What is this weed?

This weed can take over quickly if not contained persistently before it makes seeds. It starts with a base of broad leaves, then sends up tough stems with little yellow flower clusters at the base of the leaves. Those flowers turn into sticky balls that cling like Velcro to everything, spreading voraciously. The stems aren't strong enough to support the leaves and seeds, so they cling to other plants, fences, or any other vertical surface.

What is this plant, photographed in my annual bed, which is reported to have medicinal purposes? I'll post the answer tonight.