Palicourea axillaris is widespread and quite morphologically variable. Not surprisingly, its circumscription and characterization has differed among authors. Here (Taylor, 2021), Palicourea axillaris is characterized generally following Steyermark (1972), Taylor et al. (2004), and Delprete & Kirkbride (2016) but more narrowly, with two of Steyermark's infraspecific taxa excluded and no infraspecific taxa recognized. The plants included here in Palicourea axillaris show well marked morphological variation in pubescence, leaf size, stipule size, and form of the calyx limb across the range of the species, but much of this variation is found within in local regions and no groups are clearly separable morphologically. Some of its specimens share distinctive general aspects and can be sorted into morphological groups, and some of these correspond to some of Steyermark's varieties. However, an equal number of specimens are intermediate in their supposedly diagnostic features, and no morphological features or groups are consistently correlated with geographic regions. A similar range of morphological variation in supposedly diagnotic characters can be found both across the range of this species, and also within local montane areas in Costa Rica. See Taylor (2021) for detailed discussion.
Steyermark (1972) recognized a number of varieties and forms of this species, but some of these were not validly published apparently due to a misunderstanding of the nomenclatural rules.