In article , Stewart Robert Hinsley
writes
Most of the plants called thistles belong to a sub-group classified as
Cardueae, which also includes Cardoons, Cornflowers, Knapweeds, et alia.
Sow-thistles are an exception; they are related to Dandelions, Lettuces,
Hawkweeds et al (Lactuceae).

Thanks Stewart. The plot re. plant relationship thickens here a bit, but
all of those you mention are in the Compositae family, though in
different genera and species of that group. Some of them are also
mentioned as species or sub-species within the Asteraceae family, and
use the synonym Lactuca for differentiation. Plantfinder gives twenty
varieties of Sow-thistle, with the common Sow-thistle listed as Sonchus
oleraceus.
--
Alan & Joan Gould - North Lincs.