Looks like a begonia but I can't tell you which one.

Lovely find - lucky you.

Begonia?

Thanks for the feedback. After checking other photos and details for the plant I am sure you nailed it. In Michigan it is actually considered a Noxious weed and quite toxic to livestock, especially horses; in Europe it is considered an ornamental plant good for landscaping purposes so I guess I will still include it in my collection of wildflowers even though most people in Mich. might disagree.