I really need to see the foliage and stem to be sure but it looks like a member of the alfalfa family based on the shape of the bloom. Did you add a hay or straw mulch? This weed comes from animal feed but it does have the advantage of fixing nitrogen from the air and returning it to the soil so it is not all bad. I suggest you cut back the flowers with a weed strimmer then cover it all with damp newspaper and a wood chip or bark mulch if you cannot hand pull or hoe all the plants out. Always try to remove weeds before they bloom and set seeds. Keep growing, Marianne Binetti