WordNet sense 1 (any of several Old World coarse prickly-leaved shrubs and subshrubs having milky juice and yellow flowers; widely naturalized; often noxious weeds in cultivated soil):

sow thistle

WordNet sense 2 (tall Old World biennial thistle with large clasping white-blotched leaves and purple flower heads; naturalized in California and South America):

blessed thistle, holy thistle, lady's thistle, Our Lady's mild thistle, Silybum marianum