Vervain is a common name for many plants in the genus Verbena and the family Verbenaceae. V. bonariensis would be called a Vervain, but not all Vervains are V. bonariensis or even Verbenas. Your salesperson may now know as much as she believes she does. Did it come with any identification (or a tag) other than what the salesperson told you?

- Steve

google Verbena hastata / American Blue Vervain and see if this is the plant you purchased.

Hi Laurie,
I have Verbena hastata in my garden and it does sound like what you purchased. It will get very tall, and though the blooms aren't real showy, it blooms for a very long time. It has the same habit as V. bonariensis that it will come back as a perennial and self-seed around. The seedlings are easy to remove if unwanted. I don't have powdery mildew problems in my garden, so I can't comment on that. Butterflies do like it as you can see in the pic below.
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The photo was a shot I took standing at 5'2".
Remy

You might be able to get seeds for verbena bonarensis. Maybe ask on the seed exchange, someone may have a big stand of it. The vervain is pretty too but the dancing purple spheres of the Verbena bonarensis are really unique. I had it the last 2 years as a pass-along. I didn't know what it was at first but really liked it when it first bloomed and I was able to ID it here. This year I am still waiting for the seedlings to show up: I hope I have not lost it.

Thanks everyone! My Google search shows the plants I bought are Vervain. I did collect some V. bonariensis seeds before moving, but I've been too busy to sow them. I bought plants for instant gratification. LOL Perhaps I should sow the seeds after I finish planting. I might get a nice combo with the Vervain and the V. bonariensis.

Laurie, I think Vervain just means Verbena as others have said - but there are many sorts. The most common two border varieties here in the UK are V Bonariensis (about 5'), and V. Rigida (about 2' and a slightly differant colour). V Rigida is also known as the sandpaper Verbena, because its leaves are so rough. V Hastata is almost as tall as v Bonariensis, but is not seen so often here, though you can get it in about 4 different colors (white and shades from pink to purple), compared to 2 colors of V Rigida (purple and very pale blue), and only one color of V Bonariensis.

Laurie, I have just re-read your first post and seen your spcific queries. V Bonariensis is not that hardy, and may not survive your zone 6. But that need not be a problem, its seeds around a lot, so you will never be without it. And it grows the same height and flowers virtually as well in the first year. It can get powdery mildew it it gets too dry, but it isn't a problem in the UK, which is probably cooler in the summer - though it went into the 90's last summer. You don't need to pich it back. By its nature it is a very see through plant. And no you don't need to deadhead. Butterflies love it more than anything else in my garden.