This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Club 19: 354. 1892. Persicaria opelousana Small, Fl. SE. U. S. 378. 1903.
Perennial, glabrous or nearly so throughout,, stem slender, erect or ascending, sparingly or considerably branched, 1°-3° tall, becoming woody below. Leaves linear or linear-lanceolate, 1 1/2'-5" long, sessile, ciliate; ocreae cylindric, strigose, fringed with slender bristles; spicate racemes panicled, erect, often geminate, 1/2'--2' long, not densely flowered; calyx greenish-white, pedicelled, 5-parted to below the middle; stamens 8 or fewer, included; style deeply 3-parted; achene 3-angled or rarely 4-angled, broadly ovoid or obovoid, 3/4"-1" long, black, smooth and shining.
P. hydropiperoides Michx. Fl. Bor. Am. 1: 239. 1803. Polygonum mite Pers. Syn. 1: 440. 1805. Persicaria hydropiperoides Small, Fl. SE. U. S. 378. 1903. Polygonum hydropiperoides Macouni Small, Mem. Dept. Bot. Col. Coll. 1: 81. 1895.
Perennial, glabrous or strigillose, stem erect, decumbent or prostrate, simple or branched above, slender, 1°-3° long. Leaves narrowly lanceolate or oblong-lanceolate, varying to linear-lanceolate, 2'-6' long, short-petioled, ciliate, pubescent with appressed hairs on the midrib beneath; ocreae cylindric, loose, strigose, fringed with slender bristles; racemes panicled, terminal, erect, narrow, more or less interrupted, 1 1/2-3' long; calyx white, pink or purplish-pink; stamens 8; style 3-parted to below the middle; achene 3-angled, ovoid or oblong, 1"-1 1/4" long, smooth, shining.
In swamps and wet soil, New Brunswick to Minnesota and California, south to Florida and Mexico. June-Sept.
Polygonum setaceum Baldw.; Ell. Bot. S. C. & Ga.
1: 455. 1817. Persicaria setacea Small, Fl. SE. U. S. 379. 1903.
Perennial, appressed-hirsute, stem 2°-4° high, erect, simple or sparingly branched; leaves lanceolate or oblong-lanceolate, 2'-9' long, mostly short-petioled, acuminate at both ends, ciliate, conspicuously pubescent on both sides, inconspicuously punctate; ocreae cylindric, long, hirsute or appressed-hirsute, fringed with very long, stout bristles; racemes few, terminal, l'-2 1/2' long, erect, linear-oblong, sometimes geminate, rather loosely flowered; calyx white or pink; stamens 8; style 3-parted to below the middle; achene 3-angled, oblong or obovoid. short, thick-pointed, 1"-1 1/2" long, minutely reticulated and rather dull, or smooth and shining.
In swamps, Massachusetts and Missouri, Louisiana and Florida. June-Sept.