Polygonum cuspidatum fall foliage

Polygonum, or knotweed, is a genus in the buckwheat family Polygonaceae. The name is probably derived from the Greek poly, "many" and gonu, "knot". There is another theory which states that the second part of the name is derived from Greek gonos, "children".

The genus primarily grows in northern temperate regions. Appearance varies widely, with some species remaining low shrubs about 1 m high while others become up to 3 m high. Stems are upright and bamboo-like, often hollow, accounting for some of the common names of the family, such as "Elephant ear bamboo" and "Mexican bamboo". The large, smooth-edged leaves range from an elongate triangle through heart shaped to huge, 'elephant ear' type leaves. The stems are often reddish or red-speckled. Young shoots look similar to red asparagus. Small white or greenish flowers form in July and August and grow in dense clusters from the leaf joints. Although it dies back to the ground after hard frosts, the stems may persist through the winter as bare, reddish-brown stalks.

Many knotweed species, particularly Japanese, Giant and Himalayan knotweeds, are considered noxious, invasive pests. Like many such weeds, Japanese knotweed was introduced from Japan first into the U.K., then into North America in the 19th century as a landscaping ornamental.

Knotweed grows extremely quickly during the spring; Giant knotweed can reach 4.5 meters by June, Japanese knotweed 3 meters, and 'dwarf' Himalayan knotweed 1.5 - 2 meters. In Japan, Japanese knotweed is known as itadori, or 'strong plant'. Knotweed is a creeping perennial that can spread rapidly due to its ability to reproduce vegetatively. It develops an extensive network of rhizomes (roots that can sprout) spreading from 7 to 20 meters from the parent plant and at least 2 meters deep. Root and stem fragments as small as 1 cm can form new plant colonies. Floods and high water events wash whole or partial plants into rivers and creeks, which then break up, dispersing pieces of knotweed throughout the flooded area and banks, which then sprout into new plants. The freshly disturbed soil allows the rapidly growing young knotweed plants to outgrow other plants and take over the area, suppressing other species. Cutting, mowing, digging and some herbicide treatments, especially in early to mid growing season, fail to curb knotweed growth and in fact stimulate the production of shoots from latent buds dispersed on the root crown or rhizomes.

Species recognized by ITIS are:

• Polygonum achoreum
• Polygonum acuminata
• Polygonum acuminatum
• Polygonum alpinum
• Polygonum amphibium - Water Smartweed
• Polygonum arenarium
• Polygonum arenastrum
• Polygonum argyrocoleon
• Polygonum arifolium
• Polygonum aubertii
• Polygonum aviculare - Doorweed
• Polygonum baldschuanicum
• Polygonum bellardii
• Polygonum bidwelliae
• Polygonum bistorta
• Polygonum bistortoides
• Polygonum bolanderi
• Polygonum boreale
• Polygonum brasiliensis
• Polygonum bungeanum
• Polygonum buxiforme
• Polygonum caespitosum
• Polygonum californicum - California Knotweed
• Polygonum campanulatum
• Polygonum capitatum
• Polygonum careyi
• Polygonum cascadense
• Polygonum caurianum
• Polygonum chinense
• Polygonum cilinode
• Polygonum convolvulus - Climbing Bindweed or Black Bindweed
• Polygonum cuspidatum - Japanese knotweed
• Polygonum davisiae
• Polygonum densiflorum
• Polygonum douglasii - Douglas' Knotweed
• Polygonum erectum
• Polygonum fowleri
• Polygonum franktonii
• Polygonum glaucum
• Polygonum herniarioides
• Polygonum heterosepalum - Dwarf Desert Knotweed
• Polygonum hickmanii
• Polygonum hirsutum
• Polygonum hudsonianum
• Polygonum hydropiper - Smartweed
• Polygonum hydropiperoides
• Polygonum lacerum
• Polygonum lapathifolium - Kellogg's Knotweed
• Polygonum leptocarpum
• Polygonum marinense
• Polygonum maritinum
• Polygonum meisneranum
• Polygonum meisnerianum
• Polygonum minimum - Broadleaf Knotweed
• Polygonum nepalense
• Polygonum orientale
• Polygonum oxyspermum
• Polygonum paronychia
• Polygonum paronychioides
• Polygonum parryi
• Polygonum patulum
• Polygonum pensylvanicum
• Polygonum perfoliatum - Mile-a-minute weed
• Polygonum persicaria - Spotted Ladysthumb
• Polygonum phytolaccifolium - Alpine Knotweed
• Polygonum plebeium
• Polygonum polycnemoides
• Polygonum polycnenoides
• Polygonum polygaloides
• Polygonum polystachyum - Himalayan knotweed
• Polygonum punctatum
• Polygonum raii
• Polygonum ramosissimum
• Polygonum robustius
• Polygonum sachalinense - Giant knotweed
• Polygonum sagittatum
• Polygonum salicifolium
• Polygonum scandens
• Polygonum segetum
• Polygonum setaceum
• Polygonum shastense
• Polygonum striatulum
• Polygonum tenue
• Polygonum virginianum
• Polygonum viviparum - Alpine Bistort
• Polygonum X Bohemicum - hybrid of Japanese and giant knotweed