Verbena bonariensis

We planted these years ago and they're still flowering every year, although much less vigorously than originally. Whether they've run out of steam, or don't like the soil (stony clay), I don't know. They self seed but the seedlings never seem to come to anything. Perhaps I should pot some up and bring them on until they're stronger plants, then plant them out and see how they do. They're a lovely colour and the mass of tiny trumpet florets look quite other-worldly close up. It's one of those flowers that looks like the inhabitant of a rock pool or a coral reef.