I don't deadhead individual flowers (life's too short). I take off the whole spike when all the flowers on it are over. There is some seeding, but they're easy to recognise and pull out what's not wanted.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
I deadhead the whole stem too as they do self seed with gay abandon and I find the foliage looks better if deadheaded - unless I want babies. If you have a white form and a pink form the babies can be blush form.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
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I live in west central Scotland - not where that photo is...