Vascular Plants of North Carolina
Account for Swamp Smartweed - Persicaria hydropiperoides   (Michaux) Small
Members of Polygonaceae:
Members of Persicaria with account distribution info or public map:
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Section 6 » Family Polygonaceae
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Author(Michaux) Small
DistributionCoastal Plain, Sandhills, and lower Piedmont; scattered records westward, plus Polk and Swain counties in the Mountains.

N.S. and Que. to AK, south to FL, TX, and CA; Mex., C.A., S.A.
AbundanceGenerally common in the Coastal Plain and Sandhills, but somewhat less numerous in the northeastern counties. Uncommon to locally common in the lower Piedmont; rare to uncommon in the southern Piedmont, and very rare elsewhere.
HabitatWet to seasonally inundated soils of fresh marshes, beaver ponds, impoundment margins, swamp forests and openings, stream margins and riversides, interdune ponds and swales, ditches.
PhenologyFlowering and fruiting late May-November.
IdentificationSwamp Smartweed is one of several of our species which look similar in the field: leaning to erect stems 2-4 feet, with inflorescences terminal and from upper leaf axils. In this species, flowers are green and white (not pink or rosy), ocreae bristles are greater than 0.5 mm long, and sepals are smooth (not pitted or punctate). Most leaves are quite linear to narrowly lanceolate, barely 1/3-inch wide. Bog Smartweed (P. setacea) is usually taller, broader-leaved, thicker-stemmed, and flowers bright white and showy. The very common Dotted Smartweed (P. punctata) has a looser spike, with many gaps between flowers, wider leaves (up to 1 inch wide), and punctate sepals. Swamp Smartweed is a familiar wetland plant in the Coastal Plain, occurring in dense stands, often somewhat matted or leaning, at least at the base of the stem.
Taxonomic CommentsIncludes P. opelousana, which represents the narrow-leaved end of the variation, and which most NC plants are. RAB (1968) listed this species as Polygonum hydropiperoides, with two varieties; this species being the nominate one and var. opelousanum now being lumped by Weakley (2018).

Many species formerly treated in the genus Polygonum have been moved to Persicaria, the smartweeds. These are generally erect and tall plants with terminal and axillary floral spikes; most occur in wetlands. Others remain in Polygonum, the knotweeds, which are generally prostrate to ascending and with inconspicuous axillary flowers. They occur mostly in dry soils and tend to be weedy.

Attention must be paid to the small collars at the junction of the main stem and leaf stems (called ocreae) and whether they possess terminal hairs or bristles. Some keys also refer to the even smaller collars from which flowers emerge (called ocreolae). Another important ID character is the surface of the greenish sepals -- whether smooth or dotted with indentations (punctate).
Other Common Name(s)False Waterpepper, Mild Waterpepper
State RankS5
Global RankG5
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B.A. SorrieMarsh near Pee Dee River, 1999. RichmondPhoto_natural
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