 13: 310, 1911
Opuntia mackensenii is not recognized by some workers who see it as a form of O. macrorhiza or O. phaeacantha. However, Powell and Weedin (2004) described the taxon clearly. They also described O. mackensenii var. minor. Plants may have pale spines but commonly have spines grading to black at the base. Sometimes spines are distinctly black and white. O. mackensenii is a prostrate plant to 3-ft across, but it may have ascending branches. Plants are generally may be up to 12-inches tall if branches ascend. Cladodes are often nearly circular. Many specimens have a tuberous root. Britton and Rose reported that the type locality is near Kerrville, TX. See the original citation. O. mackensenii is tetraploid as well as var minor.
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Dave Ferguson reports:
O. mackensenii is similar to O. phaeacantha, but that taxon is hexaploid. O. mackensenii was renamed O. edwardsii by researchers, but their type specimen was incorrect and the name fell by the wayside. Spines are usually white or black and white. Pads are greener- and softer-looking than those of O. phaeacantha, and the fruit is a rounder and richer in color (most often redder). It is commonly found over the northern half of Texas and reaches into Kansas, Missouri, Arkansas, New Mexico, and Oklahoma. It is often confused with O. macrorhiza in Texas especially because field guides do not have a slot for it.