The oldest name available for this species is Polygonum persicaria, coined by Linnaeus in 1753. In transferring the species from Polygonum to Persicaria, the name Persicaria persicaria cannot be used because using the same word for both parts of the scientific name is prohibited by international argeement.Persicaria maculosa (syn. Polygonum persicaria) is an annual plant in the buckwheat family, Polygonaceae. Common names include lady's thumb,spotted lady's thumb, Jesusplant, and redshank. It is widespread across Eurasia from Iceland south to Portugal and east to Japan. It is also present as an introduced and invasive species in North America, where it was first noted in the Great Lakes region in 1843 and has now spread through most of the continent.
Persicaria maculosa is an annual herb up to 80 cm (31 in) tall, with an erect rather floppy stem with swollen joints. The leaves are alternate and almost stalkless. The leaf blades often have a brown or black spot in the centre and are narrowly ovate and have entire margins. Each leaf base has stipules which are fused into a stem-enclosing sheath that is loose and fringed with long hairs at the upper end. The inflorescence is a dense spike. The perianth of each tiny pink flower consists of four or five lobes, fused near the base. There are six stamens, two fused carpels and two styles. The fruit is a shiny black, three-edged achene. This plant flowers from July to September in the temperate Northern Hemisphere.
Persicaria maculosa is native to Europe and Asia, where it can be mistaken for Polygonum minus. The latter has narrower leaves, usually less than 1 cm wide. It has been introduced to North America and is naturalised throughout the mainland continent, growing along roadsides, riverbanks, and on fallow ground. In the USA, it is very similar to Pennsylvania smartweed, but redshank has a fringe of hairs at the top of the ochrea, something which Pennsylvania smartweed lacks. The species has also been found in New Zealand and Australia.
In the United Kingdom, this plant is a weed, without natural habitat, and always associated with human activity. It likes moist soils, particularly reach ones, and acid peaty loams. It does not like lime, and liming is cited as a means to fight it.
Persicaria maculosa contains persicarin and tannins. In medicine, Redshank is used against diarrhoea and infections. The leaves and young shoots may be eaten as a palatable and nutritious leaf vegetable. It is often seen as a weed and rarely cultivated. A yellow dye can be produced from this plant with alum used as a mordant.