https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:132040-2
https://www.biotaxa.org/Phytotaxa/article/view/phytotaxa.566.2.9

"As per the Shenzhen Code Art. 11.2, “A name has no priority outside the rank at which it is published …” (Turland et al., 2018). Although the priority of the name Juncus canadensis var. brevicaudatus Englemann (1866: 436) starts from 1866, its rank is varietal, and it cannot compete with the names at other ranks. At the rank of species, the name J. tweedyi, published in 1900 has priority over the name J. brevicaudatus, published in 1904. Thus, Welsh et al.(2003, 2008, 2015) were correct in treating J. tweedyi as the correct name. It is speculated here that those who used J. brevicaudatus might have treated it as having priority from 1866 and ignored the varietal rank. The preceding concept of the publication priority regardless of the rank of a name alludes to a convention practiced in the then used American Codes, e.g., Rochester Code (1892) mandating “The Law of Priority.—Priority of publication is to be regarded as the fundamental principle of botanical nomenclature.”

It is noted here that Brooks and Clemants (2000) found no distinguishing morphology to separate Juncus tweedyi from J. brevicaudatus. Upon examination of numerous specimens by the first author, their findings are corroborated, and no morphologic characters are known to separate the two. There is a weak trend for plants of the western portion of the geographic range to have slightly larger capsules.

Given the fact that the name J. tweedyi has at least one current usage, we believe the best path forward is not to conserve the name J. brevicaudatus, but rather recognize J. tweedyi as the correct name. Furthermore, the possibility exists that future work could show a distinction, e.g., on a molecular basis, between J. brevicaudatus and J. tweedyi, and recognizing such a distinction might be facilitated by the availability of both names. For the preceding two reasons, we opt not to propose the conservation of the name J. brevicaudatus over J. tweedyi."

See also: https://www.inaturalist.org/flags/650929