In May one of the giants of the perennial world rears up and cheerfully produces a panicle of enormous lilac dahlia flowers: mystifying the horticulturally challenged, and amusing those of us who grow this triffid. This is the tree dahlia (Dahlia imperialis), which can grow up to 5m tall! Related to the usual sorts of herbaceous dahlias, everything about it is magnified. It grows from an enormous tuber and its stems are like stout canes of bamboo, and shoot up during summer. The leaves are like the lacy foliage of the typical dahlia plant, just very enlarged. It likes the same sort of growing conditions as its smaller cousins: full sun and rich, well-drained soil, and plenty of feeding to fuel its phenomenal growth.
There are other colours, including white, red and rich purple (e.g. 'Timothy Hammett', which is a cross between a tree dahlia and a normal garden dahlia) as well as double forms, but the lilac one is the most commonly seen. The tree dahlia is best grown in amongst other tall autumn-flowering shrubs, such as sasanqua camellias or tibouchinas, to give it support and to provide companion flowers. Some of the statuesque autumn/winter-flowering salvias which grow to around 3m - such as Salvia wagneriana, Salvia wagneriana, Salvia involucrata x karwinskii and Salvia purpurea - are in bloom at the same time and provide a suitable sense of scale to the tree dahlia if planted nearby. The tree daisy (Montanoa bipinnatifida), with white daisy flowers in autumn, is another possibility.
The tree dahlia is probably best suited to frost-free climates: most Sydney suburbs are ideal. It may need some support from a thick stake when it is in full bloom. It should be cut down to the ground as soon as flowering is over. New plants can be grown from lengths of these cuttings taken from the base of the canes and containing a couple of nodes. Lay the cuttings horizontally in a shallow trench and cover them with soil and they should begin to grow and will produce flowers in the following autumn. It is also possible to take firm tip cuttings in summer.
Postscript: I eventually have got tired of the ungainliness of tree dahlias, as they are so hard to stake (often snapping off in high winds) and are so out of proportion to other plants. They were a great novelty but I am going to take mine out. 'Timothy Hammett', which was supposed to grow only 1m tall, soared to 3m and was very hard to manage.