Definition of black root in US English:

black root

noun

  • 1Any of various North American medicinal plants with dark-coloured roots; especially a kind of speedwell, Veronicastrum virginicum, native to moist ground in the eastern United States.

  • 2Any of various plant diseases accompanied by a blackening of the roots.

Origin

Early 18th century; earliest use found in William Byrd (1674–1744), landowner and diarist.