Polygonum erectum L.

Erect Knotweed


Erect knotweed is a plant that bears small, starchy seeds that can be very common in some archaeobotanical samples. It is one of the four small starchy seeds of the pre-maize quartet, which also includes chenopod, maygrass, and little barley. There are many wild types of Polygonum, but their fruits (thin-shelled achenes) are usually two-sided (lenticular) in cross-section. The possibly domesticated form of knotweed (erect knotweed) is three-sided in cross-section. It is erect knotweed that I will examine here.


       The achene is teardrop-shaped, and the naked charred kernel with its embryo missing is shaped like a fat hotdog bun. Sometimes, empty pericarps (the outer fruit covering) can be found in well-preserved samples. Carbonized erect knotweed seeds range in length from 1.7 to 3.2 mm for the striate-papillose morph (shape variant) and 2.4 to 4.7 mm for the smooth morph (Fritz 1987).

       Like chenopod, erect knotweed fruits vary in form. The wild-type morph has a thick, striate-papillose (bumps arranged in snake-like ridges) pericarp. The striate-papillose morph is also shorter and wider than the other morph, which has a thinner, smoother pericarp. The striate-papillose morph (Figure 2) has a pericarp thickness of ca. 60 microns (Figure 3). The smooth morph (Figure 4), which dominates most archaeobotanical samples, has a pericarp thickness of ca. 19 microns (Figure 5).


Archaeological Distribution

       Erect knotweed has been recovered from sites in eastern Kentucky, the Arkansas Ozarks, southeastern Missouri, and the central Mississippi, and Illinois River Valleys, including the American Bottom area.

       The Marble Bluff Site, in Arkansas, has thick morph knotweed achenes among a seed concentration dating to ca. 1000 B.C. (Fritz 1987), around the end of the Late Archaic. Caches of knotweed from sites dating to the Early Woodland period have been found in eastern Kentucky. Erect knotweed was identified from samples from Cloudsplitter Rockshelter (Cowan 1985). Knotweed has also been found in samples from Cold Oak Shelter, but only a few have been identified as erect knotweed (Gremillion 1993). In western Illinois, knotweed cultivation may have begun in the Middle Woodland period (D. Asch and N. Asch 1985). Both morphs of erect knotweed were present in samples from Illinois during the Middle Woodland. By the Mississippian period, erect knotweed assemblages are dominated by the smooth morph (N. Asch and D. Asch 1985; Fritz 1987). These later assemblages are proposed as being composed of genetically altered, domesticated knotweed.

The Modern Plant and Its Distribution

       Erect knotweed is a bushy-branched annual that ranges in height from 0.2 to 1 m (Figure 6). The leaves are greenish yellow and elongated diamond-to lance-shaped, and around 5 cm long.  The stem has thick joints, giving the plant its name, Polygonum, translated from Latin: "thick knees" (Fernald 1970).

       Today, erect knotweed grows throughout the eastern United States, except the extreme southeast. Even so, modern erect knotweed plants are difficult to find. "It does occur today as scattered plants--never in harvestable frequencies--within stands of smaller-seeded knotweed, P. aviculare L., which forms low mats on hard-packed trampled ground around human habitations" (D. Asch and N. Asch 1985:185). P. aviculare is a collective species which includes weedy European introductions. It is possible that these introduced taxa out-competed the native P. erectum, thus accounting for its modern scarcity.


       Fritz (1987:12-13) states that the "ranges of archaeological achene lengths and pericarp thicknesses overlap with those of the long, thin-coat morph in modern populations. The case for husbandry, therefore, continues to rely heavily on ecology, ubiquity, and context...[M]ore complex comparison of modern wild and archaeological populations is needed to convincingly make the case [for domestication], and this work lies ahead." Obviously, more work needs to be done to clarify the domesticatory status of P. erectum.



Asch, D. L., and N. B. Asch

    1985   Prehistoric Plant Cultivation in West-central Illinois. In Prehistoric Food Production in
            North America, edited by R. I. Ford, pp. 149-203. Anthropological Papers No. 75.
            Museum of Anthropology, University of Michigan, Ann Arbor.

Asch, N. B., and D. L. Asch

    1985   Archeobotany. In The Hill Creek Homestead and the Late Mississippian Settlement in the
            Lower Illinois Valley, edited by M. D. Conner, pp. 115-170. Research Series, Vol. Center 
            for American Archaeology, Kampsville, Illinois.

Cowan, C. W.

    1985   Understanding the Evolution of Plant Husbandry in Eastern North America: Lessons from
            Botany, Ethnography, and Archaeology. In Prehistoric Food Production in North America,
            edited by R. I. Ford, pp. 205-244. Anthropological Papers No. 75. Museum of
            Anthropology, University of Michigan, Ann Arbor.

Fernald, M. L.

    1970   Gray's Manual of Botany, 8th edition. D. Van Nostrand, New York.

Fritz, G. J.

    1987   The Trajectory of Knotweed Domestication in Prehistoric Eastern North America.
            (poster) presented at the 10th Annual Ethnobiology Conference, Gainesville.

Gremillion, K. J.

    1993   Plant Husbandry at the Archaic/Woodland Transition: Evidence from the Cold Oak
            Shelter, Kentucky. Midcontinental Journal of Archaeology 18:161-189.

Steyermark, J. A.

    1963   Flora of Missouri. Iowa State University Press, Ames.

Written by: Gina S. Powell