My name is Robert Maijer, new to the forum, living in The Netherlands. 15+ years ago I worked at a bonsainursery that imported plants from Japan. As a "weed" there were some Viola growing with the bonsai. I still grow these plants (they stay with me as volunteers in other pots and garden. The small plants have dark green slender leaves, 2 cm wide at the base, 5-7 cm long. Flower are small, dark purple/blue and flowers apear early in spring when it is still cool. I do not have a photo available now.

Any suggestions for a possible species?

Thanks, Robert