I'm a sucker for adorable little violets, and this one grabbed my attention with its lovely elegant purple flowers and tidy, glossy green foliage. I collected the seeds from plants growing in a boggy area around a perennial stream near Missoula, but northern bog violet will adapt to a range of high-moisture situations: irrigated garden, streamside, rain garden...as long as the moisture is there. 
 
This plant will grow in very dark shade, but is also happy with partial sun (but moisture, remember!). Unlike Canada violet, this species has a tidy clumping habit and doesn't spread much. You can divide established plants. Like most of our violet species, it sports beautiful, pollinator-attracting flowers early in the season, and then cleistogamous flowers later on (that means closed up flowers that pollinate themselves) for a little extra reproductive insurance. Cool, right? 
 
You could plant this with pioneer violet for a neat mix of yellow and purple, although I'm not yet sure if one would eventually outcompete the other. Large-leaf avens, mertens sedge, and seep monkeyflower are other suitable companions.
 
 
Close-up photo credit: Joshua Mayer (wackybadger), CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons