I'm trying to ID a plant that grows wild in moist shade at my mother's house. It's variable in height, up to about knee-high. The leaves are opposite with long slender stalks and smaller leaves sprouting in some of the axils. The leaves are "leaf-shaped" with the widest part in the middle. There are three main veins that run from the stalk end to the tip, with smaller veins branching off of them. The edges are toothed. The leaves are soft and the top is smooth, but I think I'm feeling a few very small bristles on the underside. The main stems are juicy, like touch-me-not (jewelweed) stems.

My brother, who mows the lawn, likes this plant because it's attractive and completely covers a tricky area so he doesn't need to mow there. He says he's never noticed any flowers, so presumably they are inconspicuous. The roots systems are fairly small (with one main root and smaller branches), so I assume the plants are self-seeding annuals (though I suppose it's conceivable that they are part of a colony-type root system).

Any idea what this is?