East Asia
Japanese knotweed is an upright herbaceous perennial that grows up to 10 ft (3m) tall. It has deep taproots as well as rhizomes that may extend up to 65 ft (20 m) laterally. The above-ground stems are, hollow, smooth, and often woody at their base; they are often reddish when young, turning green with age. Stem nodes are jointed, swollen, and have papery sheaths (ocrea). The leaves are alternately arranged along the stem, 3–7 in (7½–18 cm) long and 2–5 in (5–12½ cm) wide, with a squared-off base and abruptly pointed tip. Knotweed flowers are small, greenish to creamy-white, and have five petal-like sepals that are fused at the base, forming a narrow tube. Flowers grow in branched clusters from leaf axils near stem tips. Japanese knotweed flower clusters are typically 3–6 in (7½–15 cm) long, and the leaf immediately beneath each flower cluster may be shorter than or the same length as the flower cluster. Knotweed fruits are papery and have three wings and one seed.
Knotweeds are a major concern for displacing native species in riparian areas and in native and commercial forests. Knotweeds have demonstrated allelopathic tendencies where they release compounds into the soil that inhibit the growth of other plant species. They alter nutrient cycling in soil and streams, disrupt food chains, and can exclude trees, which is potentially detrimental to fish and other stream inhabitants that benefit from tree shade. Their lack of fine surface roots can lead to increased erosion, which is problematic along waterways. Their forceful rhizomes are capable of cracking concrete and asphalt, thus causing costly damage to roadways, parking lots, and building foundations.
Grevstad, F.S., J.E. Andreas, R.S. Bourchier, and R. Shaw. 2022. Knotweeds (Reynoutria spp.): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-19-KNOTWEEDS-P. https://bugwoodcloud.org/resource/files/23198.pdf