Knotweed
Knotweed, the common name for a large number of related plants, many of which are weeds in fields or gardens. Some knotweeds are also called smartweeds. The common knotweed is a weak-stemmed, often sprawling plant that grows to a length of three feet (90 cm). It bears slender, oval leaves about one inch (2.5 cm) long. The flowers, greenish with pink edges, are very small. The swamp smartweed is a North American species that is unusual because it can grow in water or on land.
Knotweeds belong to the genus Polygonum of the family Polygonaceae. The common knotweed is P. aviculare ; the swamp smartweed, P. coccineum.