Juncus effusus (Soft Rush, though the leaf tips are stiff and painful, just not as stiff or painful as those of some other rushes) is an interesting case, I think.
First, the species level. Juncus effusus has recently been split in North America. Four or five native species have been split out of it in the Pacific Coast states. It’s also been split at the species level in the eastern states. However, the eastern vs. western botanists have been doing this work completely independently and it’s not clear how to reconcile these species from different sides of the continent, so the Flora of North America doesn’t recognize any of them. This is sad.
Juncus effusus itself has three major subspecies that I know about. Juncus effusus ssp. effusus is native to Europe. Juncus effusus ssp. solutus is native to eastern North America. Both of these have been introduced to the Pacific states & province, where J. effusus ssp. pacificus is native. Obviously, the differences matter.
These species and subspecies aren’t all that hard to tell apart if the appropriate parts are photographs. Unfortunately, the most important parts are the leaf sheath tops that are found near the base of the stems. Almost nobody photos these.