Hmm. What is this?
I thought it was a Brussels sprout plant, but it doesn't really look like what I see on the Internet, and my plant is now more than 4 feet tall, and most descriptions of Brussels sprouts have them growing up to ~3 feet.
This one isn't exactly a volunteer, but it's growing a few feet away from where I sowed Brussels seeds back in May. All my other seedling failed, probably because the slugs ate 'em, but this one popped up a few weeks later, and it's been growing strongly every since.
I just hope I haven't been tending some sort of giant weed all this time!