You must be talking about Verbena bonariensis - Brazilian verbena. I love it too.
You can cut the stalks off at the ground or just break them off when they are very dry.
Not only do the butterflies love it, have you had goldfinch hanging all over it eating the seed? They are really fun to watch.
This verbena seeds it's self around very freely, but I had a problem with powdery mildew this year;>(

Thanks for the info. Powdery mildew has been everywhere this year. Bayer systemic pretty much cured it on the verbena but has barely kept it at bay on others. I think that my two pair of goldfinces are spoiled by the nyjer seed bird feeder. I haven't noticed them on the verbena.