This popped up after cool, rainy weather- very unusual here this time of year. I've never seen it before, and my husband blames it on the organic humus I added before planting. It is growing in a localized area, but after removing it- it came back. This area is slightly shaded by plants. It starts in clumps, and spreads. After a short time, the tops open up and little black seeds are inside.

Can someone tell me what this is, how to get rid of it?