It self seeds freely here. All you have to do is pull out the ones that you don't want. Easy from cuttings too if you're somewhere colder where it doesn't set so much seed.
If you're in a wetter/colder area @Beany1971, and they don't seed so readily, you can take cuttings and grow them on over winter to plant out the following year. They're very amenable, useful plants.
It's a place where beautiful isn't enough of a word....
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https://www.gardenersworld.com/plants/verbena-bonariensis/
They're very amenable, useful plants.