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Foeniculum vulgare
Looked like a carrot/hemlock relative, and HUGE. Grew taller than me, and the main stem was so thick that it almost had a bamboo-like appearance; hollow on the inside. The stalk had no hair, the inflorescence wasn't very dense. No purple pigment on the stem that I could see. Young leaves were very aromatic with an amazing scent like anise.
Disturbed area under a bridge, growing in well-drained gravel.
Isn't Nigella sativa the fennel flower, though, and that's a different thing? That has one bloom per inflorescence and this has umbels. If not fennel, this is definitely very closely related. Thanks for the suggestion!