Violets are notoriously tricky to identify, especially the blue ones. First, some of the characteristics are rather subjective (eg “leaves somewhat rounder than other species”). Second, they are known to hybridize freely. And third, taxonomists frequently re-name them.
Nonetheless, I am reasonably confident that I have these three correct. As always, please leave a comment if you feel differently.
common blue violet
(formerly V. papillonacea)
This violet is found from Quebec to Florida and east into the Great Plains, in many different habitats, including peoples’ lawns, where it’s considered a weed. I’m more likely to think of turfgrass as weeds.
marsh blue violet
The primary way to distinguish marsh blue from common blue is to consider the habitat. This group of plants was growing on a mossy mound in the middle of a stream. I’ve seen others growing right in the water. Note also that the flowers rise well above the leaves (mostly), while in common blue flowers and leaves are about the same height (mostly). Marsh blue has a range similar to common blue, though it isn’t found quite as far west or south.
Viola sagittata var. ovata
(formerly V. fimbriatula)
You can see that this one has a somewhat longer leaf from the previous two. Also, the habitat was drier: it was growing on a slope in an area dominated by mountain laurel, with other ericaceous species (namely Vacciniums) and two different pine species nearby. This species doesn’t range as far south as the other two, and goes west only into some of the Midwest states.
One characteristic you should not rely on for identification of blue violets: color. It can vary wildly. Have a look at this:
This is common blue violet… white form.