How do I look after Verbena bonariensis?

Daphne Wells, by email This is a herbaceous perennial with tall, wiry but tough stems and heads of tiny purple flowers through summer. Simply cut the stems back to ground level in late autumn and it will come again next year. It spreads readily by runners and by self-seeding but is never invasive. I always think it looks marvellous growing in gravel, which is where I have it. Verbena bonariensis is a magnet for pollinators