What is this monster thistle-like weed?

Question

What is this monster weed? It grows back every year and I yank it, but I can't dig out the tap root because it's too close to the gas line.

It is in Norfolk, VA, USA in full sun.

I'm kind of curious about whether it will flower, and I decided to let it grow this year for identification. It's over 6 feet (>1.8 m) tall at this time, and looking rather wilted today in the heat.

Reminds me of a thistle, but it doesn't really have that many barbs, and the ones that it has don't prick my finger when I touch them. I don't want to let it grow much longer, because I'm worried about it going to seed and spreading!

Answer 1

After a bit of research, I believe my suspicion was correct. It seems to me that you have Tall Blue Lettuce (Lactuca biennis).

Wikipedia article:

Lactuca biennis is a North American species of wild lettuce known by the common names tall blue lettuce and blue wood lettuce. It is widespread across much of the United States and Canada from Alaska and Yukon south as far as California, New Mexico, and Georgia.

Lactuca biennis is a biennial herb in the dandelion tribe within the daisy family growing from a taproot to heights anywhere from one half to four meters (20 inches to over 13 feet). There are deeply lobed, toothed leaves all along the stem. The top of the stem bears a multibranched inflorescence with many flower heads. Each head is just over a centimeter (0.4 inches) wide and has many whitish to light blue ray florets but no disc florets. The fruit is a mottled achene about half a centimeter (0.2 inches) long with a brownish pappus.

Facts, characteristics and images: Lactuca biennis (tall blue lettuce)