Galinsoga parvifolia, Yellow weed or Potato weed
This is a fairly common weed of gardens and cultivated areas and is found in over thirty countries, being of South American origin and brought to Kew Gardens from Peru in 1796. It is not normally seen growing as such a dense patch as in this photo but I came across it in a place where a vegetable crop had been harvested several months ago. Normally it appears as an isolated specimen, often working its way through other plants and reaching about 40 cm in height. What I did not know about it until recently is that it has a culinary use as a dried herb in South American cooking. In Colombia it is called Guasca and is used as an ingredient in a traditional soup called Ajiaco which normally includes chicken.