This sprawling, rather weak stemmed annual was introduced from south Asia. Hairless stems have membranous sheaths (ocrea) with long bristles on their margins encircling the stem at the base of each alternate leaf (C,D). Lance-shaped to elliptical leaves with smooth margins are 3-4 inches in length by 1 inch wide (A). Some leaves may have a faint blackish smudge near the middle of the upper leaf (C). Pink flowers with 5 tepals (petals and look-alike sepals) are on slender racemes which are fairly dense above, but often more widely spaced below (E).  Flowers have prominent bristles at their base, especially the lower ones (E). Fruit is a shiny, black, three-angled seed.

As the common name suggests, Bristly Lady’s Thumb has bristles at the base of its flowers (see photo E) separating it from other smartweeds including closely related Lady’s Thumb (Persicaria maculosa). Bristles on the papery sheaths surrounding the stem at each leaf base are also longer than those of Lady’s Thumb and although one may see a faint dark smudge on the mid portion of the upper leaf, this writer has not seen the darker chevron or “thumbprint” so characteristic of Lady’s Thumb.