Prostrate Knotweed
Polygonum arenastrum
Knotweed family (Polygonaceae)

Description: This adventive annual plant is more or less prostrate, producing hairless stems up to 3' long. The alternate leaves are up to 1" long and 1/3" (8 mm.) across. They are oblong or oblong-elliptic, smooth along the margins, and hairless. At the base of each leaf, there is a membranous sheath (ochrea) that wraps around the stem and hides the short petiole. This sheath becomes ragged and brown with age. The foliage of Prostrate Knotweed is often blue-green in appearance.

A small cluster of 1-5 nearly sessile flowers occurs at the base of each leaf toward the growing point of each stem. Each tiny flower is about 2-3 mm. long, consisting of 5 sepals, a style with a tripartite tip, several stamens, and no petals. The flat sepals have white margins, otherwise they are green, pink, or purple. The flowers bloom from mid-summer to early fall for 2-3 months. These flowers open for only a short period of time when the weather is hot and sunny. A 3-angled achene develops within the sepals of each flower. This achene is reddish brown or brown and tapers at one end more abruptly than the other. A fine membrane often clings to the surface of this achene. The root system consists of a slender shallow taproot that frequently branches. This plant spreads by reseeding itself. Different varieties of Prostrate Knotweed have been described, which are sometimes regarded as separate species.

Cultivation: Typical growing conditions consist of full sun and a heavy soil that is slightly moist to dry. This plant flourishes in poor soil where there is little competition from other plants. It can withstand a fair amount of trampling.

Range & Habitat: Prostrate Knotweed is a common plant that occurs in every county of Illinois. It is adventive from Europe. Habitats include edges of roads and driveways, cracks in sidewalks and pavement, compacted soil along paths, and waste areas with barren ground. This species is found in disturbed areas, especially in and around residential areas and cities.

Faunal Associations: The small flowers probably attract flower flies (Syrphidae) and small bees, although such floral visitors are uncommon. Insects that feed on the foliage, sap, and other parts of Prostrate Knotweed and other knotweeds (Polygonum spp.) include Chaetocnema concinna (Brassy Flea Beetle), Capitophorus hippophaes (Polygonum Aphid), the aphids Aspidaphis adjuvans and Aphis polygonata, and larvae of such moths as Grammia virguncula (Little Virgin Moth), Haematopis grataria (Chickweed Geometer), Nomophila nearctica (Lucerne Moth), and Orthonama obstipata (The Gem); see Clark et al. (2004), Blackman & Eastop (2013), Covell (1984/2005), and Wagner (2005). Many upland gamebirds and granivorous songbirds eat the seeds of these plants, including the Ring-necked Pheasant, Mourning Dove, Cowbird, House Sparrow, Slate-colored Junco, and House Finch. The Bird Table has a more complete list of these species. In addition, the nutritious seeds are eaten by the Thirteen-lined Lined Ground Squirrel, White-footed Mouse, and House Mouse (Martin et al., 1951/1961; Whitaker, 1966).

Photographic Location: Several flowering plants were growing from the crack of a city sidewalk near Urbana, Illinois.

Comments: The Polygonum spp. (Knotweeds) with a prostrate or spreading habit have been subjected to various taxonomic classifications through the years. Some authorities regard Prostrate Knotweed as a single polymorphic species with several varieties, while other authorities identify several species of Prostrate Knotweed. Using the classifications of Mohlenbrock (2002) and Yatskievych (2000), the plant in the photograph is probably Polygonum arenastrum, which is by far the most common Prostrate Knotweed in Illinois. Another introduced species, Polygonum aviculare, has leaves that are more narrow (3-6 times longer than wide) and noticeably smaller in size toward the growing tips of the stems, while the leaves of Polygonum arenastrum are wider (2-4 times longer than wide) and remain about the same size along the length of the stems. There is also a native species of Prostrate Knotweed, Polygonum buxiforme, with an appearance that is even more similar to Polygonum arenastrum. For this native species, the 3 outer sepals of the flowers are longer than the 2 inner sepals and they have hooded tips, while the 3 outer sepals of Polygonum arenastrum have flat tips and they are about the same length as the inner sepals. In order to make such distinctions, it is necessary to use a magnifying glass.

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