Persicaria longiseta (Oriental Lady's-thumb)
|Also known as:||Bristly Lady's-thumb, Bristly Knotweed, Low Smartweed, Creeping Smartweed|
|Habitat:||part shade, sun; moist disturbed soil; shorelines, floodplain forest, woods, ditches, waste places, roadsides|
|Bloom season:||July - October|
|Plant height:||1 to 3 feet|
|Wetland Indicator Status:||GP: none MW: OBL NCNE: OBL|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Slender spike-like racemes up to 1½ inches long at the tips of branching stems, sometimes in the upper leaf axils. There may be gaps in the lower part of the cluster (interrupted) but not in the upper part (uninterrupted). Flowers are pink, about 1/8 inch long, with 5 tepals (petals and similar sepals). At the base of the flower is a short sheath (ocreola) with bristly hairs that are often as long as the flower, sometimes longer.
Leaves and stems:
Leaves are alternate, thin, lance-shaped, up to 3 inches long and to 1¼ inches wide, toothless, hairless except for short cilia-like hairs around the edge and sometimes on major veins on the underside. Leaves are stalkless or nearly so, and do not have a dark blotch on the upper surface, but may have a faint spot.
At the base of the leaf is a brown, membranous sheath (ocrea) that surrounds around the stem, has pale ribbing, and bristly hairs up to ½ inch long around the edge. Stems are hairless, green to red, prostrate but rising at the tips (decumbent) or ascending, often rooting at the nodes in the lower plant, creating colonies.
Fruit is a shiny, smooth, dark brown to black, 3-sided seed that is less than 1/8 inch long.
Oriental Lady's-thumb, sometimes known as Polygonum caespitosum var. longisetum, most closely resembles Lady's-thumb (Persicaria maculosa), which does not have the long bristles in the flower cluster, a trait only P. longiseta has. P. maculosa also has hairs only about 1/8 inch long on its ocrea compared to ½ inch on P. longiseta, and it always has a distinct dark blotch on the upper leaf surface, which P. longiseta lacks. P. longiseta has only been collected twice in Minnesota, back in 1993 in two southeast counties. It's not been recorded since but that doesn't mean its presence in Minnesota hasn't expanded. Au contraire, we spotted a fairly large roadside colony along Hwy 95 near Afton in Washington County. We suspect this is another under-reported weed, probably overlooked due to its similarity to the rather common P. maculosa.
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Photos courtesy Peter M. Dziuk taken near Afton, Washington County.
Have you seen this plant in Minnesota, or have any other comments about it?