Vascular Plants of North Carolina
Account for Water Smartweed - Persicaria amphibia   (L.) S.F. Gray
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(L.) S.F. Gray
DistributionScattered locations in the northern parts of the state, from the northern Mountains to the outer Coastal Plain. Gaps in the map appear to be real. See Taxonomic Comments below re treatment of P. coccinea.

Circum-temperate and -boreal, south in N.A. to SC, TN, TX, and CA. Mex., S.A.
AbundanceVery rare in the northern parts of the state. Not known how many sites are currently still extant, but likely only a few. Where found, however, it may form large colonies (RAB 1968). This is a Significantly Rare species.
HabitatFreshwater marshes and impoundments.
PhenologyFlowering and fruiting June-October.
IdentificationWater Smartweed can be a true aquatic, with stems rooted on the bottom and leaves floating at the surface; but in NC most often semi-aquatic or even terrestrial, with essentially erect stems. Leaves are broadly elliptic, up to 8 inches long, distinctly stalked, and have a pale midrib. The inflorescence is a thick spike of rosy to pink flowers, about 4-5 inches long and about 1/2 inch thick -- quite attractive and mostly a darker pink color relative to spikes/racemes of similar species. Sadly, this beautiful smartweed is very rare in the state, and hardly any living botanists have seen it here.
Taxonomic CommentsNamed as Polygonum coccineum in RAB (1968) and some other references. The taxonomy of the Persicaria amphibia-P. coccinea pair is far from settled. The former (in the strict sense) is thought to be native to Eurasia, according to FNA, and found in the New World only in New Brunswick and New York; the latter is native to the Americas. Weakley (2020), following Haines (2011, Flora Novae Angliae) recognizes P. amphibia ssp. laevimarginata and P. coccinea as both occurring in North America and both native there and both rather widespread. Alternatively, FNA discusses the differences between the 2 "species", as well as chracters noted in aquatic plants and semi-terrestrial plants, both of which have been given varietal or subspecific names. FNA lumps everything into a broadly viewed P. amphibia. The arguments of FNA and Haines cry out for modern molecular study. Meanwhile, BioDiv Taxon Editors will treat all NC material as P. amphibia, except where NCU specifically lists P. coccinea (as it also has specimens labelled as P. amphibia).

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)Swamp Smartweed
State RankS1
Global RankG5
State StatusSR-P
US Status
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USACE-empOBL link
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B.A. SorriePhoto taken at Wrentham, MA, in 1980s. Photo_non_NCPhoto_non_NC
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