Prunella thrives in damp, shady places, so if this is your lawn and you want to deter it you need to change the conditions.
This may not be easy or possible if you have lots of trees or live in a high rainfall area, as I do. I find that raising the mower blade a notch makes the grass look better and helps to conceal the less attractive bits of weeds. The flowers may be easier to stomach and to explain away!
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In the sticks near Peterborough