Wildflowers of the United States

Dotted Smartweed, Dotted Knotweed, Water Smartweed - Persicaria punctata

Genus Common Name: Smartweed

Native Status: Native

Dicot Annual Perennial Herb

Persicaria punctata - Dotted Smartweed, Dotted Knotweed, Water Smartweed.
The USDA Plants Database lists 80 species of Polygonum in the United States. However, this genus has recently had 30 species split off into the new Persicaria genus, and several other species moved into Fallopia. Persicaria punctata (Polygonum punctatum) is one of the species of Perisicaria native to the United States, although it is introduced rather than native in several states - Idaho, Montana, Colorado, Utah, Wyoming, and Hawaii. The only states where it is not found Nevada and Alaska. (The USDA Plants Database map does not show it in Utah, but BONAP, which is likely more current, does show it there. However Flora of North America does not list it Utah, so that presence remains questionable in my mind. It is found in much of Canada.

A key identification feature are the punctate dots or dotted glands on various parts of the inflorescence and perhaps the stem. Most Persicaria species do not have these. A similar species with these punctate dots is Persicaria hydropiper (Polygonum hydropiper) - Marsh-pepper Smartweed - a species introduced from Europe. It must have been an early introduction, because there are reports of its use by native Americans both as a drug for certain ailments and as a food. P. hydropiper has reddish stems vs green for P. punctata, and the achenes of the introduced plant are reported to be dull and roughened versus the smooth, shiny achenes of the native P. punctata.

Found in:
AL, AR, AZ, CA, CO, CT, DC, DE, FL, GA, HI, IA, ID, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MS, MT, NC, ND, NE, NH, NJ, NM, NY, OH, OK, OR, PA, RI, SC, SD, TN, TX, UT, VA, VT, WA, WI, WV, WY

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Distribution of Persicaria punctata in the United States and Canada:

Map courtesy of The Biota of North America Program.
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Site: Flintstone Nazarene Church, Walker County, GA Date: 2012-September-25	Photographer: Gerald C. Williamson Nikon D7000 Tamron SP 90MM f/2.8 AF Macro
The inflorescence of Persicaria punctata is usually terminal although they may occasionally be axillary. There are generally 2 to 6 blossoms connected to the stem together in a fascicle with ocrea, with many of these irregularly spaced along the inflorescence stem.

Site: Chattanooga Valley Church of the Nazarene, Walker County, GA Date: 2012-September-25	Photographer: Gerald C Williamson Nikon D7000 Tamron SP 90MM f/2.8 AF Macro
A key identification feature for Dotted Smartweed are the punctate dots or dotted glands on various parts of the inflorescence. There are generally 2 to 6 blossoms in an ocreate fascicle. The blossoms are green changing to white toward the end of the 5 tepals, although they are occasionally pink. The tepals are fused for approximately the lower 1/3 of their length.
Click on the photo for a larger image
Site: Chattanooga Valley Church of the Nazarene, Walker County, GA Date: 2012-September-25	Photographer: Gerald C Williamson Nikon D7000 Tamron SP 90MM f/2.8 AF Macro
The leaves of Dotted Smartweed are up to 6 inches long and 3/4 of an inch wide. They are usually on a short petiole, but may be sessile. The leaves may or may not be glabrous.
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Site: Chattanooga Valley Church of the Nazarene, Walker County, GA Date: 2012-September-25	Photographer: Gerald C Williamson Nikon D7000
While the round stem of Dotted Smartweed is hairless (glabrous), the sheathes at the leaf nodes have a fringe of longish hairs atop them. This is a feature that is useful in identifying species within Persicaria.
Click on the photo for a larger image
Site: Chattanooga Valley Church of the Nazarene, Walker County, GA Date: 2012-September-25	Photographer: Gerald C Williamson Nikon D7000
Dotted Smartweed has ascending stems and can be a highly branched plant. Another common name for Dotted Smartweed is Water Smartweed, indicating the affinity for wetlands, floodplains, marshes, and areas of shallow water.
Click on the photo for a larger image

References used for identification and information:

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Persicaria punctata initially published on USWildflowers.com 2012-12-22; Updated 2015-06-25

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All content except USDA Plants Database map Copyright Gerald C. Williamson 2024
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Code Update 20230302