Herbs: Is this an herb (or a weed), 1 by stillwood

stillwood wrote:
Several years ago my DH planted a small area with a variety of herb plants. Unfortunately, he did not keep a record of where each type was planted. It was his area and I paid no real attention to it. Over time his interest waned and the area got overgrown. A couple of years ago I dug up most of what was there - retaining some obvious rosemary and some of the plant pictured here which has appeared in an ever larger area each year. Now it is becoming a problem. The yellow flower is pretty, but the plant is expanding - when I pull it up, there is an impressive root system. Herb or weed - does anyone know?