Stewart Robert Hinsley wrote:
What you describe is, I think, a fasciated dandelion. What the OP
describes does not sound like the same. "Giant dandelion" suggsts to me
something in the Hieracium/Crepis/Hypochoeris area; "tight cluster of
flower heads" a Sonchus (Sow Thistle), but I find the whole of Lactuceae
difficult to identify.
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Stewart Robert Hinsley
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Oh how I hate Sonchus asper(spiny sowthistle) - dang things are
everywhere around me. Ground up with dry twigs they do make a nice
mulch :0)