Answer: True Verbena canadensis is hardy into zone 5, as long as drainage is perfect. "Homestead Purple" on the other hand, is marginally hardy in zone 6 even with excellent drainage. If it is sold as an annual in your area, then chances are that experience has shown it not to be hardy where you are.
By the end of the season your plant should be quite huge, probably too big to dig up. However, you might try it since you will probably lose it anyway. You might also try layering it or taking cuttings and growing these on in pots until the weather turns cold; then store them in a well protected coldframe or even better, a garage until spring. These methods would cause the plant to go dormant for the winter.
To be honest, I think as a houseplant it would grow leggy and fail to bloom for you, but if you are determined to try it, cut it back when you dig it, put it in a large pot with a very free draining mix, keep it in the brightest location possible and do not overfeed it. Water moderately, never leaving it soggy. Good luck with your plant!
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