These guys are popping up along the edge of my driveway, around the downspouts and in flower gardens. Seems like they spread underground, since they appear in rows. The stems are hairy.

I can pull them out by hand, but I have to be careful, since even thick stems (3/8" dia) break easily. The stems break just above ground level, so I can still pull out the roots with care.

The root system appears to be ball-shaped, carrying surrounding soil out with it.

Any ideas? They seem to be pretty hardy.