Hardy perennial plant with very pronounced rhizomes creeping below ground. Prefers soils suffering from waterlogging. Occurs in abundance in heavy loamy and clay soils depending on the conditions.
Water knotweed [Polygonum amphibium]
Up to 120 mm long lanceolate (spear shaped) true leaves, at the base rounded to cordate (heart shaped), short petiolate (Picture 1). The petioles originate from the center of the stems. The flowers are pink, in dense, cylindrical pseudo-spikes (Picture 2). The seeds are sharp-edged longitudinal, shiny brown-black (Picture 3). The seed production is very small. The plant can spread through seed germination although most commonly spread through extension of the rhizomes.