It has a unique smell, to me I like the smell. Do they grow back from roots, or only from seed?

My plants overwinter AND reseed prolifically

Yes, they are perennials which reseed up here too.
I have them in my yard because a neighbour grows them, and the seeds travel over here.

I guess they are here to stay

Most of mine act as an anual, though. But they self seed prolifically, so I always have them in my garden. I like to see them pop up where there was none before.

I enjoy the smell too, but not the way they reseed. Some of mine live 2-3 years but with the reseeded ones, there are always some around even if I don't want them.

I almost lost these this year. Typically they come back in the same spot plus reseed elsewhere, but this year I only have a few random plants. Hopefully those will reseed, and perhaps I' ll try to save some seed just in case.