This grower is sure of this plant identification.
Grown from cuttings taken last year and kept in the greenhouse.
Purple top verbena, South American vervain, Tall verbena, Vervain
Verbena bonariensis (Purple top verbena). This is a tall perennial, which is grown as an annual, and produces deep purple flowers from mid summer to early autumn. The deeply toothed leaves cluster in a mounded rosette.
They are marginally frost hardy to frost tender. Grow them in medium, well-drained soil in full sun or at most partial-shade.
Walls, rock garden, attracts bees and butterflies.