Oh dear! Mares tail is very hard to get rid of and pretty resistant to weed killer. It is waxy and has deep roots. You could try mixing some glyphosate with some wallpaper paste and painting it on but you will have to do it more than once.
Hmm, I'm trying to not use any weedkiller. It seems to be around the harder clay soil areas not where I've spread compost. I suppose I'll just weed and try and improve my soil in those areas.
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Don’t let it spread!
Good luck 🤞