Verbena bonariensis

Tall wiry stems at 1.2 to 1.5m carrying tight clusters of small electric-purple flowers, and the stems are so thin and so sparsely leafed that you can plant it at the front of a border and still see everything behind it. That see-through quality is why designers use it constantly.

For butterflies it is one of the two or three most important plants in a late-summer British garden. The flat flower heads make a landing platform, and it goes on from midsummer to the frosts.

It is a short-lived perennial and a hard winter will kill it, particularly on heavy wet ground. What saves it is prolific self-seeding: dozens of seedlings appear in spring and replace the parents without your doing anything. That is also the honest warning, because in a garden it likes it will come up everywhere, in gravel, in paving, in the middle of other plants. Pull what you do not want while it is small. Leave the dried stems standing over winter for structure, and cut back in spring once the new growth shows.