An evergreen, sub-shrubby plant with very slender, glabrous stems, self rooting. Leaves opposite, 1⁄4 to 1 in. long, heart-shaped to truncate at the base, pointed at the apex, glabrous, margin undulated; stalk about 1⁄8 in. long. Flowers terminal, mostly in pairs, very shortly stalked; corolla slenderly cylindrical, 1⁄2 in. long, four-lobed and 1⁄4 in. wide at the end, pink, becoming darker towards the tips, downy inside; calyx cup-shaped, 1⁄8 in. long, four-lobed.
Native of Japan and S. Korea, collected by the late H. J. Elwes in an Arborvitae forest near Aomori in the north, where it was growing in dense shade and flowering in July. Apparently not introduced until 1933, when it reached the late Fred Stoker, who retrieved two plants from some packing material that had come from Japan. It is very hardy and flowers a month later than its American ally, which is distinct also in its more rounded leaves. In M. undulata they often approach triangular in shape.