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Sherardia is a monotypic genus of flowering plants in the Rubiaceae family. The genus contains only one species, viz. Sherardia arvensis or (blue) field madder, which is widespread across most of Europe and northern Africa as well as southwest and central Asia (from Turkey to Saudi Arabia to Kazakhstan) and Macaronesia (Canary Islands, Azores, Madeira, Savage Islands). It is also reportedly naturalized in Australia, New Zealand, Taiwan, Kerguelen, Ethiopia, Sudan, southern Africa, Mexico, Costa Rica, South America, Bermuda, Cuba, Haiti and much of Canada and the United States (especially the Pacific States and the lower Mississippi Valley).
Sherardia arvensis is an annual plant with trailing and upright stems growing up to 40 cm long, having a square cross-section. The rough pointed bristly leaves of about 1 cm in length are in whorls of four to six (normally six at the ends of the shoots, but four nearer the root).
The tiny pale lilac or pink flowers are approximately 3 mm in diameter and have a long tube, with only the end part of the four petals free. The flowers grow in clusters of two or three together in an involucral structure formed out of a ring of six bracts.
The fruit are dry and about 3 mm long with two lobes giving rise to the two seeds (nutlets).
The four-angled stems with whorls of bristly leaves and tiny flowers are reminiscent of the Bedstraws and other related Rubiaceae, but Sherardia is distinguished by its mauve/pink flowers that are organized in clusters and having a long corolla tube.
Sherardia arvensis is a common weed of fields, pasture, grassland, and disturbed areas. The fleshy roots, though much inferior to the common madder (Rubia tinctorum), are sometimes used for the production of a red dye.
The genus and species were described by Carl Linnaeus in Hortus Cliffortianus in 1736  and also appeared in his masterwork Species Plantarum in 1753. The genus was named in memory of the prominent English botanist William Sherard (1659–1728). The Latin epithet arvensis means that it is found in fields.
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- Kew World Checklist of Selected Plant Families, Sherardia arvensis
- Altervista Flora Italiana, Sherardia arvensis
- Biota of North America Program, Galium sherardia (synonym of Sherardia arvensis)
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- Linnaeus C (1737). Hortus Cliffortianus. Amsterdam: George Clifford. p. 33.
- Linnaeus C (1753). Species Plantarum. p. 102.
- Sherardia in the World Checklist of Rubiaceae
- USDA Plants Profile
- Plants of Missouri
- Wildflowers of the United States, Sherardia arvensis
- Calflora taxon report, Sherardia arvensis
- University of California at Davis, Integrated Pest Management On-line, Field Madder (Sherardia arvensis)
- Tela Botanica, Sherardia arvensis
- News from Rockcliff Farm, Natural Gardens of North Carolina, Blue Field-Madder, Sherardia arvensis
- Herbario de la Universidad Pública de Navarra, Sherardia arvensis
- Flora Vascular, Sherardia arvensis
- Wilde Planten in Nederland dn Belgie, Blauw walstro, Blauslyt, Field Madder, Rubéole des champs, Ackerröte, Sherardia arvensis