Buttercup (Ranunculus).

Dig it all out and try to get the roots. Once it takes hold it is VERY hard to get rid of, especially if it gets into your lawn. Easier to pull out in beds. Spreads by seed and wicked long underground runners so mowing does not control it.

I have been fighting it here for ten years. Finally mulched over big chunk of lawn and turned it into a bed to try to keep it from invading from an adjacent ditch. Still have to deal with bits popping up through cardboard and 4" of wood chips but I'm finally winning the battle! Lol

Try to alleviate its favoured conditions: wet, compacted soil.

Thanks - that was quick :)

Will keep digging it out. There is some in my pasture and that isn't good as it is poison for livestock