Verbena bonariensis may not be fully hardy for you as a perennial, so I'd deal with it the same way you deal with half-hardy perennials.

I have a feeling you could probably do either, but I'm not sure how pot culture would affect the seeds over winter. Pots are more susceptible to swings in temps and moisture levels, but I have a feeling it probably wouldn't affect the seeds. They might sprout earlier next spring in the pots, so you would have to watch them carefully and make sure they have moisture when they do sprout.

Why don't you do both?

The first couple of years I grew this verbena, I always started them inside around March, but then I realized they self-sowed like crazy and there really wasn't any need to do so. The ones started inside did mature and bloom earlier, but that was about the only advantage.

One other thing: Are you sure you want to grow these in pots? I'm having a hard time picturing them growing this way because they grow so tall (over 4 feet), but maybe you're using really big pots.

Kevin

Tx. I did hear that they grow tall but just assumed that in a windowbox, it would naturally limit the root growth and hence the height??

No, they will get tall even in a small container unless you keep clipping them. With overwintered containers, I always subtract a zone or two in terms of hardiness. Brazilian Verbena probably won't survive a true Zone 6 winter in the ground, let alone in a container. I would hang onto the seeds until Spring.

Here in zone 4 it's a re-seeding annual.