Prostrate Knotweed
Polygonum aviculare
Knotweed family (Polygonaceae)

Description: This plant is a more or less prostrate summer annual, 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 (Polygonum aviculare) 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. It is more tolerant of road salt and pollution than most species of plants.

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 knotweed species (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: Knotweed species (Polygonum spp.) with a prostrate or spreading habit have been subjected to various taxonomic classifications through the years. Some authorities regard Prostrate Knotweed (Polygonum aviculare) as a single polymorphic species with several subspecies (USDA, ITIS), while other authorities identify several distinct species of prostrate knotweed (Yatskievych, 2000; Mohlenbrock, 2014). The approach that has been adopted here regards Prostrate Knotweed as a single polymorphic species. In general, the different species or subspecies that have been described are difficult to differentiate in the field. The most common subspecies of Prostrate Knotweed in Illinois and North American generally is Polygonum aviculare depressum, which is referred to as Polygonum arenastrum by the taxonomic splitters. These different subspecies or species are differentiated from each other by such factors as the shape of their leaves (either more broad or narrow), the constancy in the size of their leaves (remaining about the same size or becoming smaller as they approach the tips of the stems), the relative length of the inner and outer sepals, and the shape of the tips of the sepals (hooded or flat).