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Yellow sorrel, Oxalis corniculata, is a low, creeping weed that will happily grow anywhere - it's as happy in the bone-dry cracks in paving as it is in the damp garden and around the tops of containers. Although small, it's tricky to pull out and the roots usually stay where they are and it will regrow from them.
Solution
Organic
The best way to deal with yellow sorrel organically is to hoe it before it flowers and set seed, then remove it or leave it on soil surface on hot day to wither and die, to dig it out or smother it with mulch or a deep layer of soil. In lawns, vigorously rake the patch with a wire rake in mid-September to remove the worst of it.
Chemical
This plant is strongly resistant to lawn weedkillers, but in beds and borders you can treat it with the soil-acting granular weedkiller dichlobenil (Casoron G4 Weed Barrier), which should be applied in February/early March. Check label for restrictions of use.
