Self-seeding Verbena bonariensis (ht 1 - 1.5m) from South America is regarded as a weed by some gardeners, but few other plants have the airy, ethereal grace of its tall skinny stems topped with dainty lilac-purple posies. It is very different from the usual bedding Verbena plant.
It flowers throughout summer and autumn and gives a relaxed, uncontrived look to the garden. It looks best at the front edge of a border, to break the monotony of low plantings without obscuring other plants behind it. It also looks effective grown with ornamental grasses to create a meadowy look. It will grow simply anywhere, in any soil, but does best in full sun. If tip-pruned when young, it will branch out well. I also cut mine back by half periodically through summer, which gives them a new lease of life. Though regarded as an annual, it is technically a perennial and some plants survive through winter in Sydney - but there are also plenty of new seedlings each year, so I pull old plants out when they become very scruffy. Cut stems will last well in a vase and buds will continue to open. It can be grown from cuttings.
This plant is attractive to butterflies.
Postscript: Eventually, I tired of this plant. It just self-seeded way too much for my liking and the plants over time seem to regress to a less attractive form.