Lesser yellow trefoil

Lesser yellow trefoil (Trifolium dubium), a member of the clover family, is a small-leaved, procumbent annual weed.  It forms a flattened, roughly circular mat of interlaced thin wiry stems, with tiny trifoliate leaves. Many small multiple heads of pale yellow flowers are produced throughout the summer months. 
Lesser yellow trefoil (Trifolium dubium) is usually most troublesome on lighter, sandy soils, particularly where the grass is weak through lack of regular feeding. It forms a deeply penetrating tap root, seeds freely and if scattered plants are ignored, large colonies quickly form. It survives close mowing – a practice which can further weaken turf on poor soils.