Definition of blackroot:
(n) :
species Veronicastrum virginicum (Culver's root)
(n) :
Several North American plants with black roots of:
(n) :
genus Pterocaulon, such as Pterocaulon pycnostachyum (dense-spike blackroot) and Pterocaulon virgatum (wand blackroot)
(n) :
species Aletris farinosa (colicroot)
(n) :
Several plant diseases caused by fungi in the genus Aphanomyces causing that cause roots to rot and turn black.