Thank you all for your answers. So far, it looks like “none of the above”. I’m sure it’s not wild turnip. And it’s not common sow thistle because the stem is not hollow, does not exude a milky juice when injured, and the plant does not seem to spread from horizontal rhizome-like roots. It has a single, very deep tap root. The tap root is kinda woody, so I would say it’s not edible like a turnip or carrot.
The plant is a fast grower, up to 3 feet in 3 months or less. I’ve not let any grow taller than 3 feet, so I don’t know how tall it can eventually get. It doesn’t need any extra water. The ones I cut down yesterday were thriving despite no rain here since early April. The stems are spiny, and the leaves are deeply lobed and very dark green. Any ideas?
I know that many weeds can come back from the roots, so I try to dig up the smaller plants roots and all. When the plant gets to a certain size, it becomes very hard to dig up the entire root. So what I’ve done before was to cut the stem at soil level and pour Roundup on the mini-“stump”, this being half an inch to three-quarters of an inch in diameter. This seems to work. But I keep getting new plants over the yard. I ran out of Roundup, so yesterday I did as before but used brush killer (with 8% Triclopyr). We’ll see if that works also. Pesky plant.