Spikey Mystery Plant

What is this? two of these appeared in containers in my front yard this year... I pulled one out of wine barrel #1, but the other I let grow just to see what it was.

At first I thought it was some kind of grass, but the leaves are serrated on the edges (very finely, and sharply!), but then this happened:

here's the one I pulled out of the wine barrel a few months ago...It was hard to remove and had a large taproot.