This naturalized annual was introduced from Europe. The erect, occasionally sprawling stems, up to 3 feet tall, bear alternate lance-shaped leaves with smooth margins up to 6 inches long. Usually (not always) there is a dark purplish spot or crescent near the middle of the leaf. A papery sheath (ocrea) with short bristles along its upper margin wraps around the stem at the base of each leaf (E). Tiny pink flowers on cylindrical spikes are on erect stems above the leaves (A).

Some other smartweed species occasionally have purple smudges on their leaves. They are usually not as distinct or dark as those of Lady’s Thumb. When present, is the best means of separating it from its other smartweed look-alikes.  The very similar smartweed, Bristly Lady’s Thumb is the species you are most likely to encounter along our trails. It, too, may have leaf smudges but not dark chevrons on the leaves and, as the name implies, has much longer bristles along the upper margins of the sheaths (ocrea) that surround the stem.