Oxalis

These lovely little plants are native, but they have also become a fast favorite with shade gardeners (in fact I have some planted along the side of my own house) and are often sold around St. Patrick's day as "Shamrock" house plant. Oxalis is edible, with a slightly sour taste, but don't eat too much, the oxalic acid in them can be toxic in very large doses (same as in rhubarb leaves). The leaves are rather sensitive to the sun and will fold up into little pyramids if they get too hot.