One species each day (from my backyard)

It started to rain when I went on my nightly exploration in my backyard so was a little impatient when taking this picture. The camera was advertised as having a weatherproof body but I don't feel like testing this just yet. And even if the camera body is, I'm not so sure of the lens.
I guess this is Viola riviniana. It spreads by rhizomes and out-competes other plants in the garden. It produces lots of dark green leaves and very rarely any flowers, and until this night I have paid little attention to it except for pulling it out where I could. I'll have a little more respect for it now and will appreciate its pretty blue flowers more often.